An Investigation of the Feasibility of the Use of Gels and Emulsions in Cleaning of Gilded Wooden Surfaces. Part A: Removal of Brass-Based Overpainting

Abstract

The term ‘cleaning of gilded wooden surfaces’ should be understood broadly to account for their complexity. Each layer of a gilded surface requires assessment, and often, a separate cleaning methodology. Removal of overpainting is a part of the surface cleaning process of gilded wood. Many cleaning methods for gilded surfaces were adopted from other conservation fields, as there is little research in reference to cleaning of gilded wood. Two case studies involving nineteenth century gilded frames, focused on finding a suitable gel and application method to remove brass-based overpainting and surface dirt. This paper presents the findings from our investigation in regards to brass-based overpainting removal (Part A). The results of experiments into removal of soiling from gilded wooden surfaces are covered in a second paper (Part B). Two rigid gels reviewed in conservation literature were tested for removing brass-based overpainting from oil-gilded surfaces: Xanthan gum gel and poly(vinyl alcohol)-borax gel (PVOH-borax gel). Low-viscosity emulsions based on Pemulen TR-2 were also tested as they involve more straightforward clearing process than with viscous gels. Gels were evaluated according to application and removal methods, viscosity, texture, and cleaning efficiency. Rigid solvent gels and low-viscosity emulsions offer a viable alternative to free solvent application provided they can be easily removed from the surface, with minimal clearing requirements. Factors influencing the performance of gel systems include the thickness of the overpainting layers, the presence of a ‘barrier layer’ between the overpainting and gilding, the solubility of the overpainting binder, and the solubility characteristics of the solvent (its rate and power of swelling, evaporation rate and application method). For the removal of overpainting from gilded wooden surfaces, fast-acting solvents with moderate swelling power are more appropriate than slow-acting solvents with strong swelling power. The latter can soften gilding layers beneath the overpainting. Important factor to consider are solvent volatility and the rate it diffuses into the overpainting.

Appendix

Description of the solvent systems preparation included in the tests:

• A. 2% Pemulen TR2-TEA, pH 6.5 (stock solution) was formed according to the MCP recipe [25] as follows:

  5 g Pemulen TR2 was mixed with 90 ml of deionised water; around 4.5 ml of TEA was added to adjust pH to 6.5 (little bit more for pH 8); then another 150 ml was incorporated to achieve a stock gel of ~2%.

  1% Pemulen TR2-TEA, pH 6.5, was formed according to the Chris Stavroudis’ instructions on YouTube:

  5 ml of the 2% Pemulen TR2 stock solution was mixed with 2 ml citric system pH 6.5 (see below) and 3 ml deionised water on the magnetic plate with stirrer. The final mixture was 10 ml of 1% Pemulen TR2-TEA-citric system, pH 6.5.

• B. Citric system: Citric acid with Triethanolamine, pH 6,5

  4,8 g citric acid was mixed with 70 ml of deionized water. Triethanolamine was added to adjust pH to 6,5 (approx. 4.5 ml) then an extra 20 ml of deionised water was added to make entire solution approx. 100 ml.

Emulsions:

• C. 1% Pemulen TR2-TEA, pH 6.5, + 10% (20%, 30%) benzyl alcohol was formed according to Chris Stavroudis instructions on the GCI YouTube channel:

  5 ml of the 2% Pemulen TR2 stock solution was mixed with 2 ml citric system pH 6.5 (see above) and 3 ml deionised water on a magnetic plate with a stirrer. After approx. 5 min 1 ml (2 ml, 3 ml) benzyl alcohol was added and stirred again to form ~10% (~20%, ~30%) emulsion.

• D. 1% Pemulen TR2-TEA pH 6.5 + ~10%, (20%, 30%) benzyl alcohol + ethylenediaminetetraacetic acid (EDTA) pH 6,5 was formulated with influence of Chris Stavroudis MCP (as above):

  5 ml of the 2% Pemulen TR2 stock solution was mixed with 2 ml of EDTA (5%) (replacement of citric system) and 3 ml deionised water on the magnetic plate with stirrer. 5 min later 1 ml (2 ml, 3 ml) of benzyl alcohol was added to form ~10% (~20%, ~30%) benzyl alcohol emulsion.

• E. PVOH-Borax gels with acetone were prepared as follow ([1], p. 243).

  90:10: For ~10 g batch, 0.6 g 75PVOH was solved in 6.37 g deionised water and 0.94 g solvent (acetone) at a moderate temperature ~50 °C for approx. 1 h. In a separate container 0.1 g borax (Bare Essentials Borax) was solved in 2 g of deionised water. Borax solution was added to PVOH mixture using magnetic stirrer until combined.

  50:50: For ~20 g batch: 1 g 75PVOH was solved in 5.4 g deionised water and 9.4 g solvent (acetone) at ~50 °C for approx. 1 h, before adding the borax-water solution (0.2 g borax solved in 4 g deionised water).

• F. 8% Xanthan gum stock gel was prepared by mixing 8 g of powder with 20 ml of deionised water, pH 7, and then top up with deionised water up to the final volume of 100 ml.

  4% w/v Xanthan gum gel was used for all experiments.

  For 25% solvent gel 25 ml of solvent was mixed with 25 ml of distilled water, pH 7, added to 50 ml of stock gel and stirred.

  For 30% solvent gel 30 ml of solvent was mixed with 20 ml of distilled water, pH 7, added to 50 ml of stock gel and stirred.

  For 50% solvent gel 50 ml of solvent was mixed 50 ml of stock gel and stirred.

  Xanthan gum, with 20% benzyl alcohol 5% EDTA, pH 7, was prepared basing on an article by Doherty and Rivers 2017, p. 123.

  10% EDTA stock solution was prepared by mixing 10 g EDTA in 100 ml distilled water, buffered with TEA to pH 7.

  To make gel 50 ml EDTA (10%) was mixed with 30 ml of distilled water, pH 7, and 20 ml benzyl alcohol, and then 4 g of Xanthan powder and stirred with magnetic stirrer.

• G. Acetone poultices were prepared as follows:

  Large cotton bandage was folded to form patches; the size was determined by the area designated for treatment. The pocket was packed with cotton wool and saturated with solvent. During treatment, the poultice is covered with polyethylene sheet, to reduce evaporation. The poultice can be removed from the surface anytime and the treatment terminated immediately.