Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports

Penelope Banou; Stamatis Boyatzis; Konstantinos Choulis; Thanasis Karabotsos; Dimitris Tsimogiannis; Lamprini-Areti Tsakanika; Constantina Tzia; Athena Alexopoulou

doi:10.3390/analytica3030019

Banou, Penelope;Boyatzis, Stamatis;Choulis, Konstantinos;Karabotsos, Thanasis;Tsimogiannis, Dimitris;Tsakanika, Lamprini-Areti;Tzia, Constantina;Alexopoulou, Athena

2022-07-02 00:00:00

Article Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports 1 , 1 1 1 Penelope Banou * , Stamatis Boyatzis , Konstantinos Choulis , Thanasis Karabotsos , 2 2 2 1 Dimitris Tsimogiannis , Lamprini-Areti Tsakanika , Constantina Tzia and Athena Alexopoulou Department of Antiquities and Works of Art, University of West Attica, Ag. Spyridonos Str., 12243 Egaleo, Greece; sboyatzis@uniwa.gr (S.B.); kchoulis@uniwa.gr (K.C.); akarab@uniwa.gr (T.K.); athfrt@uniwa.gr (A.A.) School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 9 Iroon Polytechniou Str., 15780 Zografou, Greece; ditsimog@chemeng.ntua.gr (D.T.); btsakanika@gmail.com (L.-A.T.); tzia@chemeng.ntua.gr (C.T.) * Correspondence: pbanou@uniwa.gr; Tel.: +30-6977218522 Abstract: Condition assessment of works of art created with oil media on paper could be a complex matter when presenting problems of damage due to the absorption of oil binders by the paper support, since they depend on several factors and occur in variable conditions. The present work refers to the results of an investigation on the effect of linseed oils on the color, opacity, morphology, tensile strength, and chemical properties of lignocellulosic papers, in comparison to that of pure cellulosic papers. Lignocellulosic papers are involved in research on new, yet signiﬁcant, parameters that might inﬂuence the behavior of the oil-impregnated areas of the supports upon aging. The research was applied to mock-ups, made of two types of lignocellulosic paper impregnated with three types Citation: Banou, P.; Boyatzis, S.; of linseed oil and subjected to accelaratated ageing in speciﬁc conditions of relative humidity and Choulis, K.; Karabotsos, T.; temperature in closed environment. The research involved colorimetry, opacity, tensile strength, pH Tsimogiannis, D.; Tsakanika, L.-A.; measurements, SEM, FTIR, and VOC analysis with GC-MS. The results indicated that thermal-humid Tzia, C.; Alexopoulou, A. Oil Media ageing caused the gradual darkening of the oil-impregnated mock-ups, alterations in opacity, and on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper decrease of pH values, depending mainly on the formulation of linseed oil, as well as a reduction in Supports. Analytica 2022, 3, 266–286. tensile strength. FTIR analysis results indicated that the chemical changes that occur upon ageing https://doi.org/10.3390/ supported the recorded optical and mechanical alterations, while VOC emissions are both associated analytica3030019 with the paper type and the kinetics of degradation of the different types of linseed oil. Academic Editors: Anastasios Keywords: linseed oil; pure cellulosic paper; FTIR; VOC; GC-MS; colorimetry; opacity; tensile Zouboulis and Konstantinos Simeonidis strength; SEM; pH Received: 1 June 2022 Accepted: 28 June 2022 Published: 2 July 2022 1. Introduction Publisher’s Note: MDPI stays neutral Oil media on paper, such as oil paintings and oil sketches on paper and prints, but with regard to jurisdictional claims in also archival material and books, present alterations on their supports, considered as published maps and institutional afﬁl- damage by conservators, which are associated with the effect of oil binders contained in oil iations. colors and traditional oil-based printing inks when absorbed by the paper support [1–3]. Alterations include discoloration, a decrease in pH value, loss of mechanical strength and embrittlement, and cracks and losses in the oiled areas of the paper support [1–6]. The intensity and extent of this alteration/damage determine the condition of a work and Copyright: © 2022 by the authors. conservation decision-making. However, it occurs irregularly, varying from limited and Licensee MDPI, Basel, Switzerland. local to overall, as it depends on several parameters, such as the materials (type of paper, This article is an open access article distributed under the terms and oil medium, and pigments) and techniques (e.g., impasto or diluted with solvents or oil) conditions of the Creative Commons used in the creation of the works [6]. This fact often makes the evaluation of the condition Attribution (CC BY) license (https:// of a collection a composite matter. It should be mentioned that damage to works without creativecommons.org/licenses/by/ priming or a preparation layer on the paper support, or coated papers, has been recorded. 4.0/). Analytica 2022, 3, 266–286. https://doi.org/10.3390/analytica3030019 https://www.mdpi.com/journal/analytica Analytica 2022, 3 267 A small number of research works on this matter have published in the past, though they do not provide adequate results to compensate to the interpretation of alterations and problems recorded in original works [7–10]. Recently, the authors of the present work reported the initial results of an investigation into the effect of linseed oils on pure cellulosic paper [11]. Speciﬁcally, we have researched the changes in the optical, morphological, mechanical, and chemical properties of pure cellulosic paper mock-ups impregnated with three types of linseed oil, subjected to thermal-humid ageing in air-tight vessels. The study introduced a holistic methodology to study the effect of linseed oil on pure cotton paper supports for the ﬁrst time, with interesting results for paper conservators to support condition assessment. Three formulations of linseed oil were selected for the preparation of mock-ups, as linseed oil was the most representative oil medium used in oil painting, printing, and typography for over ﬁve centuries (from the 15th century AD up to the 20th century) [12–15]. The current work will present the results of the application of the same methodology for the investigation of linseed oils in lignocellulosic paper supports, a signiﬁcant category of paper. In the 19th century, wood pulp was introduced to industrial papermaking, which became widely used after 1850 to produce several types of commercially available lignocellulosic paper supports for writing, drawing, painting, printing, and printmaking, still in use today [16]. The method of wood-pulp processing and the ﬁbre and pulp content provide additional factors that might inﬂuence the changes of the paper–oil system upon ageing. A comparative study of the results could provide scientiﬁc evidence for the effect of linseed oils both on pure cellulosic and lignocellulosic papers upon ageing, of sub- stantial value for the condition assessment of works. The research aims to study the alteration/damagesthat occurs on paper supports due to the absorption of oil binders upon ageing and determine the possible stages of deterioration. 2. Materials and Methods 2.1. Materials Three formulations of linseed oil were selected for the preparation of mock-ups: Cold- pressed linseed oil, alkaline reﬁned linseed oil, and stand-oil (Windsor and Newton), which are the same ones used for pure cellulosic papers. The difference in the methodology of oil manufacture provides these formulations with different physicochemical properties, such as wetting power, drying rate, yellowing or darkening, viscosity, rheology, acid value, and degree of polymerization [13,14,17,18]. Mock-ups were made of two types of paper with different lignin content. A typical ® ® watercolor paper, Canson Montval , white color, cold-pressed, acid-free, made of wood pulp (soft and hardwood ﬁbres) and limited lignin content, without optical brightness additives, 185 gsm (complying with ISO Standard 9706 requirements for permanence ® ® (Montval |Canson https://en.canson.com/watercolour/canson-montval, accessed on 25 May 2022) (Art & Hobby, Athens, Greece) and a wrapping paper, Kraft paper (Dion- isopoulos, paper distributor, Greece), brown color, 100 gsm, buffered, made of soft and hardwood ﬁbres, containing lignin, ﬁllers, additives, and metallic contamination. Applica- tion of optical microscopy, EDX analysis, and FTIR analysis provided data on the ﬁber and pulp content. 