Are limes created equally?
This is the crucial point. How many times have you heard you should use 'lime'?
It isn't a one product fixes all problems affair.
The lime revival saw a huge push for the use of Natural Hydraulic Lime (NHL), not the least of which came from the manufacturers. Mortar specifications have been extremely generic ever since:
NHL 2: Internal work only
NHL 3.5: Exterior repointing and rendering
NHL 5: Exterior use in exposed positions
Case study research carried out by the likes of Nigel Copsey - a leading figure in the field of conservation mortars, has identified that NHL 3.5 in particular has shown a failure to perform in the manner expected from a lime mortar. Its strength continues to build year on year and its ability to deal with moisture is limited.
The case is true of all NHLs.
This evidence is slowly changing conservation best practice standards within Historic England, Historic Scotland and SPAB guidance. At Russet and Sage we expect their use to be extremely limited in the coming years.
Natural hydraulic lime in occasion has its place but its use should be limited to an NHL 2 with a high free lime content and only in cases where its use can be reasonably justified.
The importance for homeowners to engage with craftspeople that understand lime in all its forms and its appropriate application has never been more important because architects, building surveyors and conservation officers are still writing/ accepting lime repointing NHL 3.5 specifications for listed and other historic buildings.
The conservation of historic buildings is a specialist field and it's one that is still learning. Most of the knowledge was handed down from person to person and a lot of it was lost in the great wars. For this reason a substantial amount of ongoing research is being carried out using historic books and documents.
At Russet and Sage we strive to be at the fore-front of current best practice which means constantly evolving our practices to offer our built heritage the best possible chance of survival.
We have been successfully using hot-mixed quicklime mortars for years and are working towards educating home owners in its benefits over hydraulic limes and lime putty.
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