Context 154 - May 2018

20 C O N T E X T 1 5 4 : M A Y 2 0 1 8 DAVID ODGERS Progress with stone consolidants Developments in stone consolidants show that they have a role as a treatment of last resort, to be used only after other options for slowing the rate of deterioration have been considered. In an article in The Spectator in 1860, the writer was pondering the options for treatment of the recently constructed but already decaying Anston stone of the Houses of Parliament. ‘There is however a dispute between rival preservers – a contingency at which we need not wonder considering what innumerable processes have been lauded as possessing the chemical properties for perfecting stone which has rotted by the decomposing action of the elements.There seems to be very little room for confidence in any of these processes’. Those words remain relevant, as the consolidation of external weathered stone mostly continues to be a frustrating exercise of hope over experience. For many centuries stone has been treated with a variety of materials, including limewash, oil, waterglass, copperas and wax. Some were applied to provide an aesthetic effect and some to provide protection. They should all be considered as surface treatments and not confused with consolidants,which are intended to restore cohesion and give mechanical strength to deteriorated stone without adversely affecting the behavior of the stone, depositing by-products that may harm it or affect its appearance. To achieve this, they need to penetrate through the full depth of the decayed zone but not fill the pores of the stone; and as though all of this were not difficult enough to achieve, the consolidant should not prejudice subsequent treatment or affect future maintenance of the stone. The search for a suitable consolidant has been going on for centuries.The synthesis of silicon-based compounds in the first half of the 19th century was largely unheralded but in 1861 Hoffman suggested a form of ethyl silicate (very similar to modern day silanes) for the consolidation of decaying stone. Since then many products have been proposed and used, including ‘fluate’ (fluorosilicate), ‘Baryta water’ (barium hydroxide), ‘siassic liquid’ (sodium silicate/arsenic acid), ‘silicon ester’ (a form of ethyl silicate), ‘Zerelemy liquid’ (sodium silicate) and even ‘Bakelite’ (phenol formaldehyde). The government-appointed Stone Preservation Committee reported in 1927 that it was unable to recommend any stone preservative treatment. The Ministry of Public Building and Works carried out experiments in 1964/65 using a variety of silicon-based products and limewater.The application of these was well recorded and the effect of the consolidant was assessed over several years, and reported by Clarke and Ashurst in 1972. The results were largely negative. All of the work with silica-based consolidants acknowledged the fundamental fact that they are more compatible with siliceous stones (sandstones) and chemically incompat- ible with calcareous stones (limestones). During the 1970s there were a number of developments in stone consolidation: first, the refinement of some of the silane-based materials (Wacker OH, for example) to improve performance and make them more compatible with limestone; second the extensive and generally successful treatments pioneered by Hempel and Larson at the V&A (and used for a couple of the external figure sculptures at Wells Cathedral); and, third, the The south court ofWoburn Abbey, showing areas treated in 1985 Areas 1–4 treated with biocide and brushed down Areas 5–10 dressed back before treatment E35 1 E34 E33 2 3 4 5 6 7 8 9 10 1 Brethane 2 Control untreated 3 Lime poultice and limewater (including grouting and mortar repair) 4 Lime poultice, limewater and shelter coat (including grouting and mortar repair) 5 Lime poultice, limewater and shelter coat 6 Lime poultice and limewater 7 Boiled linseed oil 8 Microcrystalline wax 9 Control untreated 10 Brethane