Silvia G. Fernández-Villa, José M. de la Roja, Margarita San Andrés, Colour Change Caused by Dry Cleaning Process on Painted Surfaces, e-conservation Journal 5, 2017
Available online 15 March 2017
Colour Change Caused by Dry Cleaning Process on Painted Surfaces
Silvia G. Fernández-Villa, José M. de la Roja, Margarita San Andrés
Over the last few years, the dry cleaning process has become an excellent alternative to solvent or water-based mixtures, which prevents diffusing the dirt into the painting structure during the cleaning process. The use of this kind of cleaning is especially effective on contemporary paintings as it prevents the solubilisation of additives such as the surfactants contained in polymeric dispersions (e.g. in acrylic and PVAc-based paints). In addition to some reported side effects that could be caused by dry cleaning processes on paintings (such as changes in the integrity of the paint or any residual materials which may be left behind) one parameter that should be tested is the colorimetric change seen on treated surfaces. In some cases, the use of certain dry cleaning materials can cause burnishing or roughening of the painted surface. This effect results in a change in brightness and chroma and subsequently to a change in the colour of the treated area, especially on dark shades. This article evaluates the colorimetric change of different kinds of painted surfaces caused by the dry cleaning process, such as gouache, commercial formulated acrylic, fish glue tempera, casein tempera, PVAc and primal acrylic paint. The cleaning materials tested include different kinds of sponge cloths, malleable materials and erasers used in the field of heritage conservation/restoration. The change in colour is measured by means of a spectrophotometer and the chromatic values are processed in a spreadsheet. The colorimetric changes have been calculated using the advanced equation recommended by CIEDE2000 and the chromatic values obtained have been studied following the standard recommendations.
The removal of dirt from paint surfaces is an increasingly frequent problem, especially in the case of the unvarnished or matte paint usually seen in the field of contemporary painting. The porosity and high absorption capacity of these types of paints makes the removal of the surface dirt that accumulates on them highly complex, as these deposits are usually highly adhesive on porous bases. The options for use of organic solvents are very limited as in many cases there are painted layers which are highly sensitive due to their low binder content and in which the pigment is poorly binded and more solvent-sensitive. In other cases, the most significant risk lies in the fact that the solubilised dirt could diffuse through the paint layer during the cleaning process, due to its high porosity.
The alternative use of water-based methods on certain types of paint may also pose other types of risk due to the water-based sensitivity of some soluble components, such as additives like dispersing aids, thickeners or surfactants. In particular, it has been reported that surfactants which are present in small quantities in acrylic emulsion paint formulations can be affected by water-based cleaning systems [1-3]. An interesting alternative to consider in view of the problems described above is the use of dry cleaning systems.
However, the use of several dry cleaning materials may lead to some undesirable side effects, such as a colour change in the painting. Burnishing or roughening of the painted surface can critically influence its appearance and coherence, especially on matte paints. This kind of effect is especially noticeable on dark shades, as it occurs in the colour change caused by applying a layer of varnish on a matte or semi-matte paint .
In previous research different dry cleaning systems have been tested on contemporary matte paint layers prepared in our laboratory , to which both surface dirt and a synthetic soiling mixture had been applied . The effectiveness of these cleaning systems, the possible residue left by each of the tested materials as well as the superficial alterations which may have occurred on the painting surface were evaluated. Painted surfaces were studied using scanning electron microscopy (SEM), and in some samples a significant burnish effect was seen due to the rubbing process associated with these cleaning systems. The change in surface gloss was measured using a reflectometer and the results obtained confirmed that the majority of the dry cleaning materials tested tended to burnish or roughen the grounds or painting surfaces. Furthermore, by means of a colorimetric analysis, certain differences in colour were determined, although it is not clear whether the changes detected were caused by surface burnishing or roughening or by dirt residue.
In order to determine the effect of the surface rubbing action on these colour changes, this project includes a colorimetric analysis of diverse paint samples prepared in our laboratory, before and after they have been rubbed with each type of dry cleaning material.
