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Analysis of facing materials used as remoistenable temporary supports for facing on canvas paintings

Paola Alba, Susana Martín-Rey and MaríaTeresa Doménech-Carbó

Abstracts

This paper includes a critical review of the most commonly used facing techniques and a description of an innovative system based on the use of remoistenable temporary supports which allows superficial protection with adequate bond strength and easy reversibility. The article describes the part of the research focused on the evaluation of the characteristics of a selection of adhesives and temporary supports and their compatibility for developing of remoistenable temporary supports suitable for the protection of canvas paintings.

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This investigation was carried out within the framework of the project CTQ2017-85317-C2-1-P supported by the Ministerio de Ciencia, Innovación y Universidades, Fondo Europeo de Desarrollo Regional (ERDF) funds and Agencia Estatal de Investigación (AEI). The authors wish to thank for their cooperation: Dr. José Luis Moya López, Alicia Nuez Inbernón and Manuel Planes Insausti (Electron Microscopy Service of the Universitat Politècnica de València - Spain), Professor Annarosa Mangone and Dr. Lorena Carla Giannossa (Chemestry Department of the Università degli Studi di Bari “Aldo Moro” - Italy), Professor Hans Poulis (Adhesion Institute of TU Delft – Netherlands) and Professor Antonio Iaccarino Idelson (Equilibrarte srl, Roma – Italy).

Introduction

1From the seventies onward, the theoretical and technical review of conservation materials and methods as well as the awareness of the importance of concepts, such as reversibility and minimal intervention, increased steadily. Although facing is widely used for multiple purposes, there is still little information about it.

2Facing is a temporary measure during a painting’s treatment that supports and prevents crumbling or fracturing of paint layers during a treatment. Its application further involves the cleaning of the entire surface. At first glance, facing seems to be an almost simple and neutral intervention. Its application, however, has complex consequences involving changes to a painting’s strata and it can have potential repercussions on the conservation of the paint itself.

3The impregnation of porous structures is thus an irreversible measure. Restoration materials can interact with the paint, or influence latter operations. Facing also implicates a subsequent cleaning of the entire surface. Furthermore, facing adhesives can cause changes in both the color and the refraction index of the paint layer. The contraction of the facing support in association with strong adhesives can stress or even damage the paint, especially in case of a long stay in unstable environmental conditions. This could also affect the reversibility of the adhesive.

4This paper offers a critical review of the most popular application systems, comprising traditional and synthetic materials. Aspects considered fundamental for the understanding of facing mechanisms will be described. The paper also presents research on basic materials used for preparing remoistenable temporary supports, an alternative method that enables higher control of the adhesive penetration in the substrate, and consequently easier removal of the residues.

State of the Art

5Facing has always been considered one step in a multi-step intervention, which is why written documentation and didactic literature about facing are hardly found. Restoration handbooks and conservation journals offer little to no information on the materials and techniques used for facing or the reasoning behind the decision face a painting. The lack of historical information on the use of this technique is due to the fact that it was rarely considered necessary to write, share, comment, or transmit experiences and opinions on the topic. Especially because a facing is usually removed during an intervention, making it difficult to obtain data through diagnostics.

6Recently, however, more and more conservators have been starting to re-think the process by designing facings with characteristics for the specific needs of a painting’s surface. Unfortunately, only few reflections and experiences arepublished in conservation journals and congress proceedings, so that the debate remains connected with oral tradition.

  • 1 The survey was carried out for the Master’s Thesis: Alba, Paola. 2015. “La velinatura. Riflessioni (...)

7These examples represent an exception. A survey, submitted to conservators with different cultural and geographical backgrounds (Italian, Belgian, Spanish, French, Estonian)1, emphasizes the professionals’ close and possibly acritical connection with the tradition. Many conservators confirmed that they frequently use traditional adhesives for facings, such as rabbit skin glue, bone glue, colletta, sturgeon glue, starch or wax-resin applied with brush. Another interesting aspect confirmed by the survey is that conservators use the same application methods for synthetic facings.

Some Clarifications

8The English word facing is a generic term, which indicates that an outer layer is literally covering a surface. This term is merely functional, as it refers to the act of covering a surface with something or gluing something onto a surface. It does not reference the reasoning behind the decision that leads the conservator to face a painting, or to the measure’s aim.

9There are two principal purposes in facing: a protective one and a consolidating one. These fundamental aspects are of uttermost importance and an often neglected distinction. In the first case (Fig. 1), the conservator wants to avoid displacement, or the loss of detached fragments through temporary glueing: the detachment of the paint flakes is temporarily stopped, and it is possible to postpone the solution of the problem. The polymer should ideally settle at the interface between the interim support and the paint layer without penetration and eventually be reversible.

Fig. 1. Protective and consolidating facing

Fig. 1. Protective and consolidating facing

Distinction between protective facing and consolidating facing. The temporary support is marked in yellow, the adhesive is marked in orange.

