Developing Innovative Apolar Gels Based on Cellulose Derivatives for Cleaning Metal Artworks

Abstract

The use of organic solvents, particularly those of a non-polar nature, is a common practice during cleaning operations in the restoration of polychrome artworks and metallic artifacts. However, these solvents pose significant risks to the health of operators and the environment. This study explores the formulation of innovative gels based on non-polar solvents and cellulose derivatives, proposing a safe and effective method for cleaning metallic artworks. The study is focused on a toxic apolar solvent, Ligroin, identified as one of the most widely used solvents in the cultural heritage treatments, and some "green" alternatives such as Methyl Myristate and Isopropyl Palmitate. The main challenge lies in overcoming the chemical incompatibility between non-polar solvents and polar thickening agents like cellulose ethers. To address this problem, the research was based on a hydrophilic-lipophilic balance (HLB) system and Hansen solubility parameters (HSPs) to select appropriate surfactants, ensuring the stability and effectiveness of the formulated gels. Stability, viscosity, and solvent release capacity of gels were analyzed using Static Light Multiple Scattering (Turbiscan), viscometry, and thermogravimetric analysis (TGA). The efficacy of cleaning in comparison with Ligroin liquid was evaluated on a metal specimen treated with various apolar protective coatings used commonly in the restoration of metallic artifacts, such as microcrystalline waxes (Reswax, Soter), acrylic resins (Paraloid B44), and protective varnishes (Incral, Regalrez). Multispectral analysis, digital optical microscopy, FTIR spectroscopy, and spectrocolorimetry allowed for the assessment of the gels' ability to remove the different protective coatings, the degree of cleaning achieved, and the presence of any residues. The results obtained highlight the ability of the formulated gels to effectively remove protective coatings from metallic artifacts. Cetyl Alcohol proved to be the most versatile surfactant to realize a stable and efficient gel. The gels based on Methyl Myristate and Isopropyl Palmitate showed promising results as "green" alternatives to Ligroin, although in some cases, they exhibited less selectivity in the removal of protective coatings.

Keywords: HLB; cleaning polychrome artworks; green chemistry; innovation; physical gel; sustainability.

Figure 1

Pie chart of the most common solvent used in cleaning practices nowadays by restorers.

Figure 2

Macroscopical observations of the tubes prior to turbidity measurements. A stands for “Apolar phase” while P stands for “Polar phase”.

Figure 3

Global Destabilization Kinetics of the formulated gels.

Figure 4

Termogravimetric curves of the different gels.

Figure 5

First derivative curves of the thermogravimetric curve related to all the different gels formulated.

Figure 6

Spectra acquired on the copper frame treated with Regalrez varnish.

Figure 7

Spectra acquired on the copper frame treated with Reswax protective coating. In magenta the Reswax spectrum (a), in orange the liquid Ligroin (b), in blue Gel 1 (c), in purple Gel 3 (d), in green Gel 2 (e), in turquoise Methyl Myristate liquid (f), in light purple Gel 6 (g), in blue Gel 4 (h), in dark green Isopropyl Palmitate liquid (i), in dark yellow Gel 7 (l), in light blue Gel 5 (m), in violet Gel 9 (n) and in red Gel 8 (o).

Figure 8

Spectra acquired on the copper frame treated with Reswax + Paraloid B44 multilayer protective coating. Starting from the top: in turquoise not aged Reswax + Paraloid B44 spectrum (a), blue aged Reswax + Paraloid B44 spectrum (b), in dark green Gel 7 (c), in dark yellow Isopropyl Palmitate liquid (d), in blue Methyl Myristate liquid (e), in purple Gel 4 (f), in light green Gel 5 (g), in cerulean Gel 8 (h), in magenta Gel 9 (i), in dark blue liquid Ligroin (l), in green Gel 3 (m), in orange Gel 1 (n), and in red Gel 2 (o).

Figure 9

Spectra acquired on the copper frame treated with Soter protective coating treated with the gels formulated with Ligroin solvent.

Figure 10

Spectra acquired on the copper frame treated with Soter wax + Incral varnish multilayer protective coating after the gel cleaning test. Starting from the top: in dark yellow the multilayer protective coating spectrum (a), in blue the multilayer coating aged (b), in purple liquid Isopropyl Palmitate (c), in light green Gel 9 (d), in turquoise Gel 8 (e), in magenta Gel 7 (f), in dark blue Gel 5 (g), in dark green Gel 1 (h), in orange Gel 3 (i), in light blue liquid Ligroin (l), in light purple Gel 1 (m), in greenish Gel 6 (n), in turquoise liquid Methyl Myristate (o), and in red Gel 4 (p).

Figure 11

Spectra acquired on the copper frame treated with Zapon protective coating after the gel cleaning test. Starting from the top: in dark blue the not aged Zapon spectrum (a), in green aged Zapon spectrum (b), in purple liquid Ligroin (c), in dark green Gel 3 (d), in dark yellow Gel 1 (e), in light blue Gel 2 (f), in purple liquid Methyl Myristate (g), in light green Gel 5 (h), in turquoise Gel 4 (i), in magenta Gel 8 (l), in dark blue Gel 6 (m), in dark green Gel 7 (n), in orange Gel 9 (o), and in red liquid Isopropyl Palmitate (p).

Figure 12

Oxidated copper frame with the application of products as described in Table 10. Sx: all surfaces; dx: area subjected to aging, in particular the red frames indicates the area that were covered during the aging period.