Products tested
New strengthening and deacidifying products based on nanofibres and nanoparticles from three different institutions were tested at the University of Barcelona (UB). The products were from: Chalmers University of Technology (CUT, Gothenburg, Sweden), the Consorzio Interuniversitario per lo Sviluppo a Grande Interfase (CSGI, Florence, Italy) and the Zentrum für Bucherhaltung GmbH (ZFB, Leipzig, Germany).
The following is a description of the products tested from each of these three institutions (see Table 1 for the exact compositions of the materials).
CUT products
CUT produced three water-based consolidants: CUT I (+) and CUT II (−), which are nanosilica-based and consolidate at the fibre level (numbers 1 and 2 in Table 1, respectively), and CUT CNF (number 3 in Table 1), which contains cellulose nanofibrils and consolidates at the thread level (the consolidant stays more on the surface of the canvas) [33].
Both CUT I (+) and CUT II (−) contain the colloidal silica Levasil CS40-213b (spherical diameter of 21 nm), obtained from AkzoNobel Pulp and Performance Chemicals in Sweden, as well as branched polyethylenimine (PEI) (average molecular weight of 25,000 g/mol), obtained from Sigma-Aldrich. CUT II (−) has a second added layer of the sodium carboxymethyl cellulose (CMC) Akucell AF0305 (average molecular weight of 650,000 g/mol), obtained from AkzoNobel Pulp and Performance Chemicals in Sweden.
CUT CNF is composed of cellulose nanofibrils (CNF) provided by Stora Enso AB (Sweden) in the form of an aqueous dispersion of 3.3 wt%. Further details on these materials and products are described elsewhere [32].
CSGI products
CSGI provided three deacidifying agents, two of these (CSGI I (+) (number 4 in Table 1) and CSGI II (−) (number 5 in Table 1) were provided in a water/ethanol mixture, while CSGI nanoP (number 6 in Table 1) was 100% ethanol-based.
As CSGI II (−) contains a deacidifying agent and a cellulose derivative, it is better for providing mechanical strength to the canvas than CSGI I (+).
Calcium carbonate nanoparticles were used at a concentration of about 2% in ethanol (CSGI nanoP). CSGI I (+) was prepared by mixing calcium carbonate nanoparticles in ethanol with PEI (0.05%) and adding water to obtain an ethanol/water mixture of about 45% vol. CSGI II (−) was obtained by adding CMC (0.4%) to CSGI I (+).
A solvothermal process starting from calcium metal and ethanol has been recently proposed for the preparation of calcium hydroxide nanoparticles [35]. A modification of this process, based on the use of water and diethyl carbonate to convert calcium ethoxide into calcium carbonate, was used to prepare calcium carbonate nanoparticles. Branched PEI with an average molecular weight (Mw) of 25,000 g/mol was purchased from Sigma-Aldrich. Sodium CMC supplied by Nouryon, Bohus, Sweden (formerly AkzoNobel Pulp and Performance Chemicals) had a degree of substitution of 0.77 and a viscosity of 12 mPa s (at 1%). The Mw of the CMC was 650,000 g/mol (determined by size exclusion chromatography).
Combination of CUT and CSGI products
CUT and CSGI designed products that could be mixed together. CUT I (+) can be mixed with CSGI I (+) (number 7 in Table 1) and CUT II (−) can be mixed with CSGI II (−) (number 8 in Table 1). This enables the application of a deacidifying agent and a consolidant (at the fibre level) in a one-step process.
After applying the mixture of CUT I (+) and CSGI (+) or CUT (−) and CSGI (−), a nanocellulose-based consolidant (CUT CNF) can then be added if desired for further consolidation (numbers 9 and 10 in Table 1). This would entail the application of three products in a two-step process.
ZFB products
Six products were developed by ZFB. Two of them were deacidifying agents (a calcium carbonate-based one in water (ZFB 1a, number 11 in Table 1) and a magnesium oxide-based one in heptane (ZFB 2a, number 14 in Table 1)) and two of them were nanocellulose-based consolidants (one in water (ZFB 1b, number 12 in Table 1) and one in heptane (ZFB 2b, number 15 in Table 1)).
The remaining two products were mixtures of the abovementioned deacidifying agents and consolidants. One of them was a mixture of the two water-based products (ZFB 1c, number 13 in Table 1) and the other one was a mixture of the two heptane-based products (ZFB 2c, number 16 in Table 1).
The deacidifying agents, ZFB 1a and ZFB 2a, were developed and produced by ZFB and are sub-µm dispersions in water and heptane, which means that the calcium carbonate and magnesium oxide used have a particle size of under 1 µm [34].