Credits: Paola Alba.

10In the second case, the purpose of the treatment is the consolidation of superficial layers. The temporary support is supposed to act like a barrier on the surface against external mechanical action. The adhesive should partially penetrate the paint layers and consolidate them superficially. At the end of the intervention, only adhesive residues on the surface should be removed.

11It is, therefore, necessary to make the distinction between protective facings and consolidating facings, thereby declaring the specific pursued purpose of each facing. This remains a theoretical distinction, as the adhesive’s application by brush and the use of a low-viscosity adhesive results in the adhesive’s arbitrary use as a superficial, temporary and removable protection or a consolidant. Furthermore, there is no distinction between the protective and consolidating effects when traditional application systems are used, because the porosity of materials and the presence of cracking in the paint layers leads to its partial penetration, making it impossible to have a superficial action.

Materials and methods

12This research is aims to design an alternative method for the use of superficial and protective facings and to translate the theoretical distinction between the two kinds of facing into practice. It enables the facing of canvas paintings with adequate and superficial adhesion while being easily removable.

13The research was organized in different steps, focusing on:

  • The assessment of ideal characteristics for facing materials and the selection of the most suitable adhesives.

  • The analysis of the different characteristics of facing adhesives and temporary supports, and their compatibility for the development of remoistenable temporary supports.

  • The evaluation of the adhesive properties for facings.

  • The assessment of the adhesive penetration.

  • An estimate of the reversibility of adhesive residues and the alteration of the surface of the painting after removing the protective facing.

Remoistenable temporary supports

14Remoistenable temporary supports (RTS) have been around for a long time. Similar methods are used in paper conservation for tear mending of water-sensitive or solvent-sensitive materials (Pataki 2009; Lechuga 2011; Olender, Young and Taylor 2017; Bedenikovic, Eyb-Green and Baatz 2018). The only one related publication in the area of easel painting is the one of Borgioli, Boschetti and Tortato (2016).

15For this investigation, it has been proposed a novel method of preparation, application and removal of RTS on paintings combining some of the above mentioned methodologies with some innovation that improves the method and makes it suitable for paintings. Figure 2 shows a scheme of the different steps in the preparation, application to the paint layer and removal of the RTS from the paint layer.

Fig. 2. RTS: preparation steps

Fig. 2. RTS: preparation steps

Scheme of RTS preparation and reactivation for application and removal.

Credits: Authorship of Paola Alba and MaríaTeresa Doménech-Carbó.

16In particular, application of the adhesive on the interim support (cellulosic tissue or nonwoven tissue) is improved by lying it on a foil of Mylar or Teflon. In this way, the adhesive impregnates the interim support and migrates at the interface between the temporary support and the Mylar, and dries forming a uniform film (Fig. 3-4).

Fig. 3. RTS: preparation

Fig. 3. RTS: preparation

Scheme of the application and settle of the adhesive on the interim support.

Credits: Paola Alba and MaríaTeresa Doménech-Carbó.

Fig. 4. RTS: adhesive film

Fig. 4. RTS: adhesive film

RTS after the application and the drying of the adhesive.

Credits: Paola Alba.

17Application of the RTS on the paint layer requires reactivation of the dried remoistenable tissue with a solvent. For this purpose we were inspired by the mist lining used by Van Och and Hoppenbrouwers (2003) and modified by Iaccarino Idelson (Iaccarino Idelson and Serino 2014). In fact, it was decided to use a ‘reactivation cloth’ instead to apply directly the solvent by spraying on the remoistenable tissue.

18The cloth is made of nonwoven tissue (TNT 30/B), which is rolled up and wrapped in a polyethylene foil. Then a precise amount of buffer solution is introduced with a needle (Fig. 5 left top). After some minutes, the buffer solution has uniformly spread in the cloth and it is apt to reactivate the dried RTS.

19The application of the RTS on the paint is carried out as follows:

  • The dry remoistenable tissue is applied with the adhesive side in contact with the painting. Then, the cloth is un-wrapped from the polyethylene foil and superimposed to the dried remoistenable tissue during a specific time, for reactivating the adhesive and assuring a satisfactory adhesion to the paint surface.

  • A Mylar foil, which has the function of delaying the evaporation of the buffer solution applied on the cloth, is overlapped to the cloth.

  • For improving the adhesion of the RTS to the paint, is also possible to rub the surface with a sponge (Fig. 5 right top).

20The time of this operation depends on the temporary support (depending on the nature of the support it can vary from less than a minute to two minutes).

21To remove the remoistenable tissue from the paint the is followed a similar procedure to that used for its application: the reactivation cloth is overlapped to the RTS, then the Mylar foil is superimposed and after an adequate time it is possible to proceed with the mechanical removal of the remoistenable tissue (Fig. 5 left bottom). If necessary, after the elimination of the facing, it is possible to quit the residues of adhesive with a cotton swab with a small amount of buffer solution (Fig. 5 right bottom).

