Context 154 - May 2018

28 C O N T E X T 1 5 4 : M A Y 2 0 1 8 inappropriate mortar being used without considering whether it is fit for purpose. All methods of analysis provide information about the mortar, but there are limitations which specifiers and those commissioning analysis should be aware of. Taking mortar samples is usually a destructive process (unless the mortar is already detached or has failed). Statutory consent may be required if taking samples from a scheduled monument. To avoid any possible misunderstanding, permission should always be gained in advance if a building is listed. Mortar analysis can sometimes help to provide information about the date of the building, particularly if supported by archival evidence, but care has to be taken in the sampling method and interpretation of results for this purpose. Often the estimated date of construction (provided by the sampler) is taken as the date of the mortar. This also creates the potential for error, as the information supplied with the mortar sample may not be representative or accurate. Off-site mortar analysis (as carried out in a labora- tory) will provide information only about the sample provided, not the building as a whole. Therefore, the value of the analysis is dependent on accurate information being provided with the sample. To gain a representative view of the mortar(s) used in the building, several samples may be required from different areas of construction, different phases of the building, and different applications (such as building, harling, pointing mortar or plaster). Informing specifications A mortar analysis report is not a specification, although it is often mistaken as such.Typically, it will include a sec- tion suggesting how the mortar can be best ‘replicated’, but this is subjective, and comes with qualifications, including availability of materials. Furthermore, there The gable wall was re-harled with a traditional hot-mixed lime mortar using materials to match the locally distinctive harling tradition, and to provide a robust weatherproof finish. (Photo: Jessica Hunnisett, Historic Environment Scotland) are many other factors to consider, including the nature of the substrate and its condition, and the exposure of the structure.2 For example, the original mortar used to construct the wall of a now-ruined property is unlikely to be appropriate to consolidate an exposed wallhead devoid of a roof and weathering details to protect it. If it is specification that is desired, the starting point is what is required for the building to perform and meet the necessary expectations, including authenticity of materials or application method.Analysis may form part of the information used to develop a repair specification, but it should not take the place of appropriate condition assessment and evaluation of the structure. Clients may be surprised at the basic nature of some of the tests undertaken by laboratories, and the assumptions that follow. It is not uncommon for example for a ‘snap’ test to be undertaken (simply breaking the sample by hand) on which is then based a supposition about the hydraulicity of the mortar.The danger is that this is largely subjective, may vary between analysists or technicians, and depends on the quality and condition of the sample. It is unlikely to provide any meaningful guidance towards a specification where the strength of the mortar is an issue. Equally, acid digestion is routinely used to give an indication of binder-to-aggregate ratios, but it is unreliable as it also dissolves calcium-carbonate- based aggregates and inclusions.These tests are therefore indicative and can not be used to reliably inform a repair specification. A valuable element of a mortar analysis is the aggregate identification. The importance of specific aggregates to the appearance and performance of a mortar can be overlooked and may even be missed out of repair specifications. Having original aggregate graded, identi- fied and matched as part of a mortar analysis is of use in identifying a suitable aggregate for the repair mortar. Basic mortar analysis will not normally identify additives References 2 For further guidance on mortar specification, see Historic Environment Scotland, Short Guide: lime mortars in traditional buildings www.engineshed. org/publications/publica tion/?publicationId=85c ed9f0-474d-4ec6-8dd6- a59100fc306f and Roz Artis (2018) Specifying Hot-Mixed Mortars , HES Technical Paper 28.