2.1. Sample preparation
2.1.1. Paint samples
Six sets of paint samples were prepared. Two of them were commercial paint formulations and the others were specifically prepared using different kinds of binders and 0261 Burnt Umber pigment (CTS) (Table I) in order to obtain a dark shade. With these types of shades, the colour changes caused by rubbing with the selected dry cleaning material were much more noticeable. The first set of samples included a commercial gouache formulation (ref. 409 - Burnt Umber, Talens Holland) and the second included an acrylic commercial paint formulation (ref. 18 - Burnt Umber, Acrylic Studio Vallejo). The other samples were specifically prepared to ensure a matte finish by using a low binder ratio. For this purpose, different binder/pigment ratios were tested, set at between 1/1 and 1/3 according to the binder strength. A set of samples was therefore prepared with fish glue tempera paint, casein tempera paint, vinyl paint and an acrylic paint binded with primal.
The application of the different paints was carried out on a gesso sottile background with a light plain brush in an attempt to achieve the most homogenous layer possible. Once the layers had dried, protection was applied using an acetate sheet and the respective windows (15x15mm) were made so as to enable the specific area established for measurement of the chromatic change in the colorimetric examination to be located, both before and after rubbing with the tested materials (Fig. 1).
2.1.2. Cleaning materials
In the context of this research, several dry cleaning materials, such as microfiber cloths, sponges and malleable materials were tested. Some of them have been developed for general use and others are specifically for the conservation/restoration field. The selected materials include microfiber cloth (3M), make-up sponge (Sephora), melamine foam (3M), Akapad white (AKA Chemie Wishab), Groomstick (Picreator Enterprises Ltd) and smoke sponge (Conservation by Design Ltd). Table II provides relevant information on the materials selected for testing: product, stereoscopic microscope caption (equipment used: Leica MZ125), composition data extracted from Daudin-Schotte et al.  and the manufacturer or supplier.
Table I. Paint samples to be tested.
Table II. Selected dry cleaning materials.
Each of the six dry cleaning materials was tested by means of rubbing on the different painted surfaces mentioned above for 30 seconds (Fig. 2). This resulted in a total of 6 standard samples which underwent a colorimetric study, some of which had highly noticeable burnishing or roughening (Fig. 3). Their chromatic changes were measured.
Left to right:
Figure 1. Paint samples prior to testing.
Figure 2. Paint samples during the dry cleaning process with the selected materials.
Figure 3. Burnishing and roughening of samples caused by the dry cleaning process.
2.1.3. Colorimetric change evaluation
In order to determine the chromatic changes undergone by the painted surfaces, a Konica Minolta CM 2600d spectrophotometer was used. It has a range of 400-700nm, a 10nm measuring interval, an 8-degree viewing angle and is equipped with simultaneous measurement of SCI (specular component included)/SCE (specular component excluded) and a 3mm measurement surface area. Values are indicated in CIELAB coordinates (L* a* b*), SCE and SCI, based on a CIE standard illuminant D65 and a CIE 1964 10º standard observer. The CIELAB coordinate values taken into account are the average of at least three measurements. The chromatic values were obtained using SpectraMagic CM-S100w 1.91.0002 software and processed in a spreadsheet. The colorimetric changes were calculated according to the advanced equation recommended by CIE CIEDE2000 :
The colorimetric changes were determined by means of a comparison of the reference values obtained before rubbing with the selected dry cleaning materials in the same paint area. Moreover, colour differences at or above the perception threshold were considered according to the standard international recommendations for colour measurements .
In addition to the chromatic change caused on paint samples by the dry cleaning process, shine – as a perceptual attribute - was considered in order to determine the change on a matte or satin finish. The results were evaluated with regard to the colour and shine changes in the paint samples before and after rubbing with the selected material. Factors such as cleaning efficiency were not evaluated, as this has been studied in previous research .
With regard to the chromatic change, cleaning is considered to be less safe from a perceptual point of view when the colour difference is greater, as it leads to a bigger chromatic change on the sample surface.
In general, rubbing effects with the selected materials differed according to both the paint sample tested and the dry cleaning material used (Fig. 4). Chromatic changes were around the perception threshold when cleaning was carried out using Groomstick (AC E00=0.17±0.03; CO E00=0.73±0.13; CAS E00=0.40±0.08; VIN E00=0.66±0.09; PRI E00=0.32±0.07; GO E00=0.46±0.11) and for all the materials tested when cleaning was carried out on commercial formulated acrylic paint (E00 fluctuated between 0.17±0.03 and 0.80±0.30). Chromatic changes were above the perception threshold when microfiber cloth 3M was used (CO E00=2.04±0.05; CAS E00=2.32±0.09; VIN E00=1.90±0.02; PRI E00=2.13±0.07; GO E00=2.34±0.05), make-up sponge (CO E00=2.88±0.15; CAS E00=1.80±0.28; VIN E00=2.80±0.13; PRI E00=1.65±0.11; GO E00=2.20±0.03) and smoke sponge (CO E00=2.26±0.20; CAS E00=2.18±0.07; VIN E00=1.76±0.18; PRI E00=0.86±0.11; GO E00=2.68±0.18).
