BCD 2019

PROTECTION & REMEDIAL TREATMENT 4.1 141 C AT H E D R A L C O MM U N I C AT I O N S 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 9 MODERN LIME ROZ ARTIS T HROUGHOUT HISTORY the building limes of the day were the modern limes of their time, and the basic building limes we use now (primarily natural hydraulic limes) are the modern limes of our time. It may be that the method of processing limestone into building lime has become more modern and calcination is better controlled, but it still employs the same raw ingredients of limestone and fuel. While coal is still favoured by the larger sites on the continent for the production of natural hydraulic limes, the kilns are more usually gas fired where clean high calcium limes are concerned. Historically, the primary objective was to make lime more efficiently, using less fuel and less labour. Of the 14 kilns at Charlestown, Fife, six date from the 1760s and are of modest size, but these were soon superseded in both size and efficacy by the building of the first of the larger kilns (Kiln 11), which allowed larger volumes to be processed. This led to the construction of three more kilns – each bigger and better than the last. Confidence in lime burning had clearly increased exponentially, and ambitions had increased. Research published by Historic Scotland (in Charlestown Limeworks Research and Conservation , 2006) highlighted the case of William Shaw, an architect of Bo’ness who wrote to John Grant in 1787 “to inform him that one third of his cargo of limeshells had had no lime in it all. Grant blamed this occurrence on the improper manner of burning the stone.” Clearly, the room for error was huge back in the day. As each lime burner was paid by piece work (how much they could produce), the temptation to speed up the process meant that they would throw more coal into the equation resulting in over burnt lime, of no use to anyone. (A possible exception is the icehouse at Broomhall, the seat of the Earl of Elgin, which is decorated with the glassy nodules of over burnt limestone.) In a recent joint project with Historic Environment Scotland (not yet published), an analysis of the Scottish Lime Centre’s archive of over 4,000 samples of mortar showed that, over time, the use of quicklime as the basis for making mortar was gradually superseded by the use of lime hydrate (quicklime slaked to a powder) and the early cements of the late 19th century onwards. With a growing array of means of transport, previously limited to boat, ship and horse and cart, the transport of hydrated lime (twice the weight of quicklime for the equivalent volume) was made much easier and would have been less hazardous than transporting large volumes of quicklime (not the favoured cargo of ship masters). At Charlestown Limeworks, there was a hydrating plant at the kiln heads dating from the early 1800s, where the quicklime was treated with a controlled amount of water over several days to produce a powder, which would then be bagged and sold. Many of the Scottish ports, for example Leith, Perth and Montrose (and other ports around the UK) had lime shades, covered areas where loads of slaked lime (in powder form) could be stored and sold casually through the year. It wasn’t always the case that mortars were hot mixed by slaking quicklime with water and sand for immediate use. In many cases and particularly for the stronger hydraulic limes, like that produced at Charlestown, the quicklime was slaked with a limited amount of water to a dry hydrate and left for a few days (it’s only the free lime that slakes, not the hydraulic components) to ensure the slower slaking particles were fully slaked, otherwise, it risks remaining as an unstable material. This same method was used recently by Scottish 0 10 20 30 40 50 60 70 80 90 Pre-16th 16th 17th 18th 19th Early 20th Later 20th 21st The changing composition of building mortars Quicklime Hydrate Cement % Cement Hydrate Quicklime CENTURY Analysis of the Scottish Lime Centre’s database of mortar samples shows the declining use of quicklime as the basis for making mortar as it was gradually superseded by the use of lime hydrate and the early cements of the late 19th century onwards. One of the modern vertical shaft kilns at the Saint-Astier limeworks in the Dordogne, an area with a long history of hydraulic lime production. Limestone from the same geological source was exploited by the Romans at Perpignan over 2,000 years ago, and this plant dates from the 1850s. Here the company is still “cooking the lime” (as they would say) using coal from South Wales, but in a more controlled environment which maximises fuel efficiency. (Photo: CESA)