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28 April 2011

Wednesday 27th April 2011; Stone the Unstable Building Material

Week 3 of 12
Module 3: Material Characteristics and as a Building Material

Our lectures continued with David Jefferson today on the topics ‘Stone the Unstable Building Material’ and ‘Identifying and Sourcing Stone for Historic Building Repair’. The majority of minerals in rocks are unstable because they were formed either under heat or pressure or both and have been transported to the Earth’s surface; David equated this to taking a block of ice to the Tropics. The most common building stone in Goa where the Indian participant is located is laterite, which is one exception to this rule because it was formed under surface conditions.

David holds a few (I believe) controversial views in terms of stone deterioration because he does not believe that salts are responsible for the initial mechanical breakage of stone, and likewise frost damage. It is widely accepted that many forms of salt crystallising within a pore space or ice forming in a pore space under certain circumstances will cause pressures (due to expansion) that result in the mechanical deterioration of the stone. I'm sure we will be revisiting this discussion at a later stage in the course and I'm interested to follow this up.

Lichens are very common in Scotland and therefore I was particularly interested in two case studies that David discussed about lichens on rock art in Scotland. Lichens can last for thousands of years and are dependent on being anchored to the stone. The evidence from the case studies seems to show that once anchored the lichens will cause no (further?) damage. When in place lichens are thought to create a waterproof surface and a thermal barrier. Removal of lichens could expose the surface of the stone to unknown levels of deterioration. This is the evidence from sandstone rock art in Scotland, however it is possible that in other climates and on other stone types lichens could be damaging. Moses on the other hand are widely believed to be damaging as they absorb and hold water.

The decision to remove deteriorating stone from a surface should always be approached with caution. It is necessary to identify the form of deterioration before removing the outer layer, because this could simply expose the underlying layer of stone to the same form of deterioration.

David showed some evidence that there can be problems when combining sandstones and limestones in a building due to the fluids that may pass from one stone to another causing a chemical reaction and deterioration of the stone. Equally there has been evidence that combining carbonate limestone with magnesium limestone results in deterioration, but so far no-one has come up with an explanation why!

David believes that combining a fat lime (non-hydraulic lime / high calcium lime) with sandstone can lead to the deterioration of the stone. Calcium limes have however been used in Scotland for as long as we know and I do not know of any known damage in this form. Later David said that he understands it to be the geological age of the sandstone that dictates if this form of deterioration occurs- sandstones which are Carboniferous (299 million years ago) or older- as most are in Scotland- either do not contain reactive silicas or only have a low level. Sandstones younger than this contain reactive silicas which react with high calcium lime. I am interested in reading further about this and would be interested to hear if anyone has experience of this or if you agree or disagree with this concept. David does not believe that Natural Hydraulic Limes (NHLs) exist, which I think is an debate he'll have to take up with the lime producers- it appears to be a disagreement about the terminology.

In week 11 of the course we will spend a week in the Non-Catholic Cemetery in Rome working on six of the tombs; creating a conservation plan and carrying out the work. This cemetery contains the tombs of many famous people including the British poets Shelley and Keats. We have been split into groups for this and I will be working with Anita (the Australian participant), who is an architect, on a tomb for Florence Baldwin (deceased 1919).

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