2.2. Preparation of Mock-Ups and Artiﬁcial Ageing Oil-impregnated mock-ups were air-dried at room conditions of 22 C and 52% RH for 40 days, hanged in racks using pure cotton threads. Mock-ups were prepared and subjected to accelarated ageing in air-tight vessels in conditions of 77% relative humidity and 80 temperature, the same used for pure cellulosic papers [11]. Analytica 2022, 3, FOR PEER REVIEW 3 Analytica 2022, 3 268 2.3. Methods The research followed the exact methodology applied to the investigation on pure 2.3. Methods cellulosic paper supports reported in previous work carried out by the authors of this The research followed the exact methodology applied to the investigation on pure work [11]. Experimental work included colorimetry, opacity, tensile strength, pH cellulosic paper supports reported in previous work carried out by the authors of this measurements, SEM, FTIR, and VOC analysis with GC‐MS. work [11]. Experimental work included colorimetry, opacity, tensile strength, pH measure- ments, SEM, FTIR, and VOC analysis with GC-MS. 3. Results and Discussion 3.1.3. Color Results Chan and ges Discussion 3.1. Color Changes After 40 days of drying, oil‐impregnated white Montval mock‐ups take on a yellowish After hue, 40 while days oflight drying, brown oil-impr Kraft egnated mock‐ups white appea Montval r darmock-ups ker. For oil take‐impregnat on a yellowish ed Montval hue, while mocklight ‐ups,br it own is evident Kraft tha mock-ups t the intensi appear ty of darker the yellow . For oil-impr hue varies egnated among Montval the three mock- ups, it is evident that the intensity of the yellow hue varies among the three sets (M + CP, sets (M+CP, M+RFm and M+Stl) (Figure 1). This fact has also confirmed that oil processing M + RFm and M + Stl) (Figure 1). This fact has also conﬁrmed that oil processing during during manufacture influence the hue of the formulations of linseed oil. manufacture inﬂuence the hue of the formulations of linseed oil. (a) (b) (c) (d) Figure 1. Visible light photography: (a) sets of plain mock‐ups, Kraft (left) and Montval (right); (b) Figure 1. Visible light photography: (a) sets of plain mock-ups, Kraft (left) and Montval (right); sets impregnated with cold‐pressed linseed, K+CP (left), and M+CP (right); (c) sets impregnated (b) sets impregnated with cold-pressed linseed, K + CP (left), and M + CP (right); (c) sets impregnated with refined linseed oil, K+RF (left) and M+RF (right); (d) sets impregnated with stand‐oil, K+StL with reﬁned linseed oil, K + RF (left) and M + RF (right); (d) sets impregnated with stand-oil, K + StL (left), and M+StL (right). (left), and M + Stl (right). Accelarated ageing resulted in limited changes to plain Montval mock‐ups, mainly Accelarated ageing resulted in limited changes to plain Montval mock-ups, mainly discernable in those subjected to 21 and 28 days of ageing, while plain Kraft mock‐ups discernable in those subjected to 21 and 28 days of ageing, while plain Kraft mock-ups presented notable changes after 14 days of ageing (Figure 1). The visible observations presented notable changes after 14 days of ageing (Figure 1). The visible observations were were confirmed by ΔΕ* difference measurements, as the values of plain Montval mock‐ conﬁrmed by DE* difference measurements, as the values of plain Montval mock-ups were ups were lower than 2 for those subjected up to 14 days of ageing (the minimal detectable lower than 2 for those subjected up to 14 days of ageing (the minimal detectable difference difference is between 1 and 2 ΔE [19,20]), and higher than 5 for those subjected to 21 and is between 1 and 2 DE [19,20]), and higher than 5 for those subjected to 21 and 28 days. The 28 days. The values of plain Kraft became higher than 2 for those subjected to artificial values of plain Kraft became higher than 2 for those subjected to artiﬁcial ageing for 14 and ageing for 14 and 21 days, through the value of 3 for those of 28 days. Color changes upon 21 days, through the value of 3 for those of 28 days. Color changes upon ageing could be ageing could be attributed to the oxidation of paper in the conditions of accelarated attributed to the oxidation of paper in the conditions of accelarated ageing, especially in ageing, especially in the final stages (21st and 28th day). the ﬁnal stages (21st and 28th day). For oil‐impregnated mock‐ups, thermal‐humid conditions of ageing resulted in a For oil-impregnated mock-ups, thermal-humid conditions of ageing resulted in a grad gradual ual dardarkening, kening, both both for for Montval Montval andand Kraft, Kraft, with with the the first ﬁrst to present to present more mor distinct e distinct variations variations upon upon ageageing ing (Figu (Figur re 1)e. Oil 1).‐Oil-impr impregna egnated ted setssets of Kraft of Kr mock aft mock-ups ‐ups appea appear red to ed be to be darker after 14 days of ageing (Figure 1). However, Montval mock-ups impregnated with Analytica 2022, 3, FOR PEER REVIEW 4 darker after 14 days of ageing (Figure 1). However, Montval mock‐ups impregnated with stand‐oil (M+StL) presented color irregularities, possibly due to the uneven oil distribution over the paper surface because of stand‐oil’s high density and viscosity, as Analytica 2022, 3 269 well as limited changes between the ageing stages, a fact that could be attributed to oil processing during manufacture [13], p. 41. In addition, they appeared to be lighter than stand-oil (M + Stl) presented color irregularities, possibly due to the uneven oil distribution the other two sets at all stages of ageing, while the mock‐ups impregnated with refined over the paper surface because of stand-oil’s high density and viscosity, as well as limited linseed oil (M+Rf) were darker than those impregnated with cold‐pressed linseed oil changes between the ageing stages, a fact that could be attributed to oil processing during (M+CP). This observation also applied to the white cotton mock‐ups [11]. Oil treatment manufacture [13], p. 41. In addition, they appeared to be lighter than the other two sets at during manufacture could cause differences in yellowing and darkening upon ageing all stages of ageing, while the mock-ups impregnated with reﬁned linseed oil (M + RF) were darker than those impregnated with cold-pressed linseed oil (M + CP). This observation [17]. also applied to the white cotton mock-ups [11]. Oil treatment during manufacture could Visible observations on plain and oil‐impregnated Montval and Kraft mock‐ups were cause differences in yellowing and darkening upon ageing [17]. supported by the reflectance spectra (Figures 2 and 3) and color difference ΔΕ* values Visible observations on plain and oil-impregnated Montval and Kraft mock-ups were upon ageing (Figures 4 and 5). The trend of changes of all sets of oil‐impregnated Montval supported by the reﬂectance spectra (Figures 2 and 3) and color difference DE* values mock‐ups is quite si upon milaageing r with(Figur few es va4ria and tio 5n ).sThe . trend of changes of all sets of oil-impregnated Montval mock-ups is quite similar with few variations. Figure 2. Reﬂectance spectra of all sets of Montval mock-ups: (a) plain ones; (b) impregnated with Figure 2. Reflectance spectra of all sets of Montval mock‐ups: (a) plain ones; (b) impregnated with cold-pressed oil, M + CP; (c) impregnated with reﬁned linseed oil, M + RF; (d) impregnated with cold‐pressed oil, M+CP; (c) impregnated with refined linseed oil, M+RF; (d) impregnated with stand-oil, M + Stl. Measurements of mock-ups subjected to ageing for 0, 2, 4, 7, 14, 21, and 28 days. stand‐oil, M+StL. Measurements of mock‐ups subjected to ageing for 0, 2, 4, 7, 14, 21, and 28 days. The reﬂectance spectra of the three sets, which were derived from the average values, indicated differences in color changes among the ageing stages (Figure 2). For Montval mock-ups impregnated with cold-pressed linseed oil (M + CP), reﬂectance spectra indicated gradual darkening with distinct stages, while for those impregnated with reﬁned linseed oil (M + RF) and stand-oil (M + Stl), darkening appeared to be irregular. Color changes could be mainly attributed to oil changes upon ageing. Treatment of oil during manufacture inﬂuences the drying rate and consequently the chemical changes associated with color changes [17]. Differences in color changes between the three sets at the various ageing stages could be attributed to the slower rate of drying of reﬁned linseed oil and stand-oil than that of cold-pressed oil [13,17]. Limited changes were recorded for the mock-ups impregnated with reﬁned linseed oil (M + RF), presenting limited changes between the 4th and the 7th day as well as between the 21st and the 28th day, while there was a more intense change between the 2nd and the 4th day than those recorded on the other two Analytica 2022, 3 270 sets. In comparison, those impregnated with stand-oil (M + Stl) presented smaller changes Analytica 2022, 3, FOR PEER REVIEW 5 between the 7th and the 14th day, as well as between the 21st and the 28th day, while a Analytica 2022, 3, FOR PEER REVIEW 5 more intense change was recorded between the 4th and 7th day. Graphic representation of DE* values upon ageing showed a common trend between the three sets, indicating the differences in color changes (Figure 4). Figure 3. Reflectance spectra of all sets of Kraft mock‐ups: (a) plain ones; (b) impregnated with cold‐ Figure 3. Reﬂectance spectra of all sets of Kraft mock-ups: (a) plain ones; (b) impregnated with Figure pressed 3. Reflectance oil, K+CP; ( cspectra ) impreg ofnated all se tswith of Kraft refined moc likns‐ups eed: (oia)l, plain K+RF; ones; (d) im (b)p impre regnated gnate with d with stand cold‐oil‐ , cold-pressed oil, K + CP; (c) impregnated with reﬁned linseed oil, K + RF; (d) impregnated with pressed K+StL. Mea oil, K+ surements CP; (c) im ofpreg mock nated ‐ups with subjecte refined d to liageing nseed for oil, 0,K+RF; 2, 4, 7,(d 14 ) im , 21p,r eg and nated 28 day with s. stand‐oil, stand-oil, K + StL. Measurements of mock-ups subjected to ageing for 0, 2, 4, 7, 14, 21, and 28 days. K+StL. Measurements of mock‐ups subjected to ageing for 0, 2, 4, 7, 14, 21, and 28 days. Figure 4. DE changes of all sets of Montval mock-ups upon ageing. Figure 4. ΔΕ* changes of all sets of Montval mock‐ups upon ageing. Figure 4. ΔΕ* changes of all sets of Montval