30 C O N T E X T 1 5 4 : M A Y 2 0 1 8 ALISON HENRY Hot-mixed mortars: the new lime revival Appreciation of the limitations of lime putty mortars and concerns about the strength of natural hydraulic limes have triggered renewed interest in traditional mortars, and in hot mixing. The last five years or so have seen a revival in use of hot- mixed mortar.While many people have welcomed this as a way of making more authentic mortars for conservation, others are sceptical or even hostile to what they see as a new fad. Before exploring the pros and cons of hot-mixed mortars, it is worth reflecting on the past 40 years of lime use and how we have arrived at the current situation. The lime revival As the damage caused by hard, impervious cement mortars became apparent from the mid- 20th century, the revival in use of lime from the 1970s was naturally welcomed by conservation practitioners. Emerging practice borrowed from materials and methods used in various stonework conservation programmes, particularly the restoration of the west front at Wells Cathedral from 1974. Here non-hydraulic lime putty, typically blended in a ratio of 1:3 with aggregates, was used to make sacrificial mortars for conservation of fragile limestone sculpture; pozzolans were added where additional strength was needed. Mortar design was based on practical experi- ment, and ignored historic source materials.While such mortars were eminently suitable for this specialist application, and proved durable for re-pointing and rendering in sheltered locations, there were some failures when they were used in more exposed locations. During the 1990s attention began to shift towards hydraulic lime, which it was hoped would prove more durable. Hydraulic lime began to be imported from the continent and there was a brief renaissance of production in the UK. Mixes generally comprised 1 part powdered lime to 2 or 2½ parts aggregate. These mortars were indeed faster-setting and more resistant to salt and frost damage than ones made with lime putty. However, by the early 21st century alarm bells were ringing in some quarters. Many practitioners were concerned that the new breeds of hydraulic limes (termed natural hydraulic limes [NHLs] under the When non-hydraulic lime-putty mortars are used in exposed locations or, as here, for demanding applications such as on wall tops, failure due to frost damage is not uncommon. Quicklime made in traditional kilns often contained under- or over- burned limestone which did not slake when the mortar was made, and remained in the mix as rounded whitish particles, often referred to as ‘lime lumps’. Fragments of black fuel ash from the lime kiln sometimes found their way into the mortar too. These are clearly visible in many hot-mixed mortars. However, for very high- quality work, quicklime was ‘BHP’ – ‘best hand-picked’ – and did not contain lime lumps or ash.