BCD 2017

138 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 Hot lime washing in a threshing barn at Thornton Dale (Photo: Sam Baxter) is the process of hot mixing that delivers an enhanced general performance with a strong bond between lime, aggregates and water– the method of slaking is key. Similarly, limewashes slaked in the traditional way were not always used hot, but our experience has been that hot use offers genuine advantages. Whether used hot or cold, the evidence would seem to be that they were generally used freshly made and applied much thicker than recently. Historically, two coats were usually specified. ADDITIVES There are several traditional formulae consisting of lime (not whiting) thoroughly slaked and thinned to a cream after slaking to which various additions are made, such as salt, alum, powdered glue, oils and fats and casein (skimmed milk), and a selection of formulae is shown in the table opposite. The effect of salt is probably to hold the moisture and facilitate the carbonation of the lime, while the addition of a small quantity of alum improves the working qualities and is thought to increase the hardness of the surface. Caseins and glues give greater binding properties to the mix. Although done with cold putty limes, NPA research (Jackson 2005) demonstrated that the most efficient and durable limewashes were those composed of lime and water (with pigments as desired), and not those containing tallow or molasses or linseed oil. It was concluded that most additions were to help with the adherence of the material to the substrates, not durability. While the addition of salt to lime washes may seem counter-intuitive, there is clear evidence that it improves the bond with the substrate. In 2014, even a hot limewash struggled to uniformly attach to the repaired and previously abused timbers of the Jesuit Mission Church in Fort McMurray, Alberta, Canada, but the addition of common house salt (which was often specified in limewashes coloured with copperas/ferrous sulphate without salt addition) effected good attachment to the timbers. The addition of house salt to limewashes for masonry structures remains common in many countries but is usually unnecessary, and in the UK’s damp climate the risk of salt migration into the fabric may outweigh any benefits. Common limewash pigments include copperas (ferrous sulphate) and various natural earth pigments (iron oxides) such as ochre. Copperas was a by-product of the alum industry of North Yorkshire, and in London and Yorkshire it was not uncommon for an alum- copperas wash to be applied to masonry as a thin coating in the 19th century. All of the above indicates that craftspeople in the past – when exterior limewashing with or without additives was routine – understood far more than we do today. To give them and the buildings they created due respect and recognition, we should use the materials they used, manipulated and applied in the same way. Not only will this offer compatibility and something more truly ‘like-for-like’, but it will make the lives of masons, plasterers and conservators easier and less frequently frustrating. York House, Malton: a hot-mixed and applied sheltercoat with copperas pigment Earlier copperas pigmented wash to York House Early copperas pigments on the stonework of Crowland Abbey church cold mixing. Hot water was used to slake some hydraulic quicklimes in the past, accelerating an otherwise slow process, especially when hydraulic quicklime had been previously pulverised. Likewise, added fats or oils will be melted and properly engaged with the other ingredients in a way that simply does not occur when the limewashes are mixed cold, something that has led to the spectacular failure of such treatments in recent years. Hot limewashes and sheltercoats dry out less readily than cold-mixed varieties, carbonate efficiently and give very good coverage. They are cost-effective and have proved appropriately durable. The north elevation of York House in Malton was given a Hamstone dust, fine sand, copperas and quicklime sheltercoat in 2007. This continues to require no replenishment. It should be said, however, that many of the beneficial properties of hot limewashes and sheltercoats endure when they have cooled – there are advantages in hot use, but good bond and good coverage remain available after cooling. As with hot-mixed mortars, it