mock‐ups upon ageing. Analytica 2022, 3, FOR PEER REVIEW 6 Analytica 2022, 3 271 Figure 5. DE changes of all sets of Kraft mock-ups upon ageing. Figure 5. ΔΕ* changes of all sets of Kraft mock‐ups upon ageing. Likewise, the reﬂectance spectra of the oil-impregnated Kraft sets presented differences The reflectance spectra of the three sets, which were derived from the average values, in color changes among the ageing stages (Figure 3). For Kraft mock-ups impregnated with cold-pressed linseed oil (K + CP), a gradual darkening with distinct stages was indicated, indicated differences in color changes among the ageing stages (Figure 2). For Montval while those impregnated with reﬁned linseed oil (K + RF) presented limited changes mock‐ups impregnated with cold‐pressed linseed oil (M+CP), reflectance spectra between the 4th and 14th day, as well as between the 21st and the 28th day. On the other indicated gradual darkening with distinct stages, while for those impregnated with hand, those impregnated with stand-oil presented insigniﬁcant changes up to the 4th day, refined linseed oil (M+RF) and stand‐oil (M+StL), darkening appeared to be irregular. an intense change until the 7th day, limited changes between the 7th, 14th, and 21st day, Color changes could and abe distinct main change ly attri until buted the 28th to oil day ch . Again, anges the upon differ ag ences eing. could Trebe atment attributed of oi tol the difference in properties of the three formulations, and in particular, in the drying rate and during manufacture influences the drying rate and consequently the chemical changes yellowing/darkening. associated with color changes [17]. Differences in color changes between the three sets at Consequently, differences could be due to the rate of oxidation and degradation the various ageing stages could be attributed to the slower rate of drying of refined linseed of each formulation of linseed oil. The standard deviation of DE* values conﬁrmed the oil and stand‐oil than that of cold‐pressed oil [13,17]. Limited changes were recorded for abovementioned and indicated a common trend for Kraft mock-ups impregnated with cold-pressed linseed oil and reﬁned linseed oil, while for those impregnated with stand-oil, the mock‐ups impregnated with refined linseed oil (M+RF), presenting limited changes a smaller number of changing stages were indicated (Figure 5). However, the changes between the 4th and the 7th day as well as between the 21st and the 28th day, while there of the oil-impregnated Kraft mock-ups upon ageing and between ageing stages were was a more intense change between the 2nd and the 4th day than those recorded on the comparatively limited in comparison to those of the oil-impregnated cotton and Montval other two sets. In comparison, those impregnated with stand‐oil (M+StL) presented mock-ups, indicating that Kraft mock-ups were more opaque. smaller changes between The tr the end of 7th color and changes the 14 ofth oil-impr day, egnated as wellKraft as between and Montval thepapers 21st and were the similar to those of pure cotton paper in the means of darkening upon ageing, indicating that oil 28th day, while a more intense change was recorded between the 4th and 7th day. Graphic is the principal factor in color changes, while variations depend mainly on oil treatment representation of ΔΕ* values upon ageing showed a common trend between the three sets, of formulations. However, the differences in color changes upon ageing between oil- indicating the differences in color changes (Figure 4). impregnated cotton, Montval, and Kraft mock-ups could be attributed to the color of the Likewise, the reflectance spectra of the oil‐impregnated Kraft sets presented plain papers, pulp content and oil penetration, as well as to paper weight (ratio of paper and oil volume). differences in color changes among the ageing stages (Figure 3). For Kraft mock‐ups impregnated with cold‐pressed linseed oil (K+CP), a gradual darkening with distinct 3.2. Opacity Changes stages was indicated, while those impregnated with refined linseed oil (K+RF) presented Both plain Montval and Kraft mock-ups presented insigniﬁcant changes in opacity limited changes between the 4th and 14th day, as well as between the 21st and the 28th upon ageing (increase of opacity of up to 2%). These results indicated that artiﬁcial ageing day. On the other has han ad limited , those ef impregn fect on theaopacity ted with of stan the plain d‐oil papers. presente On the d in contrary signific , application ant change ofs the three formulations of linseed oil to Montval and Kraft mock-ups, after 40 days of drying, up to the 4th day, an intense change until the 7th day, limited changes between the 7th, resulted in a notable reduction of the opacity of the oil-impregnated mock-ups, but to a 14th, and 21st day, and a distinct change until the 28th day. Again, the differences could be attributed to the difference in properties of the three formulations, and in particular, in the drying rate and yellowing/darkening. Consequently, differences could be due to the rate of oxidation and degradation of each formulation of linseed oil. The standard deviation of ΔΕ* values confirmed the abovementioned and indicated a common trend for Kraft mock‐ups impregnated with cold‐pressed linseed oil and refined linseed oil, while for those impregnated with stand‐ oil, a smaller number of changing stages were indicated (Figure 5). However, the changes of the oil‐impregnated Kraft mock‐ups upon ageing and between ageing stages were Analytica 2022, 3, FOR PEER REVIEW 7 comparatively limited in comparison to those of the oil‐impregnated cotton and Montval mock‐ups, indicating that Kraft mock‐ups were more opaque. The trend of color changes of oil‐impregnated Kraft and Montval papers were similar to those of pure cotton paper in the means of darkening upon ageing, indicating that oil is the principal factor in color changes, while variations depend mainly on oil treatment of formulations. However, the differences in color changes upon ageing between oil‐ impregnated cotton, Montval, and Kraft mock‐ups could be attributed to the color of the plain papers, pulp content and oil penetration, as well as to paper weight (ratio of paper and oil volume). 3.2. Opacity Changes Both plain Montval and Kraft mock‐ups presented insignificant changes in opacity upon ageing (increase of opacity of up to 2%). These results indicated that artificial ageing has a limited effect on the opacity of the plain papers. On the contrary, application of the three formulations of linseed oil to Montval and Kraft mock‐ups, after 40 days of drying, resulted in a notable reduction of the opacity of the oil‐impregnated mock‐ups, but to a different extent (Figures 6 and 7). In particular, the opacity of M+CP mock‐ups reduced by 26%, M+Rf mock‐ups by 35%, and M+StL by 48%, while K+CP mock‐ups reduced by 39%, K+RF mock‐ups by 38%, and K+StL mock‐ups by 43%. However, application of linseed oils caused an increase in the opacity of the mock‐ ups upon ageing, reaching up to 48% at the final stages of ageing, with variations follow‐ ing the formulations of linseed oil. Those impregnated with cold‐pressed linseed oil and refined linseed oil showed similar trends of change in opacity upon ageing. The opacity Analytica 2022, 3 272 of Montval with cold‐pressed linseed oil and refined linseed oil (M+CP and M+RF) increased quite rapidly up to 7 days of ageing, and then to a milder pace up to 28 days of ageing. The trenddif was ferent qui extent te similar (Figur for es 6 Kraft and 7). mock In particular ‐ups (K+CP , the opacity and K+ of M RF +),CP but mock-ups the change reduced s by 26%, M + RF mock-ups by 35%, and M + Stl by 48%, while K + CP mock-ups reduced by were more gradual. 39%, K + RF mock-ups by 38%, and K + StL mock-ups by 43%. Analytica 2022, 3, FOR PEER REVIEW 8 Figure 6. Opacity changes of all sets of Montval mock-ups upon ageing. Figure 6. Opacity changes of all sets of Montval mock‐ups upon ageing. Figure 7. Opacity changes of all sets of Kraft mock-ups upon ageing. Figure 7. Opacity changes of all sets of Kraft mock‐ups upon ageing. However, application of linseed oils caused an increase in the opacity of the mock-ups In comparison, the range of changes in the opacity of mock‐ups impregnated with upon ageing, reaching up to 48% at the ﬁnal stages of ageing, with variations following the formulations of linseed oil. Those impregnated with cold-pressed linseed oil and reﬁned stand‐oil (M+StL and K+StL) were smaller, as the opacity of the M+StL mock‐ups linseed oil showed similar trends of change in opacity upon ageing. The opacity of Montval increased up to 14% and that of K+StL mock‐ups up to 25% by the final stages of ageing. Specifically, the opacity of the mock‐ups remained almost stable for up to the 4 days of ageing, increased on the 7th day and remained almost stable up to the 21st day, and then presented a notable increase until the 28th day of ageing. Processing of stand‐oil during manufacture results in pre‐polymerization and high density, which are possibly responsible for the differences in the opacity upon ageing. It could be suggested that changes in the opacity of the mock‐ups can be mainly attributed to linseed oil formulations and their different properties. Taking into consideration the changes in opacity on the oil‐impregnated mock‐ups of cotton [11], which has similar weight to Kraft, results show that oil‐impregnated Kraft mock‐ups are less opaque, indicating differences in linseed oil’s penetration into the paper pulp. This also confirms the observations in color changes (3.1). On the other hand, the weight difference of Montval paper, and differences in the ratio of paper and oil volume respectively, had an effect on the trend of changes in opacity in the various ageing stages. 