Week 9 of 12
Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation
George Wheeler, Director of Conservation Graduate School of Architecture, Planning and Preservation Columbia University and a Conservation Consultant for several organisations, joined us today and will be with us for the next few days to discuss stone consolidation. David Odgers also rejoined us to discuss consolidation using water-based techniques.
George started by giving us a reference for a book which he said should be at the top of our reading lists; 'Stone in Architecture' edited by Siegesmund & Snethlage.
The two common aims for consolidation are to increase the strength of a deteriorated stone and to limit loses by certain forms of deterioration. Consolidants were used extensively in the UK in the '70s and '80s but their use is now much less, most likely due to the ill effects being experienced since then due to the methods used. As I understand it, the technology is moving slowly, but research is continuously being carried out and new methods are being developed and old methods improved. Consolidants are able to restrengthen certain forms of deterioration, but they will never mend stones which are flaking, scaling or spalling and other techniques would be required to save these surfaces (e.g. grouting). For this reason certain types of stone such as slates which break apart in 'sheets' cannot be consolidated.
Most types of stone are receptive to consolidation, but many different factors need to be considered when deciding on the method; chemical sensitivity, water sensitivity, porosity, pore size distribution, strength and frequency of use. For this reason initial conditions surveys are essential to allow you to characterise the substrate. Consolidants should also be selected based on the type of consolidation that you wish for them to perform; the two highlighted were by a 'depositional process' whereby the spaces between grains are filled to reduce movement and by an 'adhesive process' which creates a chemical link across the grains. Throughout history a wide variety of organic and inorganic consolidants have been used and George described the use of consolidants as a 'fashion industry' where you can see the trends for use decade to decade.
We were given a list of the properties of the ideal consolidant (which doesn't (yet) exist!) including factors such as light stable, high glass transition temperature (the temperature at which a material changes from a glassy substance to a plastic one where it becomes sticky), ease of use, compatibility with other conservation activities, limits future deterioration- the list continues.
In the second half of the morning we had a session with David Odgers on 'Water and Lime-Based Consolidation'. We discussed the 5 main types of 'inorganic' consolidants; calcium hydroxide (lime water), nanolime, barium hydroxide (baryta water), colloidal silica and calcium oxalate. I have made a few observations on the lime-based techniques below.
The lime water technique is theoretically an ideal way of re-cementing a deteriorating limestone because essentially the method reintroduces a calcium carbonate cement around loose grains. The technique works by combining a lime putty with water in the same way as you would produce a limewash, however the aim of this technique is to retain the original colour of the stone rather than producing a coating layer in the way of a limewash. The water and lime putty mixture are allowed to settle out until the liquid overburden is clear; this clear over burden is the lime water which can be siphoned off. The container must remain sealed during this process to prevent any air carbonation of the lime and it is preferable to keep the container cool because lime is more soluble in cold rather than hot water. The lime water has a concentration of c.1.4g of lime per litre and for this reason multiple applications (around 40) are necessary to produce a consolidating effect. One of the serious downsides of this technique is that you must introduce a very large quantity of water into your stone which may not be desirable.
Nanolime is one of the current 'fashions' in the consolidation industry and the technique is constantly being developed and refined. Nanolime is so called because it has particles of lime which are on the nanometre scale (1 nm = 1 thousandth of a micron). The benefits of the small particle sizes are that they have the possibility to penetrate deep into the damaged stone, they have a high reactivity and a high purity. Very old (well matured) lime putty can actually be on the nanometre scale, but it differs from the nanolime due to its water content; nanolime instead uses alcohol. The concentrations of nanolime available vary from 5g/litre to 50g/litre and it can therefore be many times more concentrated than limewater.
This afternoon George Wheeler continued with an introduction to consolidants and then looked at 'Thermoplastic & Thermosetting Organic Polymers for Consolidating Stone'. We were shown various forms of 'reactive' consolidants which work by dissolving/digesting a part of the substrate to create the consolidant such as ammonium tartrate which when added to a limestone reacts with the calcium in the stone to create calcium tartrate. George explained that many such techniques are essentially surface, rather than penetrating, consolidants.
Thermoplastic resins were described as those that dissolve in organic solvents (e.g. Paraloid), and thermosetting resins as those which once set cannot be dissolved in organic solvents (e.g. epoxy or polyester resins). George listed some of the wide range of organic materials that have been applied to stone to act as consolidants from waxes and drying oils (e.g. linseed oil) to polyethylene glycol and cyclododecane. The latter is described as being a 'sublime' so it essentially 'disappears' at room temperature making it a very useful temporary consolidant, sometimes used when a deteriorating object needs to be transported.
An aside from this lecture; David Odgers described the use of 'Sheltercoat' in his lecture on 27th May and yesterday during the lab session he applied a sheltercoat to a block of sandstone. Today he showed us the results under a microscope and you can see the results below.
Two blocks of the same stone but the one on the left has been treated with the sheltercoat.
Close-up of the sheltercoat-treated stone (left) and untreated stone (right). The images are c.25mm in width.