BCD 2017

136 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 LIMEWASHES and SHELTERCOATS NIGEL COPSEY T HE REVIVAL in the use of hot-mixed lime mortars has been gathering pace across the UK and Ireland over the past three years, and the use of quicklimes in conservation and repair is now becoming increasingly routine. This has injected new energy and insight into the wider lime revival, affecting not just construction mortars but also decorative finishes and surface treatments. As craftspeople began to use hot-mixed lime mortars again they initially struggled with procedures which had not been used for 40 years or so, and historic accounts and the experiences of others were not as readily available as they are now. Rules for producing and using hot-mixed lime mortars had to be re-discovered, initially by trial and error. Hot-mixed mortars are forgiving of inexperience, however, and historic texts now provide a mine of expert opinion and direction. It should perhaps be no surprise that using the right materials, manipulated as they were in the past, allows compatible like-for-like repairs to be carried out with comparative ease and efficiency. While mortars and limewashes have a long history, with documentary sources dating back to Pliny, shelter coats are a product of the lime revival, and of Baker’s lime method in particular. They can be defined as ‘aggregated limewashes’ for the treatment of friable or previously soiled stonework of particular importance and significance, offering a sacrificial layer that fills pores and seeks to match the tone and character of the stone when newly carved. In addition to their decorative function, limewashes also play an important role in protecting the fabric. Recent research has demonstrated that the application of limewash to any masonry substrate, from the least to the most porous, enhanced the drying of the fabric at the surface and to surprising depth and with greater efficiency than when there was no limewash at all. This is due to the pore structure of calcium carbonate which is ‘perfect’ for the poulticing of moisture. It is the high free-lime content of limewashes and mortars that provides the key to effective and efficient breathability (Wiggins 2016). THE VALUE OF HOT SLAKING Many problems experienced in recent years stem from the slaking and mixing of lime. Since the lime revival began, limewashes and sheltercoats have been made by diluting putty limes, themselves made by slaking quicklime in an excess of water. Historically termed ‘drowning’, this method was considered by all commentators to deliver a weakened lime with deficient bond strength and less cohesion and ‘tenacity’ – the key standard for a good mortar, along with good workability. The importance of matured lime putty was asserted by Vitruvius, Pliny and others, but always in the context of high status stucco finishes and fine colour washes. These putties were slaked alone but, as when lime was slaked with sand for mortars, the water was always added to the lump lime and in just sufficient quantities to effect the slake plus a little more water to deliver a stiff, dough-like paste. This was laid down to allow all slaking to occur. Lime putty, used neat as a mortar, was the norm for gauged brickwork and perhaps for bedding the finest ashlar. It was rarely used as a binder (Langley 1750). More generally and in more recent centuries, running lime to putty was an efficient way of removing unslaked lumps which would otherwise impede the achievement of very fine surface finishes. This is made explicit by many sources: Fine-stuff is pure lime, slaked with a small portion of water, and afterwards well saturated, and put into tubs in a semi-fluid state, where it is allowed to settle, and the water to evaporate. A small proportion of hair is sometimes added to the ne-stuff. Stucco, for inside walls, called trowelled or bastard stucco, is composed of the ne-stuff above described, and very ne washed sand, in the proportion of one of the latter to three of the former. All walls, intended to be painted, are nished with this stucco. (Nicholson) Running quicklime to putty became more common for plastering and building during the 20th century because it was a quick and easy way of slaking large volumes of lime. However, it was always used in the knowledge that the resulting mortars would be gauged with gypsum or ordinary Portland cement, which were assumed to counter the inherent weakness of mortars made with this material while speeding the initial set, although not the carbonation of the lime. It was a response to the demand for mass housing and increasing time pressures in the construction industry. The idea of laying lime putty (albeit Traditional thick limewash on Tetbury Town Hall