LITERATURE REVIEW: THE USE OF PARALOID B-72 AS A SURFACE CONSOLIDANT FOR STAINED GLASS
SASHA CHAPMAN, & DAVID MASON
3 NEW MATERIALS
There has been much focus in recent years on the potential application in the stained glass field of two newly developed materials: ORMOCER (heteropoly-siloxane compounds) and silicon-zirconium alkoxides (SZA). ORMOCER, developed originally as protective coatings, have been found to have some potential application in hybrid treatments, though they adhere poorly to glass (Mueller-Weinitschke 1996) and are therefore not suitable as consolidants.
SZA, however, appeared to offer some promise during early tests (Romich et al. 1995). Though not reversible, SZA treatment, which relies on sol-gel chemistry rather than mechanical adhesion to bind together friable particles, is chemically more suited to the consolidation of silicate materials than the “film-forming” synthetics. After application, the silicon and zirconium alkoxides react with moisture in the air to form an inorganic gel. SZA appears to have no impact on the optical properties of treated glasses, and it has good penetration properties. Practical experience of SZA, however, has been discouraging (Mueller-Weinitschke 1996). It has been found to be very sensitive to humidity, not hydrolyzing fully unless relative humidities exceed 50%, and difficult to control owing to its very low viscosity. It is less effective as an adhesive, so loose flakes or detached parti-cles—i.e., those areas of a corroded glass surface that are most at risk—cannot be effectively re-adhered. In short, SZA appears to offer some potential but lacks the ease of handling and versatility of B-72. Further comparative studies are being undertaken, and observations on the field trials, begun in 1990, are awaited.
3.1 OPTIMIZATION OF PARALOID-BASED TREATMENTS
Examples in the literature report some of the recent attempts to refine one or more of the properties of Paraloid-based consolidants. They typically fall into three categories:
- silane coupling agent pretreatment to improve contact between the substrate surface and the consolidant or adhesive
- Paraloid-ORMOCER combination systems (in which improvements to the mechanical properties of the resin or in the protective properties of resin films are enhanced by using it in combination with an organic-inorganic hybrid polymer)
- solvent chemistry (in which the mechanical and functional properties of the resin are altered by using solvent-diluent mixtures that are responsive to the particular subject material).
3.1.1 Silane Coupling Agents
The adhesive bond of polymers to glasses can be improved by the use of coupling agents. The performance of silane coupling agents in several studies in the 1980s indicated the conservation benefits that may be derived from their use in combination with Paraloid resins for the consolidation of loose paint. Errett et al. (1984) described the process of silane pretreatment and, to examine the effectiveness of the silane in promoting wetting and spreading, treated a number of slides with a methacrylate functional silane in methanol, then applied a solution of B-72 on the following day. On average, the consolidant flowed twice as far in the silane-treated interface as in the untreated interface.
Reviewing the approach to the treatment of stone using similar methods, Jones (1988) adapted the technique for the treatment of remaining unstable paint on 14th-century stained glass panels at the Victoria and Albert Museum. A number of tests were carried out using a pretreatment of 5% methyltrimethoxysilane, 5% water, and 90% methanol, followed by 10% B-72 in acetone (B-72 was ultimately rejected in favor of B-44, a harder resin, which was felt to provide a firmer cushion for the paint layer).
Like silanes, ORMOCER, which as heteropolysiloxane compounds are well-suited to bonding chemically with silicate surfaces, offer significant potential as additives to Paraloid resin consolidants. Trümpler and coworkers (1996) are among those who have found a 50: 50 mixture of ORMOCER Or-G and 10% Paraloid B-72 in ethyl acetate to possess good characteristics in terms of both handling and penetration power. The ORMOCER-Paraloid combination is also being tested for the treatment of flaking enamel (Müller et al. 2000). Pilz (1999), meanwhile, has reported on the positive outcome of laboratory tests on simulated medieval glasses in which the combination of SZA as consolidant and 50: 50 ORMOCER-Paraloid B-72 as a protective film, applied after the glass samples have been subjected to hypercritical drying, have been deployed on glass darkened as a result of manganese oxidation. The transfer of the technique to the studio, however, remains problematic.
3.1.3 Solvent Chemistry
The third approach, which seeks to improve the wetting properties and penetration of the solution and also to control more finely the evaporation rate of the solvent, has yielded interesting results on some types of glass (De Henau 1996). Fontaine has described the treatment of corroded archaeological glass using 10% Paraloid B-72 in a 3: 2 solvent: diluent mixture of methanol and ether (Fontaine-Hodiamont 1993).