Fig. 5. RTS: application and removal

Fig. 5. RTS: application and removal

Steps of the application and the removal of RTS, using a reactivation cloth. Left top: preparation of the reactivation cloth wrapped in a polyethylene foil and embedded with the buffer solution. Right top: rubbing the surface after the superposition of the remoistenable tissue, the reactivation cloth and the Mylar foil. Left bottom: mechanical removal after the reactivation with the application of the reactivation cloth and the superimposed Mylar foil for a specific time. Right bottom: cleaning of the residues with a cotton swab embedded with the buffer solution.

Credits: Paola Alba.

22With this method, it is quite possible to obtain a superficial protection. It allows the use of a very small and defined quantity of adhesive. It is also known the quantity of the reactivation solvent applied on the reactivation cloth, thus improving the reproducibility of the method. This reduces the risks connected to this intervention, minimizing the extent of the penetration and superficial residues. The use of a very small amount of solvent further limits the expansion and the contraction of the temporary support, reducing the mechanical stress on the paint.

Ideal characteristics

23The behavior of a facing applied to a complex system, such as a canvas painting, is difficult to determine through hypothetic reasoning. However, the study of facing mechanisms, and the variables that can be changed to control the penetration, allow for the application of some theorems to select materials with suitable characteristics.

24As a general rule, for the conservator, it is fundamental to use materials which are compatible with the painting and also easily reversible. All materials used have to be chemically and physically stable, and of low toxicity for human health and the environment. Furthermore, they should not be too sensitive to thermo-hygrometrical changes and selected to their availability and affordability, in order to permit an easier reproducibility of the proposed facings.

25The adhesive’s viscosity and wetting properties are relevant factors for a superficial adhesion. During the preparation of the remoistenable tissues, the adhesive’s viscosity should not be too high in order to allow a uniform distribution at the interface between the support and the Mylar® foil. For this purpose, it is possible to choose an appropriate solvent and the polymer concentration. It would be better to use a solvent with low surface tension to obtain a higher penetration of the adhesive in the support. If this is not possible, the concentration of the adhesive can be modified.

26The penetration of the adhesive depends also on its molecular weight (MW). Firstly, an adhesive with a high molecular weight has a lower penetration into the substrate because of the dimension of its molecules. Secondly, the MW influences the viscosity: the higher it is, the higher the viscosity is.

27The adhesive and mechanical properties of a dried polymer are fundamental. It would be appropriate to calibrate the adhesion strength according to the kind of paint, its state of conservation and the purpose for which the facing is applied. In general, a polymer with lower medium strength should be used, in order to ensure the integrity of the paint layer, also when mechanical interventions on the back are required. The adhesive bond should be thin and moderately flexible, in order to follow the paint movements without being too resilient. Furthermore, the polymer should not contract too much when drying.

28The support should be adaptable and flexible, in order to guarantee greater surface contact with the paint surface. This would reduce the amount of adhesive needed for adhesion and, consequently, the thickness of the bond. Its mechanical properties should be evaluated according to the characteristics of the paint, its dimensions and subsequent interventions. The support should also have good wet-strength to prevent leaving adhesive residues after the facing removal.

29The support should be thin and with low density, to facilitate the evaporation of the solvent. It should be also absorbing to hold in the adhesive. All these characteristics contribute to reduce the penetration of the polymer.

30The aspect of dimensional changes is also important. Every support (Japanese paper, English Tissue, Eltoline Tissue, Papier Bolloré, TNT, Holytex®) presents a characteristic dimensional change when it is wetted. Most of the supports used for facings are non-woven tissues, made of superimposed and pressed fibre. Since they are not woven, supports tend to expand with the increase of humidity and to contract with its decrease. When facing, the drying stage is the most critical. During the drying process, the viscosity of the adhesive increases until it reaches a gel state, which is when its sliding capacity decreases. The contraction of the support provokes a tensile force on the adhesive film, which is simultaneously transferred to the paint layer. Therefore, slow and slight dimensional changes are desirable traits especially during the last stage of drying to avoid deleterious mechanical effects.

  • 2 The return migration refers to the stage in which the polymers penetrated in the substrate starts t (...)

31Understanding the role of the solvent is hereby imperative. The solvent used during the reactivation step needs to have high superficial tension and good wetting properties in order to facilitate the adhesion. A high evaporation speed facilitates the return migration phenomenon2. During the removal of a facing removal, it is of advantage to choose a solvent with a high affinity to the adhesive in order to guarantee its adequate reactivation while reducing the stress inferred to the paint. On the other hand, during this stage, it is very important to control the penetration of the solvent. The facing support chosen, which has a specific porosity, enables to control the penetration of the solvent. As an alternative to liquid solvents, gelling solutions can be used. Combination of suitable support and gelling solutions results in a better control of the migration of the solvent.

