BCD 2017

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 7 137 PROTECTION & REMEDIAL TREATMENT 4.1 differently made) down for a minimum of three months is indicated by Alberti (1460), but he is unique in this prescription, which appears in his chapter on ornament. Millar (1897) indicates that plasters should be hot mixed and the resulting coarse stuff laid down to allow for late slaking for up to three months, although two weeks seems to have been the norm for both coarse stuff and, for fine finishes, lime putty. During the 19th century, lime was also sometimes run to putty for convenience, but with the expectation of gauging with natural cement. The following mid-19th- century account, for example, describes the preparation of mortars at Fort Warren (Boston Harbour) mixed by mortar mill and lime slaked by being thrown into a surplus of water: The lime, thus deluged with water, loses probably some portion of its binding qualities, but the mortar at Fort Warren almost always contains hydraulic cement; and as this substance sets rapidly, it is highly essential that the lime should be thoroughly slaked before the admixture of the ingredients... the lime is reduced to [a] milky consistence, and allowed to remain in the vat as long as possible. It should be remembered, that the above method applies only when cement is added to the lime. When no cement is used, the lime must be slaked in the ordinary way, as the drenching of the lime would greatly impair its binding properties. (Wright 1845) Limewashes too were made by adding just enough water to lump quicklime for it to slake. More water was then added and stirred in to achieve the desired consistency, typically thick enough that a dipped brush does not drip. The limewash was used immediately, first run through a sieve or otherwise, depending upon the situation, while still hot, for maximum effect. Slaking itself takes a matter of minutes but significant heat remains for longer – sometimes for days if the mortar volume is large. Great store was placed on maximising the temperature of the slake in this and other mortar-making methods, whereas drowning the lime minimises the potential temperature. Quicklime slaked with too little water may reach 300°C before the addition of more water, which will bring the temperatures down below 100°C, typically to around 70°C in limewashes and to around 55°C in mortars. With precise water addition, the slaking temperature will be around 100°C. Too little risks ‘chilling’ the lime, reducing the effective binder content. In modern, scientifically controlled hydrated air lime production, it is demanded that quicklime slakes at temperatures of 85–98°C to achieve the optimum surface area and porosity of the dry hydrate. Historically, significantly higher temperatures were generated. Van Der Kloes (1914) is unique in spelling out the perceived importance of maximising the temperature of the slake (others presumably took it as read). These temperatures sound alarming, but are easily managed and are, in truth, no more than those encountered in the average domestic kitchen. A minimum of 100°C is required; significantly greater temperatures than this will indicate insufficient water to effect the slake. In our own practice, we have made limewashes by this method, as well as by incrementally feeding powdered quicklime into small volumes of water to produce a paste the consistency of ‘extra thick’ double cream, which, upon cooling overnight would appear as a putty lime. Stiffening only occurs upon cooling – used hot, the liquid flows. (Historically, grouts were always used hot to take advantage of this.) As a result, a hot limewash may be applied in thicker coats without crazing, effectively becoming putty on the wall. In historic specifications, two or three coats of lime white (as limewash was called during the 19th century) is indicated. No ‘seven water-thin coats’ specified here – nor, indeed, are they likely to be found on historic surfaces, where limewashes can easily be confused for fine finish coats. In our experience (gained through using quicklimes over the past ten years, both for hot- mixed mortars for all uses and for limewashes), limewashes and sheltercoats are best mixed directly from quicklime, made thicker than modern convention, and applied while still hot. This results in a strong and immediate bond to masonry and other substrates, and even 24 hours later they would be very difficult to remove. The intimacy of the bond between lime, water and aggregates as a result of hot mixing is particularly well-demonstrated in the case of sheltercoats. Even coarse aggregates will be held in suspension in the liquid. If left for some days, these aggregates will sink to the bottom, but stirred they will once more remain in suspension for as long as required. Compare this to modern sheltercoats; mixed cold from matured lime putty, the aggregates compact at the bottom of the bucket, requiring a hammer and chisel to remove them. Application is often a race against time. It has been a maxim of the lime revival (Schofield) to use hot water in the dilution of putty lime for limewashes, the better to engage any pigments. However, when mixing directly from quicklime, the heat comes free. Furthermore, the addition of the aggregates and pigments before or (when using powdered quicklime) during slaking locks all of the ingredients together more effectively than Recent conservation to the figures at Crowland Abbey church, Lincolnshire by Alex Carrington using hot-mixed mortars and shelter coats Hot applied sheltercoat to protect eroding sculpture at St Mary’s Church, Old Malton