Week 3 of 12
Module 3: Stone: Material Characteristics and as a Building Material
David Jefferson, a geologist and building stone speciallist from England, will be instructing us for the next couple of days on stone as a material. This morning he gave us lectures titled 'The Nature of Stone' followed by a session in the ICCROM laboratory this afternoon.
'Each and every stone is different and there are no universal remedies for the repair of stone' was the opening message of the lecture. In connection with Giacomo Chiari's lectures last week, David made the point that all stones are composed of minerals, some including basic elements and some including organic matter. We were shown samples of the wide variety of stone used historically in England and the issues now faced in correctly matching the stone for replacement. Apparently 80-85% of the conservation work that we now carry out is repairing previous restoration work.
In a discussion about quarrying we were shown how the natural bed height often dictates the course heights in historic buildings; these bedding planes were exploited to reduce the amount of cutting necessary because this meant that just the ends and sides would need to be cut to size. Apparently c.50% of wastage should be accounted for when specifying stone from a quarry as uncut 'block' because the excess needs to be removed to produced the required dimension stone. Historically limestone quarries were able to produce practically 0% wastage as the 'good' stone could be used as dimension stone, the wastage for rubble and fill, and any excess could be burnt for lime production.
This afternoon we were introduced to visual analysis of stone by the naked eye and by petrography.