Selected materials

32In order to have a low penetration and good adhesion, four adhesives were combined to form polymer dispersions. The condensation polymers Klucel® G and Tylose® MH300 (CTS Europe) were selected to increase the thickness of the adhesive dispersions. These cellulose ethers have a high molecular weight, are water-soluble (Klucel® is also soluble in polar organic solvents), have good thickening and wetting properties. Their pH is almost neutral and they form elastic and thermoplastic films while not being too sensitive to humidity changes.

33Polymers selected to improve adhesion properties of the adhesive dispersions were Plextol® B500 (CTS Europe) and Aquazol® 500 (Polymer Chemistry). Plextol® is an acrylic emulsion containing 60 % of ethyl acrylate (PEA) and 40 % of methyl methacrylate (PMMA) with polymer microdrops of 0.1-0.2 μm. It has already been combined with cellulose ethers both for nap-bond system linings (Mehra, 1972) and facing (Martín-Rey et al., 2013). Aquazol® (poly (2-ethyl-2-oxazolina) has already been tested for the preparation of remoistenable tissues used for facing of canvas paintings (Borgioli et al., 2016). Aquazol® 500 is chemically stable and it has a high molecular weight (500,000). It is soluble in water and other polar organic solvents and has an almost neutral pH as well as a good water vapour permeability. It has medium bond strength and good mechanical properties. Furthermore, is perfectly miscible with other polymers. Its sensitivity to humidity fluctuations in the environmental, however, is its biggest disadvantage.

34The chosen temporary supports were:

  • Japanese paper Bib.Tengujo (CTS Europe), made with Manila fibres (weight: 12 g∙m-2 pH 7.1)

  • TNT tissue (CTS Europe), made of 80 % Viscose and 20 % di Polyester (weight: 25 g∙m-2)

  • Holytex®, a tissue made of Polyester (weight: 17 g∙m-2)

35The preliminary selection of adhesive mixtures was based on each adhesive’s ability to distribute homogeneous on the temporary supports mentioned above (Fig. 6). This also enabled the specification of the necessary adhesive and solvent quantity for the respective preparation of the remoistenable tissue and the reactivation cloth.

Fig. 6. Preliminary selection of the adhesives

Fig. 6. Preliminary selection of the adhesives

Optical microscopy analyses carried out to verify the homogeneous distribution of the polymers on the temporary supports.

Credits: Paola Alba.

36A series of aqueous mixtures was prepared. Remoistenable tissues were reactivated with a buffer solution adjusted to a similar pH of the paint layers. In fact, water is a versatile solvent that is innocuous for the operator. Aqueous systems, however, have to be used with accuracy because of their potential danger to the paint, but can be adjusted by several parameters, such as pH and conductivity, and are therefore highly versatile (Cremonesi, 2011).

37This research is restricted to a small selection of adhesives, temporary supports and remoistenable tissues, in order to explore the efficacy of the proposed method. Therefore, three different mixtures were selected. These preliminary results will enable a more accurate selection of materials for the proposed method in the future.

38The three adhesive mixtures are TP1, KP1 and KA1. TP1 and KP1 are 5% of Plextol® (volume-volume) dissolved in water and mixed with an aqueous 3% (weight-volume) solution of Tylose® or Klucel®. The KA1 is made up from an aqueous 3% Aquazol® solution and a 3% Klucel® (both weight-volume) solution. Table 1 summarizes the composition of the final dispersions obtained.

Table 1 - Composition of the adhesive dispersions

Table 1 - Composition of the adhesive dispersions

In the table are summarised the proportion used to prepare the selected dispersions.

Credits: Paola Alba.

Preliminary tests

39As previously mentioned, during the first experimental stage, analyses of the different classes of materials (adhesives and temporary supports) and their compatibility for the preparation of remoistenable tissues were carried out:

  • Adhesives: mass loss after drying, hardness, viscosity, pH.

  • Temporary supports: pH, dimensional changes.

  • Remoistenable tissues: pH, evaporation speed, dimensional changes.

Mass loss after drying

40Thermogravimetry was used for the determination of mass loss during the drying process. Adhesive films were prepared by pouring the liquid polymers in non-stick moulds and letting them dry in stable environmental conditions (25 ºC; 50 % HR) for a month. Two sets of samples of 3x3 cm were prepared. The first set was exposed to heat over a duration of 30 minutes while increasing the temperature uniformly from 25 to 65 °C. The percentage of water loss during conservation treatments, which include the application of heat not exceeding 60 °C, was simulated. The second set was exposed to heat over a duration of 30 minutes while increasing the temperature from 25 to 105 °C, with the purpose of assessing the total loss of moisture content.

  • 3 The analytical balance, the durometer and the viscometer belong to the Laboratories of Investigatio (...)

41To be sure the obtained measurements did not include other volatile substances (that are byproducts from the thermal decomposition), samples were weighted after 48 h to assess whether the samples recovered their initial weight. This meant that the measured loss after drying referred to the water only. For this test, a thermobalance PCE-MB 50 (PCE Group)3 was used.