3.3. Morphological Changes SEM combined with EDX mapping indicated the presence of alkaline buffer for plain Montval and Kraft papers, but also the presence of fillers and impurities for Kraft paper. The particles of CaCO3 were spread throughout the fiber net of Montval paper without filling out the space/voids between the fibers (Figure 8). On the other hand, the fiber net of Kraft paper was loaded with fillers, additives, and impurities, creating a dense mass (Figure 9). Figure 8. SEM images of plain Montval mock‐ups at 0 days of ageing: (a) 130× magnification; (b) 200× magnification; (c) 500× magnification; and (d) 1000× magnification. Analytica 2022, 3, FOR PEER REVIEW 8 Analytica 2022, 3 273 Figure 7. Opacity changes of all sets of Kraft mock‐ups upon ageing. In comparison, the range of changes in the opacity of mock‐ups impregnated with with cold-pressed linseed oil and reﬁned linseed oil (M + CP and M + RF) increased quite stand‐oil (M+StL and K+StL) were smaller, as the opacity of the M+StL mock‐ups rapidly up to 7 days of ageing, and then to a milder pace up to 28 days of ageing. The increased up to 14% and that of K+StL mock‐ups up to 25% by the final stages of ageing. trend was quite similar for Kraft mock-ups (K + CP and K + RF), but the changes were Specifically, the opacity of the mock‐ups remained almost stable for up to the 4 days of more gradual. ageing, increased on the 7th day and remained almost stable up to the 21st day, and then In comparison, the range of changes in the opacity of mock-ups impregnated with stand-oil (M + Stl and K + StL) were smaller, as the opacity of the M + Stl mock-ups presented a notable increase until the 28th day of ageing. Processing of stand‐oil during increased up to 14% and that of K + StL mock-ups up to 25% by the ﬁnal stages of ageing. manufacture results in pre‐polymerization and high density, which are possibly Speciﬁcally, the opacity of the mock-ups remained almost stable for up to the 4 days of responsible for the differences in the opacity upon ageing. ageing, increased on the 7th day and remained almost stable up to the 21st day, and then It could be suggested that changes in the opacity of the mock‐ups can be mainly presented a notable increase until the 28th day of ageing. Processing of stand-oil during attributed to linseed oil formulations and their different properties. Taking into manufacture results in pre-polymerization and high density, which are possibly responsible consideration the changes in opacity on the oil‐impregnated mock‐ups of cotton [11], for the differences in the opacity upon ageing. It could be suggested that changes in the opacity of the mock-ups can be mainly which has similar weight to Kraft, results show that oil‐impregnated Kraft mock‐ups are attributed to linseed oil formulations and their different properties. Taking into consider- less opaque, indicating differences in linseed oil’s penetration into the paper pulp. This ation the changes in opacity on the oil-impregnated mock-ups of cotton [11], which has also confirms the observations in color changes (3.1). On the other hand, the weight similar weight to Kraft, results show that oil-impregnated Kraft mock-ups are less opaque, difference of Montval paper, and differences in the ratio of paper and oil volume indicating differences in linseed oil’s penetration into the paper pulp. This also conﬁrms respectively, had an effect on the trend of changes in opacity in the various ageing stages. the observations in color changes (Section 3.1). On the other hand, the weight difference of Montval paper, and differences in the ratio of paper and oil volume respectively, had an effect on the trend of changes in opacity in the various ageing stages. 3.3. Morphological Changes SEM combined with EDX mapping indicated the presence of alkaline buffer for plain 3.3. Morphological Changes Montval and Kraft papers, but also the presence of fillers and impurities for Kraft paper. SEM combined with EDX mapping indicated the presence of alkaline buffer for plain The particles of CaCO3 were spread throughout the fiber net of Montval paper without Montval and Kraft papers, but also the presence of ﬁllers and impurities for Kraft paper. fillin The g ou particles t the space/voids of CaCO wer between e spread the throughout fibers (F the igure ﬁber 8) net . On of Montval the other paper hanwithout d, the fiber net ﬁlling out the space/voids between the ﬁbers (Figure 8). On the other hand, the ﬁber net of Kraft paper was loaded with fillers, additives, and impurities, creating a dense mass of Kraft paper was loaded with ﬁllers, additives, and impurities, creating a dense mass (Figure 9). (Figure 9). Analytica 2022, 3, FOR PEER REVIEW 9 Figure 8. SEM images of plain Montval mock-ups at 0 days of ageing: (a) 130 magniﬁcation; Figure 8. SEM images of plain Montval mock‐ups at 0 days of ageing: (a) 130× magnification; (b) (b) 200 magniﬁcation; (c) 500 magniﬁcation; and (d) 1000 magniﬁcation. 200× magnification; (c) 500× magnification; and (d) 1000× magnification. Figure 9. SEM images of plain Kraft mock-ups at 0 days of ageing: (a) 100 magniﬁcation; (b) 200 Figure 9. SEM images of plain Kraft mock‐ups at 0 days of ageing: (a) 100× magnification; (b) 200× magniﬁcation; (c) 500 magniﬁcation; and (d) 1000 magniﬁcation. magnification; (c) 500× magnification; and (d) 1000× magnification. After 40 days of air-drying of oil-impregnated mock-ups, SEM images showed that a After 40 days of air‐drying of oil‐impregnated mock‐ups, SEM images showed that a gelatinous ﬁlm covered the surface of Montval and Kraft mock-ups following the relief of gelatinous film covered the surface of Montval and Kraft mock‐ups following the relief of the pulp/ﬁber net (Figures 10–13). When the liquid linseed oil turns to a semi-elastic gel upon drying, it appears to ﬁll the ﬁber net. Upon ageing, the oil ﬁlm recesses gradually the pulp/fiber net (Figures 10–13). When the liquid linseed oil turns to a semi‐elastic gel while holes open locally, and, by the ﬁnal stages, the ﬁbers became exposed and particles upon drying, it appears to fill the fiber net. Upon ageing, the oil film recesses gradually while holes open locally, and, by the final stages, the fibers became exposed and particles of fillers and additives evident. Oil impregnated mock‐ups with cold‐pressed and refined linseed oil presented similar changes for both papers, providing similar images. In comparison, images indicated that recess of the highly viscous stand‐oil was comparatively limited upon ageing, without exposing the fiber net at the final stages Figures 11 and 13). Figure 10. SEM images of cold‐pressed linseed‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. Figure 11. SEM images of stand‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magni‐ fication: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. Figure 12. SEM images of cold‐pressed linseed‐oil‐impregnated Kraft mock‐ups upon ageing, at 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. Analytica 2022, 3, FOR PEER REVIEW 9 Analytica 2022, 3, FOR PEER REVIEW 9 Analytica 2022, 3, FOR PEER REVIEW 9 Figure 9. SEM images of plain Kraft mock‐ups at 0 days of ageing: (a) 100× magnification; (b) 200× magnification; (c) 500× magnification; and (d) 1000× magnification. Figure 9. SEM images of plain Kraft mock‐ups at 0 days of ageing: (a) 100× magnification; (b) 200× magnification; (c) 500× magnification; and (d) 1000× magnification. Figure 9. SEM images of plain Kraft mock‐ups at 0 days of ageing: (a) 100× magnification; (b) 200× After 40 days of air‐drying of oil‐impregnated mock‐ups, SEM images showed that a magnification; (c) 500× magnification; and (d) 1000× magnification. gelatinous film covered the surface of Montval and Kraft mock‐ups following the relief of After 40 days of air‐drying of oil‐impregnated mock‐ups, SEM images showed that a the pulp/fiber net (Figures 10–13). When the liquid linseed oil turns to a semi‐elastic gel gelatinous After 40 film days covered of air the ‐drying surface of oi ofl‐ Montval impregnated and Kraft mock ‐mock ups, SEM ‐ups fo imllow agesing show theed relie thaf tof a upon drying, it appears to fill the fiber net. Upon ageing, the oil film recesses gradually the gelatinous pulp/fibe film r net covered (Figures the 10 su–13 rface ). Whe of Montval n the li qand uid Kraft linseed mock oil ‐turns ups fo to llow a sem ing ith‐elastic e relie gel f of while holes open locally, and, by the final stages, the fibers became exposed and particles upon drying, it appears to fill the fiber net. Upon ageing, the oil film recesses gradually the pulp/fiber net (Figures 10–13). When the liquid linseed oil turns to a semi‐elastic gel of fillers and additives evident. Oil impregnated mock‐ups with cold‐pressed and refined while upon ho dryles ing, ope itn a loc ppea allry, s to and, fill by the the fiber fin