building repair field, but many experienced people questioned the need and the wisdom of interfering with the historic integrity of such old buildings, especially since the practice involved wholesale raking out and repainting of any surviving mortar judged to be weak and failing. Sadly, such weak mortars may have been doing excellent conservation work for hundreds of years without one realising it. In conservation it is more usual to be considering the protective, nutritional and sacrificial function of mortars and renders rather than the strength, but it is worth noting that, whereas in the early stages ordinary lime mortars are much weaker in bond than Portland cement mortars, they can become stronger than the stone or brick if all the ingredient factors are right. We should consider the fact that trials on ordinary lime mortars indicate a variety of relative tensile strength between first making and eventual strength from X 3 to X 50. The traditional recommendation for application of lime mortars and renders is well described elsewhere and the purpose here is to consider only the water content ifl winter. Once the mortar or render has been frozen and expanded it will not :return to its earlier form. In compression the pressure of the load above will force it back but laterally, nothing but mechanical aid will force it back. The mortar or render must have thawed and further freezing should not be expected in that season. The work should be thoroughly wetted with lime water, or at leastwaterofneutral pH, and both mortar and render can be forced back by hard scouring or tamping. Further wetting and scouring can be applied. Any mortar lost can be knocked up with a very little new lime mortar and can be reapplied after pre-wetting the surviving new work Experience will teach you to work flush with wooden tools so that anything lost will leave anicely recessed porous durable face, naturally. Frequently failures in frozen mortars are due to overworking the surface with metal or plastics tools which draw the fines to the surface. This must be avoided since the lime binding will be in imbalance and unbound sand may be in the back. The best lime mortars seem to be those which are the driest in a wellgradedfineto coarse sand mixture. Drying agents have been added over the ages, including bone meal, blood, wort and brick dust. The fine parts of these intimately impart their chemical constituents and fine brick dust and certain sands provide cementing agents which contribute to irreversible binding. It must be said, however, that in conservation work the amount of strength called for is just that amount sufficient to hold things together. It must be remembered that the eventual strength may be far greater than the initial strength and it is possible for mortars to become stronger than the stones or brick they serve. The leading Swedish conservation architect Ingmar Holmstrom, an acknowledged experienced authority on using lime in a freezing climate, has asked the thought provoking question "How weak dare we make it?" Considerable study and trials are in hand in theNorthernhemisphere towards mastering the use of lime in winter. The loss of some lime mortar or render is replaceable. Lost stones or bricks are not. The alternative is the addition of Portland cement, but that is not an alternative in the interest of the life of the surviving early fabric of the building. Peter Hood is researching the mortars, plasters and for building the classical house in Scotland before 175 0 towards a PhD thesis in the University of Edinburgh. He is convenor of the Technical Panel in Scotland of the Society for the Protection of Ancient Buildings (SPAB). Ht is also a conservation and design consultant who has been a leading pioneer in the lime revival as a manufacturer. He is retained as a consultant by The Traditional Lime Company (Tradlym). THE TRADITIONAL LIME Co. • matured lime putties • mortars • renders • plasters • lime washes • pigments TRADLYM© TODAY'S BOND WITH THE PAST Church Farm, Leckhampton, Cheltenham, Glos GL51 5XX Telephone: (01242) 525444 Fax: (01242) 237727 CONTEXT49 Decorative CAST IRON Call the experts We have designed and made ornamental cast iron since the nineteenth century. Our expertise has proved invaluable for countless restoration projects. F.J.Thornton & Co Ltd, Jenner Street, Wolverhampton, WV2 2AE. Tel: 01902 352390 21
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