Hardness

42The hardness of a material is a measure that refers to its resistance to localized plastic deformation, and it depends on its Elasticity Modulus and its viscoelasticity. Adhesive films were used, overlapping subsequent layers of 1.5x1.5 cm to obtain samples with a minimum thickness of 6 mm. Five measurements for each adhesive dispersion were made, as indicated in ASTM D 2240 standard test.

43There are different durometers and every kind is suited for certain materials. In this case, a Shore A durometer (Shore TH200 - PCE Group), which is used to assess the hardness of rubbers, was used.

Viscosity

44Viscosity is a measure that describes a fluid’s resistance to flow and it is useful to understand the film-forming capacity of tested adhesive dispersions when applied on the temporary support. It further helped to have an estimate of the viscosity of the adhesive dispersion during the application process and the removal of the facing. In this case, a PCE-RVI 2 (PCE Group) rotational viscometer was used. For every dispersion, viscosity was measured after 30 seconds, one minute and three minutes. The measurements were repeated three times and every three minutes. The measurements were done with the adhesive dispersions at 23 °C +/- 1.5 °C.

pH

45When aqueous dispersions are used, it is important to control their pH. Therefore, the pH of the adhesive dispersions in their liquid state and after hardening on microscope slides was measured (Fig. 7).

Fig.7 pH measurements

Fig.7 pH measurements

Measurement of the pH of temporary supports and remoistenable tissues with a specific electrode for the measurement of surfaces.

Credits: Paola Alba.

46The pH of the supports and the remoistenable tissues were also measured. Furthermore, the pH of the remoistenable tissues reactivated with a buffer of pH 6.4 was determined.. The pH value was chosen on the basis of generic ones reported in the literature, referring to the intermediate pH of varnishes and aged oil paint layers (Cremonesi, 2011). The selected pH in the presented research here is just an approximation. Before treatment, it is advisable to check the real pH of the paint surface.

47For this investigation, a Hanna Precision (model 211) pH meter was used, which is equipped with a glass electrode for the measurement of aqueous solutions and a specific electrode for the measurement of surfaces.

Evaporation speed

48The evaporation rate measurement was carried out on samples of 3x3 cm and was replicated three times for each combination of adhesive and support. Samples were left to dry for 48 h. The loss of weight of the samples was measured periodically and then the data were displayed in a graphic. The analytical balance Precisa Serie 320 XT Model XT120A (Precisa) was used for this test.

Dimensional changes

49Three samples of temporary support of 2x14 cm were prepared for each kind of remoistenable tissue. In the case of TNT, two sets for each fibre direction were prepared. Each sample was measured before the application of the adhesive, after the application, and after 24 h. Then, the measurements were repeated after the reactivation of the remoistenable tissue and after 24 h. The test was done leaving the samples on a Mylar® foil, to reduce the friction/rubbing and to determine the maximal dimensional variation. A Stainless digital calibre was used.

Results and Discussion

Mass loss on drying

50The results of the thermogravimetric analysis (Fig. 8) show that the mass loss after drying at 65° C is lower than that at 105° C. After 48 h, all samples recovered their initial weight, which means that the loss after drying is related to the loss of water and not to loss of other volatile components. The results also reveal that a part of free water got trapped in the adhesive film during the treatment, which included the use of heat.

Fig. 8. Mass loss on drying

Fig. 8. Mass loss on drying

Results obtained with thermogravimetric analysis. Results referring to samples subjected to the heating program 25-65°C are marked in light colour; results referring to samples subjected to the heating program 25-105°C are marked in dark colour.

Credits: Paola Alba.

51Results also illustrate the influence of the polymers contained in the tested adhesive dispersions. TP1 and KP1, which are prepared with the two polymers used commonly as thickeners (Klucel® y Tylose®) but with the same adhesive polymer (Plextol®), behave differently. In fact, the samples made with the TP1 mixture have a higher mass loss (4.81 % (65ºC) - 5.46 % (105ºC)) than the samples prepared with KP1 (3.12-4.18 %), but KP1 retains more water when heated at 65 °C. In fact, the difference in mass loss of the samples heated at 65 °C and at 105 °C is higher than for TP1 (1.06 % against 0.65 %).

52Although it does not exceed the limits, the KA1 dispersion retains much more water (6.76-11.29 %) than KP1. This is attributed to the use of the water-sensitive Aquazol®. This characteristic is also reflected in the difference between the values of the mass loss at 65 °C and at 105 °C (4.53 %), which is much higher than in the dispersions prepared with the thickeners (Klucel® and Tylose®), and the Plextol®.

Hardness

53Hardness test results shown in Fig. 9 are quite similar. The TP1 film (97.88) is slightly harder than the KP1 (85.38) and KA1 (86.64) films.

Fig. 9. Hardness

Fig. 9. Hardness

Results obtained with hardness test with a Shore durometer (class A).

