Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation
Today Veronique Verges-Belmin, head of the stone department at the Laboratoire de Recherche des Monuments Historiques (one of the three state laboratories of the French Ministry of Culture), joined us to speak on the subject of desalination. Veronique started the class with two simple demonstrations to illustrate the two different ways that salt can be removed from a stone; advection and diffusion. Advection is a process whereby water is sucked from a medium by a second material with smaller capillaries. This was demonstrated by sucking up water into a small glass capillary tube and then placing a smaller tube into the end containing water- the water was immediately sucked into the smaller tube. We then did the reverse of sucking water into the smaller capillary tube and placing this at the end of the larger capillary tube- no water movement occurred. The point of the demonstration was to say that when you use an advective desalination process (e.g. poulticing), the poultice material must have smaller pores than the substrate. Diffusion is a process whereby ions move from a more concentrated solution towards a less concentrated solution; this was demonstrated by dropping a small amount of ink into a glass of water and watching the ink slowly disperse through the water and sink. An example of a diffusion desalination process is using a water bath where the higher concentrations of soluble salt move out of the stone towards the areas of lower concentrations.
The examples given above of poulticing and water baths are the two most common forms of desalination carried out but many other methods exist and are used either as stand-alone techniques or in combination with other techniques. Some of the other techniques highlighted were;
*Removal under a vacuum; water is sucked through your object by a vacuum and transporting soluble salts with it.
*Water pressure; water is pushed from one side of an object through to the other side transporting soluble salts with it.
*Dry removal; salt efflorescence is brushed from the surface of a stone.
Alison Heritage also rejoined us this morning (see Thursday 19th May 2011; Salts for details on previous lectures) to discuss the complications and risks of poultice salt extraction. Two of the issues we discussed were the potential for a poultice to leave a residue on the surface of the stone and the potential alteration of the salts that are present in the stone due to selective extraction. The poultice that provides the greatest adhesion will not always be the most appropriate poultice for a job because, especially in the case of kaolin-based poultices, there is often a problem with residues being left on a stone's surface. Cellulose poultices are generally considered to be possible to remove easily from most stones, however their capillary size may be too large for many stone substrates. The concept of selective extraction of salts is an interesting one. You will naturally remove the more highly soluble salts first, or more efficiently during a poulticing procedure and therefore you have the potential to alter the salts remaining in the stone into a more dangerous state. Alison's aim was not to put us off poulticing, but to make us aware of the issues involved, concluding that we should should not be put off poulticing, but that we should always use pre- and post-treatment assessments to inform our decisions on whether we should repeat, adapt or abandon poulticing on a case-by-case basis.
Veronique then spoke to us about the 'Influence of Components on Poultice Characteristics'. Details to follow...
This afternoon we had a practical lab session in groups of two trialling different types of poultices and different methods of application with a variety of other variables. We will be revisiting these tests in the lab session tomorrow morning to see and discuss the results of the tests.
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