The consolidants ZFB 1b, ZFB 2b, ZFB 1c and ZFB 2c contain hydroxyethyl methyl cellulose (Tylose® MH), purchased from Shin Etsu, and cellulose nanocrystals (CelluForce NCC™), obtained from CelluForce Inc. For the solvent-based products, a silylation reaction was carried out [34].
Traditionally used benchmark products
For comparison, three consolidants and one deacidifying agent that are currently used by conservators (see the Introduction section) were also assessed: Gustav Berger’s Original Formula® 371 (Beva® 371) from CTS Srl (number 17 in Table 1); Paraloid B72 from CTS Srl (number 18 in Table 1); animal glue from Lienzos Levante (Spain; number 19 in Table 1) and Nanorestore Paper® Ethanol 3 from CSGI (number 20 in Table 1).
Supports used for testing the products
The products were tested on four different types of canvas supports.
Two of them were new textiles: linen canvas, provided by Lienzos Levante (Valencia, Spain; reference number 20) with a grammage of 200 g/m2, and cotton canvas, obtained from Barna Art (Barcelona, Spain) and with a grammage of 417 g/m2.
The third type of support was an artificially degraded jute canvas from Lienzos Levante (Valencia, Spain; 300 g/m2). Degradation was achieved by immersing the canvas into 1% w/w of an alum solution prior to tacking it onto a stretcher (once it was dry). Then, one layer of animal glue (9.6% w/v) was applied onto the canvas. Once the glue was dry, samples were cut and placed in an oven (Heraeus Voetsch HC 0020) at 90 °C and 65% relative humidity (RH) for 18 days [36]. The pH of the degraded samples was 3.9.
Finally, products were also applied onto a sacrificial real painting: “Landscape” from ca. 1950. The grammage of this canvas is unknown, but based on experience with other known grammage, it was thought to be around 200 g/m2. The canvas has a light brown tonality and a pH of 5.4.
Application method and amount of product applied
All the products were applied onto the canvas samples using a brush so that the amount of product deposited could be quantified. On real paintings, products are added to the back of the painting, onto the original canvas, and are sprayed if the conservator prefers not to wet the painting excessively.
The amount of product applied resulted in a “5% weight increase” in all cases with regard to the weight of the untreated canvas. Accordingly, coarser canvasses received more product than lighter ones.
This rule was not followed for the CSGI products, since the guidelines indicated that “1 mL of product for each 20 cm2 of canvas” should be used for CSGI I (+), CSGI II (-) and CSGI nanoP. This causes a “5% weight increase” in the case of canvasses with a grammage of 200 g/m2 (which is quite a common grammage for regular paintings). The instructions for Nanorestore Paper® Ethanol 3* stated that 2 mL should be applied for each 20 cm2.
Colour analysis
Colour was measured for each sample at three different spots, with L, a and b values for the SCI (Specular Component Included) recorded using the CIELab76 system.
The instrument used was a Konika Minolta CM-26000d spectrophotometer. The aperture was set at 0.8 mm (the larger option) and a minimum of three measurements were taken at each spot. If the SD of either L, a or b was larger than 0.3, up to seven measurements were taken.
Melinex films with pen marks were used to relocate the instrument onto the same place after treatment to ensure that the same spots were measured. The same pile of white paper was placed under the canvas when taking the measurements to make sure that the background was the same for all measurements.
The conditions of the instrument were set at: mask/gloss, M/I + E; UV, 100%; Il.1, D65; Il.2, –; observer, 10°; display, Diff&ABS; colour space, L*a*b*; manual avg. times, —; auto avg., 3; and delay time, 0.5 s.
Gloss analysis
A Rhopoint Novo-Gloss glossmeter was used to measure gloss before and after treatment on the new linen canvas. Each sample was measured at three locations and each location was measured three times. Since matt surfaces were being measured and given that the values obtained with the 60º angle were below 10 GU, results obtained with the 85º angle were the ones that were recorded (as is normally done) [37].
Pencil marks on the samples and a plastic template from the instrument were used for repositioning the instrument onto the same place after treatment.
pH analysis
The pH was measured through the pH cold extraction method using an IQ160 pH meter with a pHW17-SS ISFET micro-probe both on the acidic jute canvas and on the sacrificial real painting.
Samples were taken using a scalpel and tweezers. The size of the sample obtained was about 4 mm of a thread for the sacrificial painting and about 2 mm for the jute canvas (jute threads are coarser, thus less sample is needed). This length of the threads was found to correspond to the 250–350 µg required for the miniaturisation of TAPPI 509 om-02 pH [38]. 100 µL of distilled water were added to the vial containing the sample and pH measurements were taken the day after the sample was immersed in water.
