Context 154 - May 2018

C O N T E X T 1 5 4 : M A Y 2 0 1 8 23 LIME AND STONE MICHAEL BEARE and JAMES MILLER Lime mortars: a conservation engineer’s view Choose lime; favour site mixing but do not reject pre-mixed solutions; do not focus on strength; take care in choosing the sand; if appropriate, analyse the original mix; and other good advice. It is often said by engineers engaged in conservation that masonry mortar is only there to even out the surfaces of bricks and stone: to stop any hard points from spalling when they press together and make walls easier to build. This principle certainly applies in mass walls where the whole structure is in compression, which accounts for the majority of historic buildings and structures. As we approach the modern age, walls become thinner and mortar is required to act in tension and shear, as well as compression, but this article considers the older forms of construction. For these, the physical strength of the mortar has very little relevance to the function of the wall. In the UK and across Europe our early mortar technologies were based on lime, as opposed to the gypsum and occasionally bitumen mortars found elsewhere. The quality of early burnt limes and the consistency of excavated sands were variable, limited by the technology available at the time. Unlike today, kilns did not fire at a uniform temperature and the lime produced would have different properties, depending on in which parts of the kiln it was burnt.The limestones would vary considerably across the country, resulting in particular in varying hydraulic properties. Some might set in water, some might not. As it was impractical to transport sands and other aggregates long distances before the canals or railways were built, most of those found in historic mortars were locally sourced.The properties of the resulting mortar would be dependent on the skill of the artisans in using the materials at their disposal. Conservation remedial work While it is tempting to hold up historic mortars as fine examples that are well-formulated, in fact many were made with poorly graded sands.The conservation engineer, looking at the internal structure of the mortar matrix, is faced with the philosophical question: ‘do we replicate a poor original or do we replace with a well-designed mortar?’There is no simple answer; every case must be considered in its own right. Although the focus is generally on the lime binder, the aggregate is the most significant element in the matrix of a mortar. Sand is usually the principal component, but the aggregate is often found to include fragments of waste masonry materials: crushed stone, brick and old mortar. This ‘filler’ has greater influence on durability, colour, frost resistance and strength than the choice of lime. The burnt limestone will have varied across the country. Reference to the geological maps will point to whether it may be a chalk, blue lias or a grey lime. Unfortunately, changing use and economic fortune have removed many of the historic limestones from production. On a commercial scale we are now left in this country with relatively pure calcium carbonate which is not hydraulic, such as found in Buxton, or foreign hydraulic limes. There are many good historic references for mortars and plasters, dealing with what was thought good practice and appropriate in areas, and with the materials available at that time. The buildings these applied to were generally thick walled and the mortar very rarely acted in tension. One of the best reference books on historic mortars was written byWJ Dibdin and published by the RIBA in 1911. This lists the results of a whole range of tests on different mortars, limes, sands and additives such as In most early architecture the masonry is in compression, as here at Norwich Cathedral, so mortar strength is largely irrelevant (Photo: Jonathan Taylor)