The chromatic change was well above the perception threshold when cleaning was carried out with melamine foam (CO E00=2.88±0.07; CAS E00=3.85±0.12; VIN E00=3.23±0.30; PRI E00=2.00±0.15; GO E00=4.31±0.02) and significant colour changes occurred when Akapad white was used (CO E00=3.98±0.52; CAS E00=5.32±0.14; VIN E00=4.39±0.05; PRI E00=4.37±0.04; GO E00=5.34±0.53). It can therefore be concluded that their results are not appropriate for the treatment of painted surfaces.
The lowest chromatic change was thus achieved with Groomstick (E00= 0.17±0.03 – 0.73±0.13) and for any of the materials on the layer of acrylic paint (E00=0.17±0.03 – 0.80±0.30), and the highest change occurred with melamine foam (E00=2.00±0.15 – 4.31±0.02) and Akapad White (E00=3.98±0.52 – 5.34±0.53). None of the tested cleaning materials affected the colorimetric characteristics of the commercial formulated acrylic paint (E00=0.17±0.03 – 0.80±0.30)
In general, excluding the specular component, colorimetric changes caused by the mechanical action of rubbing resulted in an increase in brightness and a decrease in chroma values. In contrast, a decrease in brightness occurred on the commercial formulated acrylic paint (Table III).
Table III. Colorimetric changes in the paint samples after rubbing with the selected dry cleaning materials.
The degree of the matte or satin finish of each of the samples was also evaluated after the rubbing tests had been carried out. For this evaluation, the existing chromatic difference between the chromatic values was determined taking into account the specular component excluded (SCE) and specular component included (SCI). The smaller the difference between these values, the greater the matte appearance of the surface; otherwise the surface will have a satin finish.
For this purpose, the gloss values before the cleaning tests had been carried out were evaluated. The values obtained show that apart from the acrylic paint surface, the other painted surfaces have a matte finish (Fig. 5a). After carrying out rubbing with the different cleaning materials, the results obtained show that there was an increase in the shine of the acrylic painted surface with all the cleaning materials used, and a slight increase in the gouache and primal acrylic paint layer when the cleaning was carried out with melamine foam 3M and Akapad white; the other painted surfaces remained matte (Fig. 5b).
Left to right:
Figure 4. Colorimetric change caused by rubbing according to the dry cleaning material used (a) and the painted surface treated (b).
Figure 5. Chromatic change value E00: specular component of the paint samples before rubbing (a) and after rubbing (b).
The colorimetric changes in the paint samples caused by the action of the selected dry cleaning materials vary according to whether they have a matte or satin finish, as well as the cleaning material used.
From a chromatic change perspective, the commercial formulated acrylic (with a satin finish) can be safely dry cleaned with all of the tested materials, as they do not significantly change its chromatic values. However, a slight increase in gloss occurs, especially when melamine foam 3M is used in the cleaning process.
The chromatic change caused by dry clean rubbing on the rest of the paint samples (all of them matte) is above the perception threshold, with the exception of Groomstick. Colour change effects are especially noticeable on the samples rubbed with melamine foam 3M and Akapad white. Moreover, in several cases these materials cause an increase of the gloss of the paint samples; they are not therefore appropriate for dry cleaning matte painted surfaces, especially those with dark shades.
This study was funded by the Spanish Ministry of Economy, Industry and Competitiveness under the Project I+D HAR2015-68680-P and the Community of Madrid under the GEOMATERIALS-2 Project (S2013/MIT-2914). Authors are also grateful to the Materials Laboratory at the Painting and Restoration Department of the UCM [Lab 397 of the RedLab Community of Madrid]. Authors are members of the Research Team Technical Documentation, Conservation and Restoration of Cultural Heritage (UCM- 930420).
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