anet. l stag Upon es, the ag fibers eing, the beca oil m ef ilm exposed reces sand es grad partually icles linseed oil presented similar changes for both papers, providing similar images. In of while filler ho s and les ope addni tloc iveasl levident. y, and, by Oil the imp finraegnated l stages, mock the fibers ‐ups wi beca th m coeld exposed ‐pressed and and part refine icleds Analytica 2022, 3 274 comparison, images indicated that recess of the highly viscous stand‐oil was lins of fieed ller soi and l p add resented itives sim evident. ilar ch Oil anges imp regnated for both mock pape‐up rs, sprovid with coinldg‐ pre simil ssed ar an imdage refine s. Ind comparatively limited upon ageing, without exposing the fiber net at the final stages comparison linseed oil ,p rimag esented es in sim dicat ilare dch anges that recess for both of pa the pe rshighly , provid viscous ing simil stan ar im d‐oil age swas . In Figures 11 and 13). com comparison parative,l y imag limited es in updicat on age ed ing tha , twi recess thout exposing of the the highly fiber viscous net at the stan fid na‐oil l stag was es of ﬁllers and additives evident. Oil impregnated mock-ups with cold-pressed and reﬁned Fig com up res linseed arat 11iv an oil eldy pr 13 esented limi ). ted similar upon changes ageing for, wi both thpapers, out exposing providing the similar fiber images. net In at compar the fi-nal stages ison, images indicated that recess of the highly viscous stand-oil was comparatively limited Figures 11 and 13). upon ageing, without exposing the ﬁber net at the ﬁnal stages Figures 11 and 13). Figure 10. SEM images of cold‐pressed linseed‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected Figure 10. SEM images of cold‐pressed linseed‐oil‐impregnated Montval mock‐ups upon ageing, at to 14 days of ageing; (d) subjected for 28 days of ageing. Figure 10. SEM images of cold-pressed linseed-oil-impregnated Montval mock-ups upon ageing, at 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected Figure 10. SEM images of cold‐pressed linseed‐oil‐impregnated Montval mock‐ups upon ageing, at 500 magniﬁcation: (a) after air-drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to to 14 days of ageing; (d) subjected for 28 days of ageing. 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected 14 days of ageing; (d) subjected for 28 days of ageing. to 14 days of ageing; (d) subjected for 28 days of ageing. Figure 11. SEM images of stand‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magni‐ Figure 11. SEM images of stand-oil-impregnated Montval mock-ups upon ageing, at 500 magni- fication: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of Figure 11. SEM images of stand‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magni‐ ﬁcation: (a) after air-drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. fication: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of Figure 11. SEM images of stand‐oil‐impregnated Montval mock‐ups upon ageing, at 500× magni‐ ageing; (d) subjected for 28 days of ageing. ageing; (d) subjected for 28 days of ageing. fication: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. Analytica 2022, 3, FOR PEER REVIEW 10 Figure 12. SEM images of cold-pressed linseed-oil-impregnated Kraft mock-ups upon ageing, at Figure 12. SEM images of cold‐pressed linseed‐oil‐impregnated Kraft mock‐ups upon ageing, at 500 magniﬁcation: (a) after air-drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected Figure 12. SEM images of cold‐pressed linseed‐oil‐impregnated Kraft mock‐ups upon ageing, at 14 days of ageing; (d) subjected for 28 days of ageing. to 14 days of ageing; (d) subjected for 28 days of ageing. 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected Figure 12. SEM images of cold‐pressed linseed‐oil‐impregnated Kraft mock‐ups upon ageing, at to 14 days of ageing; (d) subjected for 28 days of ageing. 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. Figure 13. SEM images of stand-oil-impregnated Kraft mock-ups upon ageing at 500 magniﬁcation: Figure 13. SEM images of stand‐oil‐impregnated Kraft mock‐ups upon ageing at 500× magnification: (a) after air-drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. (d) subjected for 28 days of ageing. This conﬁrms the observations in the opacity changes of oil-impregnated mock-ups with stand-oil (Section 3.2). However, on the Kraft mock-ups, the oil ﬁlm recess is even This confirms the observations in the opacity changes of oil‐impregnated mock‐ups less than on the Montval ones (Figures 11 and 13), as well as the cotton ones [11]. This with stand‐oil (3.2). However, on the Kraft mock‐ups, the oil film recess is even less than observation is applicable for all sets of oil-impregnated Kraft mock-ups, suggesting that the on the Montval ones (Figures 11 and 13), as well as the cotton ones [11]. This observation is applicable for all sets of oil‐impregnated Kraft mock‐ups, suggesting that the density of the paper pulp did not allow the penetration and recess of the linseed oil to the same extent as the other two types of paper, providing an explanation for the differences in color and opacity changes (3.1, 3.2). 3.4. Mechanical Strength Changes Results have indicated that the conditions of artificial ageing caused limited alteration to the tensile strength of plain Montval and Kraft mock‐ups (M and K) that presented a reduction of 9% at the final stages of ageing (Figures 14 and 15). Montval + Linseed oils M+CP M+RF M+STL 0 2 4 7 14 21 28 Days of ageing Figure 14. Changes of tensile strength of all sets of Montval mock‐ups upon ageing. Ageing duration (0, 7, 14, 21, and 28 days) is shown on the horizontal axis. Kraft + Linseed oils K+CP K+RF K+STL 0247 14 21 28 Days of ageing TS (N) TS (N) Analytica 2022, 3, FOR PEER REVIEW 10 Analytica 2022, 3, FOR PEER REVIEW 10 Figure 13. SEM images of stand‐oil‐impregnated Kraft mock‐ups upon ageing at 500× magnification: (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; Figure 13. SEM images of stand‐oil‐impregnated Kraft mock‐ups upon ageing at 500× magnification: (d) subjected for 28 days of ageing. (a) after air‐drying for 40 days; (b) subjected to 7 days of ageing; (c) subjected to 14 days of ageing; (d) subjected for 28 days of ageing. This confirms the observations in the opacity changes of oil‐impregnated mock‐ups with stand‐oil (3.2). However, on the Kraft mock‐ups, the oil film recess is even less than This confirms the observations in the opacity changes of oil‐impregnated mock‐ups on the Montval ones (Figures 11 and 13), as well as the cotton ones [11]. This observation with stand‐oil (3.2). However, on the Kraft mock‐ups, the oil film recess is even less than is applicable for all sets of oil‐impregnated Kraft mock‐ups, suggesting that the density of on the Montval ones (Figures 11 and 13), as well as the cotton ones [11]. This observation the paper pulp did not allow the penetration and recess of the linseed oil to the same is applicable for all sets of oil‐impregnated Kraft mock‐ups, suggesting that the density of Analytica 2022, 3 275 extent as the other two types of paper, providing an explanation for the differences in the paper pulp did not allow the penetration and recess of the linseed oil to the same color and opacity changes (3.1, 3.2). extent as the other two types of paper, providing an explanation for the differences in color and opacity changes (3.1, 3.2). density of the paper pulp did not allow the penetration and recess of the linseed oil to the 3.4. Mechanical Strength Changes same extent as the other two types of paper, providing an explanation for the differences in Results have indicated that the conditions of artificial ageing caused limited color and opacity changes (Sections 3.1 and 3.2). 3.4. Mechanical Strength Changes alteration to the tensile strength of plain Montval and Kraft mock‐ups (M and K) that 3.4. Mechanical Results Str haength ve indic Changes ated that the conditions of artificial ageing caused limited presented a reduction of 9% at the final stages of ageing (Figures 14 and 15). altera Results tion to have the indicated tensilethat strength the conditions of plaof inartiﬁcial Montval ageing and caused Kraft limited mock alterat ‐upsion (M and K) that to the tensile strength of plain Montval and Kraft mock-ups (M and K) that presented a presented a reduction of 9% at the final stages of ageing (Figures 14 and 15). reduction of 9% at the ﬁnal stages of ageing (Figures 14 and 15). Montval + Linseed oils Montval + Linseed oils M+CP M+RF M+CP M+STL M+RF 0 M+STL 0 2 4 7 14 21 28 Days of ageing 0 2 4 7 14 21 28 Days of ageing Figure 14. Changes of tensile strength of all sets of Montval mock‐ups upon ageing. Ageing duration (0, 7, 14, 21, and 28 days) is shown on the horizontal axis. Figure 14. Changes of tensile strength of all sets of Montval mock-ups upon ageing. Ageing duration Figure 14. Changes of tensile strength of all sets of Montval mock‐ups upon ageing. Ageing duration (0, 7, 14, 21, and 28 days) is shown on the horizontal axis. (0, 7, 14, 21, and 28 days) is shown on the horizontal axis. Kraft + Linseed oils Kraft + Linseed oils 120 K K+CP K+RF K+CP K+STL K+RF K+STL 0247 14 21 28 Days of ageing 0247 14 21 28 Figure 15. Changes of tensile strength of all sets of Kraft mock-ups upon ageing. Ageing duration (0, Days of ageing 7, 14, 21, and 28 days) is shown