Examples of microscopic analysis of historic mortars by means of polarising light microscopy of dispersions and thin sections

The main question to be answered by analysis of historic mortars is: what is the composition of the original mortar, either to compare different mortars from the same site, to find suitable replacement materials, or to reproduce the mortar material because it has proven to be very durable. After a general examination by the naked eye or a binocular lens with a low magnification, our first step is to analyse dispersions or thin sections by means of polarising light microscopy. Advantages and disadvantages of both investigative methods are discussed in this paper giving practical examples. The general advantage of optical microscopy over other methods is that the inorganic constituents of a mortar can be identified in their mineralogical form and in their spatial context. The major obstacle, however, seems to be the lack of good reference materials of mortars with known initial mixture and way of application.