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21 April 2011

Wednesday 20th April 2011; Porous Building Materials & Participant Presentations

Week 2 of 12 
Module 2: History and Theory of Conservation

Giacomo Chiari, Chief Scientist at the Getty Conservation Institute, today gave us lectures on an 'Introduction to Porous Building Materials'.

The two major causes of destruction of buildings are man and water. Giacomo focussed on the latter, however he pointed out that it is very seldom that there is only one cause of deterioration, and that you should always look at the object, structure or building that you're conserving holistically. A large focus of the lectures was on the chemistry of the processes of deterioration that occur in porous materials. A quote Giacomo made in the forward of Giorgio Torraca's 'Lectures on Materials Science for Architectural Conservation' is that 'everything happens at atomic level'. Following today's lectures I find myself realising how true this is. It can be hard to perceive how water can move through micropores in a porous material considering that these pores are defined as being between 1-10 microns (1micron = 1,000th of a mm). However, it becomes easier to imagine that this is possible when you consider that if a water molecule was the size of a human, a 1micron pore would be the equivalent of 18km3 (18km is the height at which many planes fly). It is worth noting that with regard to building materials, it is only open (connected) porosity that is of interest to us.

The evaporation of water from a porous building material is one of the phases of concern with regard to stone deterioration. The rate of evaporation can affect the position that any soluble salts are precipitated. Any alteration to environmental conditions (wind, sun exposure or moisture) can drastically and quickly affect this rate and all too often result in salts being deposited below the surface of the stone- subfluorescence. If the concentration of salts is large enough to fill a pore this is where pressures become exerted on the walls of the pore and deterioration can occur as this process repeats itself. Giacomo showed some great time-lapse videos of subfluorescence and efflorescence which I will try to add if I can find them as electronic resources.

Porous building materials are brittle- they have a low tensile strength. Mortar between two blocks of stone allows the load of the overlying material to be evenly distributed across the stone below and removes the pressure on particular points that would occur if a stone does not have a surface that is entirely even (which is true in the majority of cases).

When telling us about how we should assess a building holistically Giacomo asked us to look at the wall in the classroom and tell him what we could see. He pointed out that our eyes were drawn to the red pins but not the grey board that they were attached to; this is a danger that we are so drawn to a single (or the most obvious) defect on a building that we forget to assess the background or less obvious parts.

We had a discussion about stone consolidants and identified the many necessary qualities that this material should have if it is to be used including water resistance, retaining the porosity, not altering the chemico-physical properties of the stone, increase resistance to desegregation.... the list continued. Perhaps we will identify if we have yet discovered such a material in later lectures on consolidants.

This afternoon we had further presentations from participants; Korea, Netherlands and from myself (Scotland). The Korean participant focussed on the issues they're dealing with in protecting their exposed Buddhist monuments and temples and the need for appropriate solutions for creating shelters or protecting the stone. The Dutch participant talked us through some of the work he has carried out on external sculptures an some of the problems he has faced during the process. It was pointed out that we have a lot to gain from hearing about others failures/mistakes/errors and is something we should all do a lot more, although these things are often hard to admit.

Following my presentation about the work that we carry out at the Scottish Lime Centre Trust and the conservation issues we face in Scotland, Giacomo shared with me some photos he had taken during a visit to Yemen where they continue to burn lime in traditional kilns which is used on the local buildings. The visit was connected with a film that was being made by Caterina Borelli 'The Architecture of Mud'.

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