The lime technique is a generic term or the use of slaked lime (calcium hydroxide Ca(OH)2) for the consolidation, repair and protection of stone, particularly limestone. The method was refined by Professor Robert Baker in the 1970s and ’80s on several important projects, not least the conservation of the west front of Wells Cathedral.
Limewater consolidation is the application of at least thirty coats of water, rich in dissolved calcium hydroxide. This reacts with atmospheric carbon dioxide to produce calcium carbonate (CaCO3) the basic constituent of limestone.
Lime mortar repairs are carried out using an aggregate-rich lime mortar, with the aggregates carefully chosen for colour, size and appearance so that the mortar resembles the stone which it is used to repair. The mortar has to be applied carefully too, to obtain correct texture, and tended after application to ensure that it carbonates, sets, properly.
The purpose of a shelter coat is to provide protection to surfaces that suffer unduly from the effects of erosion from wind, rain and pollutants or to protect and consolidate freshly-cleaned historic masonry. Generally they are used as a last resort in the knowledge that they offer the only solution, even a short-term one, to particularly problematic surfaces on historically significant buildings. In some circumstances, friable surfaces can be consolidated by several applications of a shelter coat. Shelter coats should be sacrificial in nature, always being reversible. They usually consist of a feeble or non-hydraulic lime, with or without aggregates. In many respects they are similar to lime washes, and it could be argued that repeated lime washing of specific areas could be considered one way of applying and utilising shelter coats. Where a feebly hydraulic lime is being used and particularly where an aggregate is considered necessary, they can be applied by trowel, spray or brush, depending on the circumstances, but are usually applied as a series of thin coats of lime rich sand mixes, pigmented or not, often requiring several applications over a period of time.