MATCHING EXISTING MORTARS
Modern analytical techniques can determine whether the lime binder was hydraulic or non-hydraulic and pozzolans, such as crushed brick, can be readily identified. Usually the proportion of binder to aggregate can also be quantified, although sometimes old mortar was recycled and used as aggregate, confusing the picture.

Analysis of historic mortars, plasters and renders, often reveals that the aggregate was

sharp in particle shape (Figure 1) and well graded through from large to small particles or 'fines' (Figure 2).

Some suppliers of lime mortars and aggregate hold large collections or databases of aggregate samples that have been analysed as a source of reference for preparing matching mortars. However, databases such as these should be used with caution as very few quarries have a consistent source of standard material and, as the face is worked, minor changes may occur in the colour, grading and size of the particles. Aggregate sources used decades ago may well now be lost to landfill, and river sources may no longer be available as rivers change their courses over the years.