Credits: Paola Alba.

Viscosity

54The data illustrated in Fig. 10 shows that the TP1 (2491.11 mPa·s) and KA1 (2802.22 mPa·s) dispersions are much more viscous than KP1 (703.33 mPa·s).

55The differences between the dispersions prepared with the two cellulose ethers are easily justified by the difference in viscosity of Klucel® and Tylose®. Actually, according to the literature, an aqueous Klucel® dispersion at 2% has a viscosity of 150-400 mPa·s, while the viscosity of Tylose® at the same concentration is of 270-350 mPa·s. Furthermore, the viscosity of Tylose® has a higher increment than Klucel® with the increase in concentration.

Fig. 10. Viscosity

Fig. 10. Viscosity

Viscosity values for the tested adhesive dispersions.

Credits: Paola Alba.

56The high viscosity of KA1 (2802.22 mPa·s) must be noted. In fact, both materials used for this dispersion have a quite low viscosity. Probably the arrangement of molecules in the dispersion promotes the formation of intermolecular bonds (Hydrogen bonds, Van der Waals) which contribute to an increase in viscosity. In order to understand this phenomenon, additional tests with Fourier Transform Infrared Spectroscopy (FTIR) will be done.

pH

57The results on the pH measurements of the prepared dispersions revealed the influence of Plextol® through a slightly higher alkalinity, exhibited by TP1 (7.96) and KP1 (7.83). The KA1 dispersion reported an almost neutral value of pH (6.98).

58The pH values of the dried dispersions applied as thin films on microscope slides, and reactivated with a drop of demineralized water, revealed some changes. The pH of the two dispersions prepared with Plextol® are more acidic (TP1:5.30; KP1: 6.15), while KA1 remains almost neutral (7.08). This suggests that the alkalinity of Plextol® depends on volatile components contained in the emulsion.

59In regards to the samples that were prepared on the remoistenable tissues, the pH of the ones prepared with Bib. Tengujo exhibited almost neutral values (pH between 6.61 and 6.90). The pH found in samples prepared with remoistenable TNT tissues is slightly more acid (pH between 6.57 and 6.81). The influence of the support is evident in the case of Holytex®, the pH of the corresponding remoistenable tissue is strongly basic (pH between 8.81 and 9.42). This can be due to an alkaline reserve present in the tissue. Further analyses would be useful in order to study and understand these results better.

60The efficiency of the buffer solution was confirmed, since all the remoistenable tissues showed a pH around 6.4 ± 0.1.

Evaporation speed

61The evaporation rate of the water sprayed to simulate the reactivation process depends on the amount of water itself and on the kind of support, but not on the adhesive mixture.

Dimensional changes

62The test series that were supposed to record dimensional changes of remoistenable tissues were not satisfactory. The system used did not have the precision needed to obtain accurate results. Samples were prepared with the maximum possible length (1 cm shorter than the maximum aperture of the calibre), while increasing the number of measurements to reduce the error (samples were always measured six times).

63It seems the dimensional changes depend on the support but not on the adhesives, since no differences were found among the remoistenable tissues prepared with the same support and different adhesives. It is not possible to state with certainty if this data is correct or if it depends on the inaccuracy of the method. It, however, is possible to assume that if any difference exists, it might be negligible.

Conclusions

64These analyses permitted the definition of the moisture content, the hardness, the viscosity, and the pH of the tested materials, which are essential characteristics in the assessment of adhesive dispersions used for facing.

65According to the results, all kinds of adhesive dispersions seem appropriate for the preparation of remoistenable tissues. All have a similar hardness (between 85.38 and 97.88) and their values of mass loss after drying, between 4.18 % and 11.29 %, are tolerable. It is important to point out that the viscosity of the dispersions in its liquid form can provide an indication about the thixotropic behavior of the adhesives during the reactivation and the removal of the remoistenable tissue.

66It is also worth emphasizing the potential danger of the TP1 and KP1 dispersions when applied with a brush to face a painting. Their pH values in the liquid state (TP1: pH 7.96; KP1: pH 7.83) are quite similar to the cut-off value of pH recommended for the conservation of coatings and oil paint layers of paintings.

67The most conclusive results of the research are in reference to the influence of the supports in the pH of the remoistenable tissues, in the evaporation speed of the water used for the reactivation, and in the dimensional changes.

68In fact, Japanese paper Bib Tengujo has a sensible contraction. That will be related to the further results of the tensile test to assess the potential risk associated with the use of this support. Remoistenable tissues of Japanese paper show a neutral pH and intermediate values of evaporation speed.

69TNT tissues have a strong anisotropic contraction. That will be related to further tensile strength in the longitudinal and transversal directions. Remoistenable tissues made with this support show quite neutral pH values and a lower evaporation rate than that of the cellulosic support.