on the horizontal axis. On the other hand, all sets of oil-impregnated Montval mock-ups presented a reduction of tensile strength upon ageing with no characteristic differences in the trend of changes between linseed oil formulations (Figure 14). Possibly, the thickness/weight of the paper, and thus the ratio of paper and oil volume, had an effect on that. After 40 days of drying, mock-ups presented an increase in tensile strength by an average of 7%, attributed to the application of linseed oils, which turn to a rubbery solid material after drying and constrain the ﬁbres and pulp contents. The values remained almost stable up to the fourth day of ageing, then reduced gradually to those of the plain Montval mock-ups by the fourteenth TS (N) TS (N) TS (N) TS (N) Analytica 2022, 3 276 day, and they did not present noteworthy changes at the following stages. The values decreased up to an average of 20% (Figure 14) by the ﬁnal stages of ageing. The tensile strength measurements of all sets of oil-impregnated Kraft mock-ups presented a common trend of change in opacity with notable variations between linseed oil formulations (Figure 15). After 40 days of air drying, mock-ups presented an increase in tensile strength by 14% for those impregnated with stand-oil, 20% for those impregnated with cold-pressed linseed oil, and 23% for those impregnated with reﬁned linseed oil, attributed to the application of linseed oils and their different properties. The values of the tensile strength of the mock-ups gradually reduced by an average of 13% on the fourth day of ageing, then reduced gradually to those of the plain Kraft mock-ups by the fourteenth day, while they decreased by an average of 50% in the ﬁnal stages (Figure 15). The above-mentioned changes refer to the results derived from the mock-ups cut parallel to the machine direction of the papers, Montval and Kraft. The mock-ups cut cross to the direction of the papers presented similar trends, respectively. It could be suggested that the differences in reduction of tensile strength upon ageing depend both on linseed oil formulations and paper properties (weight) and pulp content. Differences between linseed oil formulations, attributed to their different properties, could be mainly observed in Kraft mock-ups (Figure 15), where paper weight is smaller. The limited reduction of oil-impregnated Montval mock-ups can be attributed to the sigiﬁcant weight difference between Kraft and cotton, and thus, the ratio of paper and oil volume. On the other hand, in comparing the results of oil-impregnated Kraft mock-ups with those of oil-impregnated mock-ups of cotton that have similar weight, there is a signiﬁcant difference in tensile strength upon ageing, as cotton mock-ups were reduced by up to 80%. It could be suggested that differences in oil penetration–recession into the ﬁber net inﬂuenced the mechanical properties of the paper–oil system. 3.5. Chemical Changes 3.5.1. pH Measurements The pH measurements showed that the application of linseed oil of all types causes a decrease in pH value of up to 2.4 points after 40 days of air drying. The mock-ups impregnated with cold-pressed linseed oil (M + CP and K + CP) presented a larger decrease (values range from 5.2 to 4.4 for M + CP and from 5.3 to 4.9 for K + CP), while the values of those impregnated with reﬁned linseed oil (M + RF and K + RF) were quite close (values range from 5.8 to 4.9 for M + RF and 5.7 to 5.2 for K + RF) (Tables 1 and 2). The pH values of the Kraft mock-ups impregnated with stand-oil (K + StL) were comparable to those impregnated with reﬁned linseed oil (M + RF and K + RF) (values range from 5.4 to 4.8 for K + StL), with those of Montval being slightly higher (values range from 6.0 to 5.6 for M + Stl) (Tables 1 and 2). Table 1. pH measurements of all sets of Montval mock-ups. Days of Ageing M M + Cp M + Rf M + StL 0 7.4 5.0 5.8 6.0 1 7.2 5.2 5.4 5.8 2 7.1 5.2 5.3 5.7 4 7.1 5.0 5.2 5.8 7 7.2 4.9 5.2 5.6 10 7.2 4.8 4.9 5.8 14 7.0 4.4 5.0 5.8 21 7.2 5.0 5.3 6.0 28 7.3 5.2 5.4 5.9 Analytica 2022, 3 277 Table 2. pH measurements of all Kraft mock-ups. Days of Ageing K K + Cp K + Rf K + StL 0 7.3 4.9 5.4 5.2 1 7.3 5.1 5.6 5.2 2 7.4 5.1 5.2 5.2 4 7.4 5.1 5.7 5.4 7 7.3 4.8 5.6 5.2 10 7.3 5.3 5.6 5.0 14 7.5 5.2 5.5 4.8 21 7.4 5.2 5.5 5.0 28 7.3 5.3 5.5 5.0 However, variations of pH values of all mock-ups was limited and variable (Tables 1 and 2) upon ageing. The same behavior has been noted for pure cotton mock-ups, although the decrease of the pH values was more intense, respectively. It could be suggested that the presence of alkaline buffer in Montval and Kraft papers played a role in the changes. The fact that oil-impregnated Montval mock-ups presented slightly higher values could be attributed to pulp processing and content. Generally, the results of pH values indicate that the application of linseed oil formulations establishes an acidic condition in the paper–oil system in the early stages of ageing. 3.5.2. FTIR Analysis FTIR analysis showed a typical lignocellulosic paper proﬁle, based on the results of previous works [21–25]. Wood pulp can be indicated by the lignin absorption at about 1730 (esters in hemicellulose fraction), 1590 (generally attributed to aromatics and possibly, to carboxylates), 1505 (aromatics, a lignin marker), 1450 (recorded in historic papers), ~1265 (broad absorbance due to C-O of guaiacyl ring of lignin residues), ~900 (glycosidic linkages in polysaccharide units), and 808 cm (typical of hemicelluloses). For the Montval paper, removal of lignin was indicated by the lack of all lignin markers, while it showed characteristic absorption at 1202–1204 cm (mainly due to the exocyclic CH twisting of 1 1 the glucose rings with contributions from other vibrations), 1050 cm , and 1030 cm (due to various C-O vibrations of the polysaccharide structure) [25]. FTIR analysis additionally conﬁrmed the presence of CaCO in both papers by the marked absorptions at about 1430 1 1 and 874 cm along with clay, shown at 1030–1000 and 910 cm , typical of aluminum silicates. Both materials were also detected by EDX analysis. After 40 days of drying, FTIR spectrum of oil-impregnated mock-ups additionally showed the characteristic carbonyl band at 1745 cm and the C-O stretching pattern at 1239, 1164, and 1101 cm , which is diagnostic for triglyceride ester linkages [25–28]. The typical bands due to rocking deformation of long alkyl chains at ~720 cm were observed in addition to the cis oleﬁnic C-H bending vibration at 970–980 cm and the C=C stretching vibrations at 1680–1600 cm , which are attributable to the conversion of unconjugated, disubstituted cis double bonds, as expected for the fatty ester in siccative oils, to trans upon drying [12,25,26,29–31]. As shown in Figures 16a and 17a, FTIR spectra of plain Montval and Kraft mock- ups did not display notable changes upon ageing. On the other hand, the spectra of oil-impregnated mock-ups showed moderate changes in speciﬁc bands of the spectrum upon ageing (Figure 16b–d for Montval mock-ups and Figure 17b–d for Kraft mock-ups). Stronger bands were observed due to the gradual intensity increase of the bands correspond- ing to carbonyl-containing species (such as aldehydes and ketones) and carboxyl acids (1600–1750 cm ), which are associated both with paper and linseed oil oxidation [23–26,29–32]. A marked increase was also noted in bands that correspond to oxidation compounds, such as hydroperoxides and alcohols (3200–3600 cm and 1100–1210 cm ) and the formation of conjugated bonds (such as in 1624, 1633, 950, and 723 cm ), a signiﬁcant change that we attributed to the degradation of linseed oil, Analytica 2022, 3 278 conﬁrming colorimetric measurements (see Section 3.1); additionally, oxidative polymer- ization in the ﬁnal stages of ageing, mainly expressed through additional C-C bonding (1099–1238 cm ), is typically observed [25,26,29–31]. (a) (b) (c) (d) Figure 16. FTIR spectra of all sets of Montval mock-ups upon ageing: (a) plain one; (b) impreg- nated with cold-pressed linseed oil, M + CP; (c) impregnated with reﬁned linseed oil, M + RF; (d) impregnated with stand-oil, M + Stl. Ageing duration (0, 7, 14, 21, and 28 days) is shown on each spectrum. Variations in bands between the sets upon ageing are possibly associated with the man- ufacturing processes of linseed oil (Figure 16b–d for Montval mock-ups and Figure 17b–d for Kraft mock-ups). More intense changes are recorded to the mock-ups after the 14th day of ageing and more. The changes in paper chemistry could not so far be clearly determined due to band overlaps with those of linseed oil. Further study is required for the study of the changes in speciﬁc bands. 