Wednesday, 11 February 2015

The Geological Detective - II

The Triton Stone Library

Having established that none of the samples in the Triton Stone Library could offer a good match to the stone used in the subject building, the next step was to obtain samples of stones that might be suitable to undertake the necessary repairs.

E-mails were sent off and phone calls made and I soon discovered that the quarry, which has been considered to be the primary source of this stone, has now been mothballed and that I would need to search elsewhere for a good match.

Stone matching in practice...

Time is of the essence, when planning and scheduling an extensive programme of works in the building restoration industry, and the first samples of stone were sent by a tried and tested contractor, who has a good working knowledge of the region.

The samples of Oat Hill stone from the recommended alternative marketed by various companies finally arrived, but a quick inspection with a hand lens revealed a medium grained, shelly oolite - with voids - which bears little resemblance to the subject stone.

The Art of Stone Matching is based on deep geological knowledge, and an understanding of chemistry - but it also relies on intuition, which microscopic analysis just can't reproduce. 

This particular compact, fine grained, moderately well sorted oolitic limestone is left with a very distinctive iron stain when subjected to a standard hydrochloric acid test.

Sunday, 8 February 2015

The Geological Detective - I

Every now and then, I receive a few photographs of an old building – or fragments of stone in the post – along with a request to help with the identification of the stone used in its construction, ahead of a programme of restoration work.

Having devised Triton's stone library, with samples from all over the UK - for day to day stone matching purposes - I was very curious when I received photographs of various chimney stacks at a large red brick Victorian house.

In contrast to the Ham Hill stone, which is heavily weathered, the stone in question still retains sharp lines to the mouldings and appears remarkably clean - revealing its distinctive colour.

Having been categorically assured that it was a limestone, my first thought was that it could be a Jurassic oolite from the Cotswolds. 

Compared to the many geologically similar samples of “Bath stone” and “Lincolnshire limestone” that are now buried in a landfill site - but which once formed part of my own private collection - the large cube that I once obtained from this region stood out just for its colour. A quick look through a couple of reference books reinforced my initial thoughts that this might possibly be Guiting stone.

Although planned for inclusion in the Triton Stone Library, samples were never received and - except for the Permian dolomitic limestone that has replaced it - none of the samples are a good colour match for the stone that has been salvaged from one of the collapsed chimneys. 

The Triton Stone Library