70A further evaluation of the behaviour of Holytex supports is needed. In fact, the pH of the remoistenable tissues prepared with this support is strongly basic, even if it was controllable by the use of a buffer solution. This support has very low dimensional changes and elevated values of evaporation speed. These two factors could be either positive or negative. In fact, a very low contraction can be good because it reduces the stress exerted on the paint layer. Nevertheless, a very low dimensional change can be a clue of a poor adaptability of the support to the substrate, which would limit the adhesion strength of the facing. The experienced high evaporation speed is suitable in the measure in which it facilitated the elimination of the solvent, but it could be also related to low affinity with the adhesives and the solvent used for the preparation of the RTS. In this case, it might lead to a poor retention of the adhesive in the support, facilitating its penetration and increasing the amount of residues left on the surface after removal of the remoistenable tissue.

71This work represents a starting point for the ongoing research into facing methods and materials. This first results allowed for a better understanding of the behavior of different kinds of adhesive dispersions and the potential influence of the temporary supports used for facing.

72The authors hope this investigation may be useful for the preparation of protective facings, by providing tools to make better choices regarding this intervention. In this way it will be possible to choose the most appropriate facing on the basis of experimental data and not tradition or hypothetic reasoning, as it often is the case in today’s practice.

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Bibliography

Alba, Paola, and Antonio Iaccarino Idelson. 2016. Protezione o consolidamento? La velinatura tra tradizione e innovazione. Proc. of XXXII Congresso Internazionale Scienza e Beni Culturali. Eresia ed ortodossia nel Restauro, June 28 – July 1. Bressanone: Arcadia Ricerche.

Alba, Paola, Antonio Iaccarino Idelson, and Susana Martín-Rey. 2017. Riflessioni sulla velinatura: una ricerca in corso. Proc. of XV Congresso Nazionale IGIIC. Lo stato dell’arte 15, October 12-14. Bari: Nardini.

Bedenikovic, Theresa, Sigris Eyb-Green, and Wolfgang Baatz. 2018. “Non-Aqueous Facing Methods in Paper Conservation – Part I: Testing Facing Materials”. Restaurator. International Journal for the Preservation of Library and Archival Material 39(3): 185-214.

Borgioli, Leonardo, Enrica Boschetti, Claudia Tortato. 2016. “I cerotti di Aquazol 500. Una procedura alternativa per la velinatura dei dipinti”. Progetto Restauro 73: 3-13.

Cremonesi, Paolo. 2011. L’ambiente acquoso per la pulitura di opere policrome. Padova: Il Prato.

Horie, Charles Velson. 2010. Materials for Conservation: Organic Consolidants, Adhesives and Coatings. Oxford: Butterworth Heinemann.

Iaccarino Idelson, Antonio, Carlo Serino. 2014. “L’intervento Strutturale”. In Lungo la via degli Abruzzi, un restauro per Pescocostanzo, Massimo Stanzione e Giambattista Gamba nella chiesa di Gesù e Maria, 64-75. Foligno: Etgraphiae.

Lechuga, Katherine. 2011. Aquazol-Coated Remoistenable Mending Tissues. Proc. of Symposium 2011: Adhesives and Consolidants for Conservation: Research and Applications, October 17-21, Canadian Conservation Institute. Ottawa.

Martín-Rey, Susana, et al. 2012. Requisiti e problemi delle velinature di protezione di grandi dipinti su tela: nuovi materiali utilizzati nelle opere del Palazzo Ducale di Gandia (Spagna). Proc. of VI Congresso Internazionale Colore e Conservazione. Prima, durante… invece del restauro, November 16-17, CESMAR7. Padova: Il Prato.

Mehra, Vishwa Raj. 1972. Comparative study of conventional relining methods and materials and research towards their improvement. Proceedings from ICOM-CC 3rd Triennial Meeting, March 22-24, International Council of Museums. Madrid.

Olender, Jacek, Christina Young, Ambrose Taylor. 2017. The applicability of gecko-inspired dry adhesives to the conservation of photographic prints. Proceedings of ICOM-CC 18th Triennial Conference Modern Materials and Contemporary Art, September 4-8, International Council of Museums. Copenhagen: J. Bridgland.

Pataki, Andrea. 2009. “Remoistenable Tissue Preparation and its Practical Aspects”. Restaurator 30: 51-60.

Van Och, Jos, René Hoppenbrouwers. 2003. “Mist-lining and low-pressure envelopes: an alternative lining method for the reinforcement of canvas paintings.” Zeitschrift für Kunsttechnologie und Konservierung 17(1): 116-128.

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Notes

1 The survey was carried out for the Master’s Thesis: Alba, Paola. 2015. “La velinatura. Riflessioni su un’operazione problematica.” Master’s Degree diss., Università degli Studi di Urbino “Carlo Bo” (unpublished).

2 The return migration refers to the stage in which the polymers penetrated in the substrate starts to come back to the surface, attracted by the evaporation of the solvent.