3.5.3. Analysis of VOCs through SPME-GC-MS The analysis of volatile organic compounds (VOCs) provides indirect, yet critical information on degradation attributed to scission reactions induced during the ageing process to both oil and paper. The simultaneous ageing of these materials as a result of this speciﬁc experimental design offers an insight into the mutual impact between oxidized and hydrolyzed oil triglycerides and the cellulose and lignin of paper. (a) (b) (c) (d) (a) (b) (c) (d) Analytica 2022, 3 279 (a) (b) (c) (d) Figure 17. FTIR spectra of all sets of Kraft mock-ups upon ageing: (a) plain one; (b) impregnated with cold-pressed linseed oil, K + CP; (c) impregnated with reﬁned linseed oil, K + RF; (d) impregnated with stand-oil, K + StL. Ageing duration (0, 7, 14, 21, and 28 days) is shown on each spectrum. VOCs were sampled non-destructively, using Solid Phase Microextraction (SPME) and an SPME needle cartridge with a 50/30 with DVBCAR/PDMS (divinylbenzene– carboxen/poly(dimethylsiloxane)) ﬁber coating. The same methodology has been success- fully applied for the investigation of the effect of linseed oils on pure cellulosic papers, following the equivalent process for original oil sketches and prints [11]. The results on mock-ups could be used as references for determining the stages of ageing or deterioration in original works on lignocellulosic papers. GC-MS analysis of all sets of oil-impregnated Montval and Kraft mock-ups identiﬁed the same 30 compounds recorded for the oil-impregnated pure cellulosic papers with the three formulations of linseed oil [11]. These volatile organic compounds belong to several chemical classes: saturated and unsaturated aldehydes, ketones, alcohols, carboxylic acids, lactones, and furans. The carboxylic acids encompass methanoic (formic), ethanoic (acetic), propanoic, pentanoic, hexanoic, heptanoic, octanoic, and nonanoic acids. The aldehyde range of compounds comprises hexanal, 2-hexenal, heptenal, 2-heptanal, octanal, 2-octenal, nonanal, 2-nonenal, decanal, 2-decenal, and 2-undecanal. Ketones include 2-heptanone, 2-octanone, 4-nonanone, 2-nonanone, and 2-decanone. Lactones include -heptalactone and -nonalactone, furans include 2-pentyl furan, 5-ethyl-2(5H)-furanone and 5-penty-2- (5H)-furanone, while for alcohols, only 1-octen-3-ol. Analytica 2022, 3 280 These compounds have been associated with variant types of linseed oil, pure cellu- losic and lignocellulosic papers [33–45], and their oxidation upon ageing. Saturated and unsaturated aldehydes, ketones, alcohols, carboxylic acids, and lactones have been mainly attributed to oil oxidation. It should be noted that the full range of C1-C18 acids is expected during oxidative degradation, however, the more volatile ones are detected with a speciﬁc type of chromatographic analysis (see Experimental). On the other hand, hexanal, heptanal, octanal, nonanal, and decanal have been detected in historical books with pure cellulosic ﬁber and wood pulp content [38], and they have been mainly attributed both to the lipid content of books and the natural ageing of paper [34,37]. Finally, furans mainly originate from the polysaccharide material in the paper. To study the evolution of the emission of VOCs, the results were included in sum-up graphs for every chemical group: acids, aldehydes, ketones, and furans. For all three sets of Montval and Kraft oil-impregnated mock-ups, the emission of acids was compara- tively higher than the other chemical groups and presented a common trend: a gradual decrease up to the 28th day of ageing (Figures 18a, 19a and 20a for Montval mock-ups and Figures 21a, 22a and 23a for Kraft mock-ups). However, at the ﬁnal stages of ageing, their emissions remained at a higher level in comparison with other chemical groups. Among the three sets, both Montval and Kraft mock-ups impregnated with stand-oil present com- paratively lower emission of acids and a more limited decrease (Figures 20a and 23a). This was also observed on the sets of cotton mock-ups [11], indicating that the type of oil Analytica 2022, 3, FOR PEER REVIEW 16 inﬂuences the behavior. On the other hand, all sets impregnated with cold-pressed linseed oil presented a more intense decrease up to the 7th day of ageing. Formic, propanoic, and hexanoic acids presented higher emissions than the rest for all sets of mock-ups. Figure Figure 18. 18. Sum Sum-up ‐up evolution evolution of of VOCs VOCsbelonging belonging to ato speciﬁc a specchemical ific chem grica oup, l group, emitted emitte by Montval d by Montval mocmock-ups k‐ups impreg impregnated nated with with col cold-pr d‐pressed essed linseed linseed oil oi upon l upon ageing: age(iang: ) acids; (a) ac (bi)d aldehydes, s; (b) aldehy ketones, des, ketones, and and furans. furans. Emissions of aldehydes were much lower than those of acids, and their trend, taking into consideration the standard deviation, appeared to be similar for the three sets of Montval and Kraft mock-ups. In the case of cold-pressed linseed oil, an increase up to the second day was detected for aldehydes, ketones, and furans, followed by a gradual decrease up to the 28th day of ageing (Figures 18b, 19b and 20b for Montval mock-ups and Figures 21b, 22b and 23b for Kraft mock-ups). This could be reasonable, since cold-pressed linseed oil is the least-treated oil, and thus is the more susceptible to oxidation. Aldehydes, ketones, and furans (secondary oxidation products of lipids) are being formed rapidly from the beginning of the accelerated oxidation, reaching a peak until the exhaustion of precursor compounds is reached, and then their emissions follow the same pattern as in the sets of reﬁned linseed oil and stand-oil. It should be noted that the type of lignocellulosic Figure 19. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Montval mock‐ups impregnated with refined linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. Figure 20. Sum‐up evolution of VOCs belongin to a specific chemical group, emitted by Montval mock‐ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. Analytica 2022, 3, FOR PEER REVIEW 16 Analytica 2022, 3, FOR PEER REVIEW 16 Analytica 2022, 3 281 Figure 18. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Montval mock‐ups impregnated with cold‐pressed linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, Figure 18. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Montval and furans. mock-up seems not to affect this phenomenon, while in the case of cotton mock-ups [11], mock‐ups impregnated with cold‐pressed linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, aldehydes, ketones, and furans of cold-pressed oil followed a declining trend from the and furans. beginning of accelerated oxidation. Figure 19. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Montval mock‐ups impregnated with refined linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, and Figure 19. Sum-up evolution of VOCs belonging to a speciﬁc chemical group, emitted by Montval Figure 19. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Montval furans. mock-ups impregnated with reﬁned linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, mock‐ups impregnated with refined linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, and and furans. furans. Figure 20. Sum-up evolution of VOCs belongin to a speciﬁc chemical group, emitted by Montval Figure 20. Sum‐up evolution of VOCs belongin to a specific chemical group, emitted by Montval Figure 20. Sum‐up evolution of VOCs belongin to a specific chemical group, emitted by Montval mock-ups impregnated with stand-oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. mock‐ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. mock‐ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. Hexanal presented the highest values of emission for sets. For the mock-ups impreg- nated with stand-oil, the change of emissions upon ageing was comparatively limited, thus the total reduction was smaller (Figures 20b and 23b). Emissions of ketones and furans were lower than those of aldehydes for all sets of mock-ups. They presented a common trend of a gradual, limited, yet analogical decrease up to the 28th day of ageing (Figures 18b, 19b and 20b for Montval mock-ups and Figures 21b, 22b and 23b for Kraft mock-ups). Analytica 2022, 3, FOR PEER REVIEW 17 Analytica 2022, 3, FOR PEER REVIEW 17 Analytica 2022, 3, FOR PEER REVIEW 17 Analytica 2022, 3 282 Figure 21. Sum‐up evolution of VOCs belonging to the same chemocal group, emitted by Kraft Figure 21. Sum‐up evolution of VOCs belonging to the same chemocal group, emitted by Kraft mock‐ups impregnated with cold‐pressed linseed oil upon ageing,: (a) acids; (b) aldehydes, ketones, Figure Figure 21.21. Sum Sum-up ‐up evolution evolution of of VOCs VOCsb be elonging longing to to the the same sam chemocal e chemoca group, l group, emitted emitte by Kraft d by Kraft mock‐ups impregnated with cold‐pressed linseed oil upon ageing,: (a) acids; (b) aldehydes, ketones, and furans. moc mock-ups k‐ups imimpr preg egnated nated with with col cold-pr d‐pressed essed linseed linseed oil oi upon l upon ageing,: ageing, (a) : acid (a)s; acids; (b) aldehydes, (b) aldehy ketones, des, ketones, and furans. andand furans. furans. Figure 22. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Kraft Figure Figure 22. 