As part of the conservation and post-excavation programme we commissioned the British Geological Survey (BGS) to prepare thin-section slides of small samples of stone from both altars and the altar base. The slides arrived back from the BGS this morning and we are looking forward to analysing them in the coming days.
Thin-section slides are prepared by immersing a small sample of stone in liquid resin. The resin soaks into the stone and fills any natural voids so using a blue-dyed resin will reveal the porosity of the stone. Once the resin has solidified a thin sliver of the rock is cut from the sample and polished. This film of resin-impregnated stone is then mounted on a glass slide to allow detailed analysis using a microscope.
These samples are needed for two reasons. Firstly, we need to identify whether any natural salts are present in the stones, particularly the Mithras altar, because this will affect any further conservation work. The scientists at the BGS will conduct further analysis to determine whether any natural salts are present within the stone and, if so, whether the salts pose any risk to the stone. The result of this analysis will help us to identify the best method to use to consolidate the altar.
Secondly, the thin-sections will be used for a detailed petrographic study of the stone by a geologist which may help us to identify the source of the sandstone.