An Investigation of the Feasibility of the Use of Gels and Emulsions in Cleaning of Gilded Wooden Surfaces. Part B: Cleaning of Soiled Oil-Gilding

Abstract

Experimental testing of various rigid gels and emulsions for removing soiling from gilded wooden surfaces were conducted using two nineteenth c. gilded frames. The gels and emulsions were selected according to their characteristics described in the conservation literature and involved Agar gels, Gellan gels, Xanthan gels, and PVOH-borax gel as well as low viscosity emulsions based on Pemulen TR2, with TAC and EDTA and with varied pH values. Two case studies have shown that hydrogels based on Agar and Gellan gum as well as Pemulen TR2 emulsions can be successfully used in the removal of soiling from gilded wooden surfaces, however each of the cleaning systems needs to be utilised with full understanding of the potential risk to the gilded surface. The option to increase the rigidity of gels can address one of the longstanding challenges of aqueous cleaning methods for gilded surfaces; that is the rate of water diffusion into the gilding’s foundation layers. The rigid gels proved to be a good vehicle to deliver the chelating agents for removal of soiling from oil gilding, offering surface contact without diffusion into underlying layers. These methods can also decrease contact time between cleaning agents and the gilded surface, thus providing better control of treatments as well as reducing the needs for agitation of the gilded surface with cotton swabs. Low viscosity Pemulen TR2 emulsions provide similar opportunity without the need for repetitive clearing. Further testing is essential to obtain a better understanding and confidence in the working abilities of gel and emulsion systems for the cleaning of gilded wood.

Appendix

Description of the hydrogel systems preparation included in the tests:

• A. Agar gel: 4 g (or 2 g) of Agar Agar powder was dissolved in 75 ml of deionised water over high heat (~150C) using magnetic stirrer. All particles of Agar were solved prior to adding 25 ml of TAC 5% w/v solution. Once combined the gel was allowed to cool down to approximately 40C before applying it to the dirty surface of the frame.

• B. Gellan Gum gel (low acyl or high acyl):

  • (a) ~3.2% w/v, with TAC, 0.83% w/v, pH 7.5: 5 g of Gellan Gum Powder was added to 100 ml water adjusted with TEA to pH 7.5, and left to stand overnight. The mixture was then dissolved over heat with a magnetic stirring bead. 50 ml of 2.5% w/v TAC, pH 7.5 was then gradually added.

  • (b) ~3% w/v, with TAC, 1.25% w/v, pH 8.0: 3 g of Gellan Gum Powder was added to 75 ml of hot water adjusted with TEA to pH 8.0, and microwaved for ~2 min. The mixture was then heated on hotplate at ~100C, and stirred with magnetic stirring bead for about 1 h. 25 ml of 5% w/v TAC, pH 8.0 was then slightly heated and gradually added.

  • (c) 1.64% w/v, with TAC, 1.67%, pH 8.0: 2.5 g of Gellan Gum Powder was added to 50 ml of hot water adjusted with TEA to pH 8.0, and dissolved over heat with magnetic stirring bead. 100 ml of 2.5% w/v TAC, pH 8.0 was then slightly heated and gradually added.

  • (d) 2% w/v, with TAC, 2.5% w/v, pH 8.0: 3 g of Gellan Gum Powder was added to 75 ml of hot water adjusted with TEA to pH 8.0, and left to stand for 1 h. The mixture was then dissolved in microwave and 75 ml of 5% w/v TAC, pH 8.0 was then heated and gradually added.

• C. Xanthan Gum gel was made according to the literature recommendations to use 4% for best consistency and working properties ([11], p.123): 7.5% Xanthan gum stock gel was prepared by mixing 7.5 g of powder with 20 ml of deionised water, pH 7.5 (adjusted with TEA), and then top up with deionised water up to the final volume of 100 ml. Then 5 ml of Xanthan stock gel was mixed with 5 ml of TAC, 5% w/v, pH 7.5.

• D. Pemulen TR2 emulsions and EDTA solutions: see: Sawicki M. et al. Chap. 1 in this volume, Part A, Appendix.

• E. PVOH-borax system with ~0.9% TAC (unsuccessful): 2 g of 75PVOH (Kuraray Poval polyvinyl alcohol L-10) was dissolved in 18.75 g of deionised water over moderate heat (<~50 °C) and then mixed gradually with 18.75 g g of 2.5% TAC solution. The mixture was cloudy and foamy. In a separate container 0.5 g of borax was dissolved in 10 g of deionised water. When borax was added to the PVOH-TAC solution separation of the components occurred.