Three samples were taken from each of the systems analysed and each sample was measured three times.
Star diagram
A table with five different items was designed so that conservators could evaluate the application of the products on linen and their visible results. This was a subjective type of evaluation, where conservators compared their experience of the new products with that of the materials they have already used in conservation. The quantification of the mechanical improvement and of the deacidifying ability of the products has been analysed elsewhere [31,32,33,34].
The items evaluated were:
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1.
Drying time of the product
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2.
Ease of use
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3.
Stiffness of the treated canvas
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4.
Colour change of the canvas: darkening
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5.
Colour change of the canvas: whitening
For each evaluated item, three different possible values were established, 1 being the worst score and 3 being the best. For each item, a reference sample for each of the two extreme values, 1 and 3, was established and all the samples were then rated against these. The information was then plotted in star diagrams, making it easier to visually identify the products with the best results (those with a larger surface of the star covered) in terms of the items evaluated.
The star diagram evaluation was carried out by the UB conservators participating in the Nanorestart project on linen samples. The fourteen different single products were evaluated, leaving aside the treatments requiring two layers or mixtures (systems 7, 8, 9, 10, 13 and 16). The following is a description of how each item was evaluated and the meaning of each given value.
Drying time of the product
Water-containing products pose a risk to materials such as the canvas or ground layers by causing swelling or shrinking [39]. In the case of solvent-based products, prolonged contact with solvents can create swelling or leaching of the paint layers and varnishes [3, 40]. Therefore, to avoid adverse consequences resulting from prolonged contact, products with shorter drying times are preferred.
The drying times of the different products were assessed. A score of 1 was given to samples with drying times similar to that of animal glue, which is water-based and takes about 20 min to fully dry (visually). A score of 3 was given to products that took less than a minute to dry fully, while a score of 2 was given to products that took between 1 and 20 min to dry.
Ease of use
The ease with which a product can be applied is quite often an important factor when deciding which product to use in a particular situation.
Products rated as 1 were those that had some properties that made them more cumbersome to use, such as high viscosity, which made it difficult to spray them, or the need to constantly shake the product well before application to avoid the particles from settling.
A score of 2 was given to products that required specific equipment such as a fume hood due to the toxicity of their solvents or other special requirements, while a score of 3 was given to products that were easy to use and did not need complicated preparations or equipment.
Stiffness of the treated canvas
The capacity of the products to improve the stiffness of the canvas was evaluated by determining if the added products improved resistance to the elongation of the canvas.
The canvas (with the added products) must support the ground and paint layers and limit the movements of these rigid layers to avoid permanent deformation that occurs long before the formation of cracks [33].
Stiffness can be improved either by locking the threads (interlocking weft and warp) with a layer of product that reinforces the canvas or by reinforcing threads and fibres onto a deeper level.
Samples treated with the different products were assessed by pulling the canvas in the weft, warp and diagonal directions to check the deformation of the treated canvas according to the different tensions that occur in a painting and in order to see the reinforcement of the whole.
A score of 3 was given to samples that, under tension, moved very slightly, as was observed with product 3 (CNF). A score of 1 was given to samples that, under tension, moved as much as the untreated linen. Finally, a score of 2 was given to samples in which a slight reinforcement was detected, and which fell between those rated as close to 1 and those rated as close to 3.
Colour change of the canvas: darkening
Ideally, any product added onto the canvas of a painting should not alter its original visual appearance. This is because even if the canvas side is never exposed when viewing the painting, the main conservation principle of introducing minimal changes into the object needs to be followed [7]. This being said, it is also true that a compromise between a small colour change and an improvement in the canvas condition (mechanically or regarding acidity levels) might be needed since the latter can be of greater importance.
Colour change in terms of darkening caused by the treatments was assessed by visually looking at the samples from a distance of 1.5 m and comparing the overall darkening of the linen sample with that of the untreated linen sample (score of 3) and also of the animal glue-treated sample (which had a score of 1 since this was the sample that darkened the most). A score of 2 was given to samples that were between those rated as close to 1 and those rated as close to 3 and therefore had some degree of darkening, but one that was acceptable.
Colour change of the canvas: whitening
Whitening caused by the treatments was assessed by visual inspection of the samples from a distance of 1.5 m and comparing the overall whitening of the linen sample with that of the untreated sample (score value of 3) and also of the sample treated with product combination 9 (the one that received one coat of CUT I (+) and CSGI I (+) plus a CNF coat on top), which was perceived to be too whitened (score value of 1). A score of 2 was given to samples that fell between the other two categories (some whitening, but acceptable).