22. Sum Sum-up ‐up evolution evolution of of VOCs VOCs belonging belonging to to a speciﬁc a specifi chemical c chemgr ical oup, group, emitted emitted by Kraft by Kraft mock‐ups impregnated with refined linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, and Figure 22. Sum‐up evolution of VOCs belonging to a specific chemical group, emitted by Kraft mocmock-ups k‐ups impreg imprnated egnated with with refined reﬁned lin linseed seed oil oil upon upon ageing: ageing: ((aa )) acids acids; ; ( b(b ) )aldehyde aldehydes s, ketones, , ketones, and furans. mock‐ups impregnated with refined linseed oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. and furans. furans. Figure 23. Sum-up evolution of VOCs belonging to a speciﬁc chemical group emitted by Kraft Figure 23. Sum‐up evolution of VOCs belonging to a specific chemical group emitted by Kraft mock‐ Figure 23. Sum‐up evolution of VOCs belonging to a specific chemical group emitted by Kraft mock‐ mock-ups impregnated with stand-oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. Figure 23. Sum‐up evolution of VOCs belonging to a specific chemical group emitted by Kraft mock‐ ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. ups impregnated with stand‐oil upon ageing: (a) acids; (b) aldehydes, ketones, and furans. Emissions of aldehydes were much lower than those of acids, and their trend, taking Emissions of aldehydes were much lower than those of acids, and their trend, taking Emissions of aldehydes were much lower than those of acids, and their trend, taking into consideration the standard deviation, appeared to be similar for the three sets of into consideration the standard deviation, appeared to be similar for the three sets of int Montval o conside andr at Kraft ion the mock standard ‐ups. In the devia catse io n, of co appea ld‐prred essed to linseed be similar oil, for an in the cre three ase up sets to the of Montval and Kraft mock‐ups. In the case of cold‐pressed linseed oil, an increase up to the Montval and Kraft mock‐ups. In the case of cold‐pressed linseed oil, an increase up to the second day was detected for aldehydes, ketones, and furans, followed by a gradual second day was detected for aldehydes, ketones, and furans, followed by a gradual second day was detected for aldehydes, ketones, and furans, followed by a gradual Analytica 2022, 3 283 Finally, the emissions of lactones and alcohol were signiﬁcantly low and presented a similar trend for all sets of mock-ups. In particular, lactones presented an increase of emissions up to the 7th day of ageing and then a gradual fall, while the 1-octen-3-ol presented a gradual fall up to the 14th day and then ﬂattened. As for pure cellulosic papers, the trend of emissions of the several chemical groups could be mainly associated with the kinetics involved with the drying and degradation of linseed oils [41,42]. The extensive oxidation of linseed oil and the consequent polymer- ization/degradation gradually inhibits the emission of volatile organic compounds upon ageing [11]. Also, it has been indicated that the process of manufacture of each formulation of linseed oil inﬂuences the quantity and trend of emission, as for pure cotton mock-ups [11]. Stand-oil, as mentioned already, is a pre-polymerized oil for which the process of drying is slower than in the cases of cold-pressed and alkali-reﬁned linseed oils [13]. Therefore, the three-dimensional matrix of polymerization is formed at a slower rate and allows the light volatile compounds, like aldehydes (e.g. boiling point of hexanal is 129 C), to be emitted for longer periods in the headspace of the mock-up. This phe- nomenon has been observed mainly for aldehydes in all types of paper, i.e., cotton [11], Montval and Kraft, but it seems that the type of paper also inﬂuences the intensity of the phenomenon. The rate of decline of the emissions of aldehydes was more intense for Kraft mock-ups, in comparison to that of cotton and Montval mock-ups that presented similar behavior. It could be suggested that the stand-oil ﬁlm, which remains on the surface of Kraft mock-ups up to the ﬁnal stages of ageing without recessing into the ﬁber net, might inﬂuence the emissions of the system. The emissions could mainly derive from the oil ﬁlm. Further work is required for the evaluation of the results. The processing of emission results of every compound is regarded as necessary to export data useful for the condition assessment of oil-impregnated paper areas on original works of art, hopefully enabling the determination of markers that indicate the condition of oiled areas of a paper support. 4. Conclusions The results of this work contribute to the research on the effect of linseed oils on paper support. Investigation on lignocellulosic papers contributes to the previous results on pure cellulosic paper beyond the effect of the provenance and processing of the ﬁbers, with factors like color, paper weight and paper: oil volume ratio, pulp content, and oil penetration that inﬂuence the changes recorded over time. Application of linseed oils is mainly responsible for the trend of all types of changes on mock-ups, while variations between the sets of each paper type could be attributed to the differences in oil processing during manufacture and the resultant properties respectively. The differences between the sets of the two papers sets seem to depend on the penetration and recess of the oil into the ﬁber net, which is associated with pulp content and density, and the paper weight, and thus the ratio of paper and oil volume. This was evident in the results of the color and opacity measurements of the sets. The extent and the rate of change of oil-impregnated Kraft mock-ups upon ageing were comparatively limited, probably due to its dense paper pulp preventing oil penetration and recess. Moreover, the trends of optical changes of oil-impregnated Montval mock- ups were comparable with those of pure cellulosic–cotton papers [11] (both white), with notable variations that indicate that paperweight and ratio of paper: oil volume might have inﬂuenced the outcome. The reduction of tensile strength for the two papers differs, with oil-impregnated Montval mock-ups presenting comparatively smaller changes. Taking into consideration the collapse of the tensile strength of the oil-impregnated pure cellulosic– cotton mocks, it could be suggested that the presence of alkaline buffer might restrain the chemical deterioration of the support. In addition, pure cotton paper had no sizing, ﬁllers, or additives, so linseed oil formulations penetrated through the ﬁbre net unobstructed and consequently had a major effect on the paper–oil system. FTIR spectra of oil-impregnated cotton mock-ups also indicated more extended chemical changes at the bands of peroxides, Analytica 2022, 3 284 aldehydes, ketones, and carboxylic acids [11], indicating hydrolysis of linseed oil and the consequent acidic hydrolysis of paper. On the other hand, Montval’s weight is 80% higher than that of Kraft and 100% more than that of cotton paper. This could set a hypothesis, to be investigated, that the ratio of oil and paper mass could have an effect on the mechanical behavior. Generally, the chemical changes presented in the FTIR analysis support the changes recorded for the sets of mock-ups, with other methods of study providing possible expla- nations for the changes recorded. VOC’s emissions appear to be inﬂuenced mainly by linseed oil processing. The trend of emissions presents a gradual decrease upon ageing, remaining comparatively low after the 14th day of ageing in response to the respective chemical changes. The input of paper type can be noted in the differences in the trend of VOC emissions between cotton, Montval and Kraft oil-impregnated mock-ups. Finally, a comparative study of the results of this work could not point out speciﬁc stages of deterioration upon ageing yet. It could be suggested that the changes on the mock-ups can be divided into three phases: the ﬁrst, from 0 to 4 or 7 days; the second, from 4 or 7 to 14 days; and the third, from 14 to 28 days. The research will proceed with advanced statistical processing of the results to establish the stages of deterioration with mathematical tools. This will compensate for the condition assessment of works created with oil media on paper, thus aiding in conservation and preservation decision-making for a wide range of cultural heritage objects. Author Contributions: Conceptualization, P.B.; methodology, P.B., A.A., C.T., D.T., L.-A.T. and T.K.; software, P.B. and D.T.; validation, P.B., A.A., C.T., S.B., D.T., L.-A.T. and T.K.; formal analysis, P.B., A.A., S.B., D.T., L.-A.T.; investigation, P.B.; resources, A.A.; data curation, P.B.; writing—original draft preparation, P.B.; writing—review and editing, P.B., S.B., D.T., L.-A.T. and A.A.; visualization, P.B. and D.T.; supervision, A.A., C.T. and K.C.; All authors have read and agreed to the published version of the manuscript. Funding: This work was performed in the framework of a PhD research project, and it has not received any funds from external sources. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Engram, C. Oil painting on paper and board supports. AICCM Bull. 1991, 17, 13–20. [CrossRef] 2. James, C.; Corrigan, C.; Enshaian, M.C.; Greca, M.R. 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Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports

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Analytica – Multidisciplinary Digital Publishing Institute

Published: Jul 2, 2022

Keywords: linseed oil; pure cellulosic paper; FTIR; VOC; GC-MS; colorimetry; opacity; tensile strength; SEM; pH

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Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports

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Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports

Oil Media on Paper: Investigating the Effect of Linseed Oils on Lignocellulosic Paper Supports

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