LITERATURE REVIEW: THE USE OF PARALOID B-72 AS A SURFACE CONSOLIDANT FOR STAINED GLASS
SASHA CHAPMAN, & DAVID MASON
It is widely accepted that minimum intervention is desirable in the treatment of any object, and where uncertainties exist about the long-term suitability of treatments, this maxim is especially important. The duty of the conservator is to consolidate loose and friable surface layers only where there is a risk of loss of valuable historic material and to employ any consolidating technique with restraint and sensitivity. Increasingly, the value of preventive conservation is becoming evident, and protective glazing systems will continue to play a primary role in the preservation of historic glass paint.
However, consolidation of glass paint remains a necessary and important undertaking, and the choice of consolidant system is one of the most important a conservator can make. The chemical composition of the glass, glass paint, and their corrosion products, the type and degree of alteration of the surface layers, the presence of earlier coatings or treatments, the nature of any associated interventions (re-assembly, crack repair, cleaning, surface treatment), the environmental conditions to which the treated panel will be subjected—all play a part in determining which, if any, are the most suitable consolidants.
To date, Paraloid B-72 has shown itself to be versatile and effective as well as stable under moderate conditions. There are certain practical limitations to its use as a consolidant, notably the nature of the adhesive bond with inorganic materials and the ability of the resin and its carrier to both penetrate and consolidate effectively the fine pores and fissures of a silicate matrix. A review of the literature illustrates some of the ways in which Paraloid B-72 can be optimized to reduce some of the inherent shortcomings. Paraloid B-72 possesses two singular advantages over its chief rivals: first, it has been around for many decades and has shown itself to be reliable if used in an appropriate context. Second, it is readily reversible and miscible with a wide range of solvents and can be easily combined with other materials to enhance its properties.
Reversibility, which may be crucial in an adhesive, can be an overestimated virtue in a consolidant, where the removal of the consolidant would entail significant, if not unacceptable, risks to the object and where it is re-treatability that is arguably the more viable principle. Inorganic gel treatments, polyurethane systems, or other forms of consolidation treatment may yet show that, in spite of their irreversibility, they can offer significant advantages over Paraloid B-72, but they have yet to demonstrate their value, practicality, and re-treatability over time.
The literature indicates that systematic reviews of the long-term performance of Paraloid-treated glass surfaces in the field are rare. While laboratory aging studies continue, these must be corroborated by surveys of treated panels in situ. More data are also needed from pilot studies on panels treated with alternative materials and on modified Paraloid-based systems. Techniques and methodologies for this research should be developed to enable a more comprehensive picture of the comparative performance of various treatments to be built up.