3 The analytical balance, the durometer and the viscometer belong to the Laboratories of Investigation and Analysis on adhesives materials and texile fibres in structural intervention of canvas paintings, of the Instituto Universitario de Restauración del Patrimonio (UPV).

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List of illustrations

Title Fig. 1. Protective and consolidating facing
Caption Distinction between protective facing and consolidating facing. The temporary support is marked in yellow, the adhesive is marked in orange.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-1.jpg
File image/jpeg, 76k
Title Fig. 2. RTS: preparation steps
Caption Scheme of RTS preparation and reactivation for application and removal.
Credits Credits: Authorship of Paola Alba and MaríaTeresa Doménech-Carbó.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-2.jpg
File image/jpeg, 280k
Title Fig. 3. RTS: preparation
Caption Scheme of the application and settle of the adhesive on the interim support.
Credits Credits: Paola Alba and MaríaTeresa Doménech-Carbó.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-3.jpg
File image/jpeg, 80k
Title Fig. 4. RTS: adhesive film
Caption RTS after the application and the drying of the adhesive.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-4.jpg
File image/jpeg, 2.8M
Title Fig. 5. RTS: application and removal
Caption Steps of the application and the removal of RTS, using a reactivation cloth. Left top: preparation of the reactivation cloth wrapped in a polyethylene foil and embedded with the buffer solution. Right top: rubbing the surface after the superposition of the remoistenable tissue, the reactivation cloth and the Mylar foil. Left bottom: mechanical removal after the reactivation with the application of the reactivation cloth and the superimposed Mylar foil for a specific time. Right bottom: cleaning of the residues with a cotton swab embedded with the buffer solution.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-5.jpg
File image/jpeg, 164k
Title Fig. 6. Preliminary selection of the adhesives
Caption Optical microscopy analyses carried out to verify the homogeneous distribution of the polymers on the temporary supports.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-6.jpg
File image/jpeg, 2.8M
Title Table 1 - Composition of the adhesive dispersions
Caption In the table are summarised the proportion used to prepare the selected dispersions.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-7.jpg
File image/jpeg, 116k
Title Fig.7 pH measurements
Caption Measurement of the pH of temporary supports and remoistenable tissues with a specific electrode for the measurement of surfaces.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-8.jpg
File image/jpeg, 2.4M
Title Fig. 8. Mass loss on drying
Caption Results obtained with thermogravimetric analysis. Results referring to samples subjected to the heating program 25-65°C are marked in light colour; results referring to samples subjected to the heating program 25-105°C are marked in dark colour.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-9.jpg
File image/jpeg, 40k
Title Fig. 9. Hardness
Caption Results obtained with hardness test with a Shore durometer (class A).
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-10.jpg
File image/jpeg, 32k
Title Fig. 10. Viscosity
Caption Viscosity values for the tested adhesive dispersions.
Credits Credits: Paola Alba.
URL http://journals.openedition.org/ceroart/docannexe/image/6532/img-11.jpg
File image/jpeg, 36k
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References

Electronic reference

Paola Alba, Susana Martín-Rey and MaríaTeresa Doménech-Carbó, « Analysis of facing materials used as remoistenable temporary supports for facing on canvas paintings », CeROArt [Online], 11 | 2019, Online since 07 October 2019, connection on 21 October 2019. URL : http://journals.openedition.org/ceroart/6532 ; DOI : 10.4000/ceroart.6532

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About the authors

Paola Alba

Paola Alba obtained a recognized Master’s Degree in Conservation and Restoration of Cultural Heritage at the University of Urbino (Italy) in 2015, and is an accredited conservator. In 2016 she started a Ph.D. at the Universitat Politècnica de València (UPV) on an innovative facing system based on Remoistenable Temporary Supports, carried out at the Instistituto de Restauración del Patrimonio (IRP) of UPV in Spain, the Chemistry Department of the University of Bari in Italy and the Adhesion Institute of Technical University of Delft in the Netherlands.

Susana Martín-Rey

Susana Martín Rey, PhD in Fine Arts Conservation of Cultural Heritage (2004, Universitat Politècnica València) and PhD in Chemical Sciences (2017, UNED). Since 2001 Professor in Conservation of canvas painting at UPV. Her research and publications (over 100 papers) focus on the analysis and testing of eco-sustainable and safe materials for the conservation of canvas paintings. She has directed 5 research projects. Since 2013 she has been the head of the Art and Heritage Fund UPV.

MaríaTeresa Doménech-Carbó

María Teresa Doménech Carbó, B.Sc., D.Phil. in Chemistry (1989, University of Valencia). Since 1999 professor in Science of Conservation at the Universitat Politècnica de València (UPV) and from 2005 to 2016 director of the IRP of the UPV. She has published over 200 papers on chemical and physical methods of analysis of artworks. She has supervised 19 research students successfully for the degree of Ph.D. She has directed 12 regional, national and European research projects.

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Copyright

Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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