Wednesday 8th June 2011; Consolidants- Alkoxysilanes

Week 9 of 12 
Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation

We started this morning with a short presentation by Jakub D'Oubal (the Czech participant) on his involvement in a project using/testing a specific type of nanolime. The main findings of the tests he has carried out are that application by syringe was more successful than by brush, that fast carbonation was achievable and that it is possible to remove the white haze which may appear on the surface of the stone. The chemical phenolphthalein is commonly used to test for the carbonation of lime mortars and in this context can be used (prior to carbonation) to test for the depth of penetration of the nanolime in test samples. The nanolime was found to have a good level of penetration following between 2-5 applications and showed a positive consolidation (tested by ultrasound transition measurements). In his experience, the higher concentrations of nanolime can also be used as a grout.

George Wheeler started today's lectures with a summary of the topics covered yesterday. Acrylic resins and epoxy resins; these are still used but in more controlled conditions due to their low UV stability, low glass transition temperature and difficulties with penetration. The main factor in their favour is that they provide good strength increases. Inorganic consolidants; these are one of the favoured groups due to their UV stability, low viscocity and ability to work in water or alcohol (and therefore compatibility with wet stone). Their downsides include that only low concentrations of active ingredients are possible in each application and only low strength increases are possible. Reactive consolidants; these are surface consolidation techniques which consume some of the stone (probably a disadvantage) and can only be used on carbonate stones.

We then moved on to the topic of 'Alkoxysilanes' (also referred to as ethyl silicate or tetraethoxysilane- each used in a different context). George told us that the alkoxysilanes have both low viscosity and a low surface tension which are excellent characteristics for a consolidant. Water has a high surface tension meaning that the water molecules are attracted to one another causing them to 'bead up', whereas the molecules in a substance with low surface tension will be more attracted to the substrate than to other molecules of the same material hence it penetrates a larger surface area of the stone.

Ethyl silicate was synthesised in 1846 and in 1861 is is known to have been suggested, and used, as a stone consolidant for the Houses of Parliament, London. It did not have a high level of success in this case but this is thought to be due to the very poor quality stone. In the 1920s several patents were then developed for ethyl silicate and the differences in its success on sandstone and limestones were recognised. It was in the 1960s that one of the products still in use today was fully developed known as 'sandstein festiger' (sandstone strengthening agent) later to be referred to as 'Wacker OH'. By the 1980s this consolidant was being used on all different types of stone although as this was developed for sandstone it was not known how much success would be achieved on other stone types.

We briefly touched on the use of consolidants on expandable clay-bearing stones and the issues that can be faced on a stone which has a high level of contraction and expansion. As I understand it, the current belief is that the consolidant (in the case of alkoxysilanes) should be weaker than the stone and therefore expansion and contraction of the stone would cause the consolidant to break and fracture rather than the stone. If this happens you will still in some circumstances have the benefit of the stabilisation of loose grains by the additional material (even if the bonds are broken), but in other circumstances additional consolidant will need to be added and this may cause issues with closing up the porosity of the stone. There has been recent development in 'elastified ethyl silicates' with the aim of reducing the cracking of the consolidant; these are still being trialled.

In the 1980s full architectural facades were commonly consolidated, however this is now much rarer and consolidation tends to be carried out on individual elements. George does not know of evidence to say that the alterations in the charateristics of a consolidated stone should affect the surrounding stones, but an issue that may be faced is the discolouration of the consolidated stone. Dark stones tend to be more susceptible to darkening in colour following consolidation.

George finished this lecture by highlighting some of the practical problems experienced with alkoxysilanes. Alkoxysilanes are not immiscible in water and therefore a stone must be totally dry when they are applied otherwise you will have the solidification of the consolidant where it hits water and therefore blocking of the pores will occur. It is likely that this technique therefore faces some serious hurdles in countries with high levels of rainfall such as Scotland. Due to the discolouration that can occur to stone following application of a consolidant it is essential that the movement of the consolidant is well controlled to prevent ugly drips and spills discolouring surrounding untreated stone.

Gottfried Hauff joined us this afternoon and will be with us until the end of the week to add to the discussion on stone consolidation, the lecture looked at 'Structural Stone Consolidation Methods and Treatment Conditions'. We must consider extraneous factors when considering the consolidation of stone such as the presence of soluble salts. Gottfried advised that either salt extraction must come before consolidation or you must pre-consolidate the stone, then extract the salts and then carry out the full consolidation.

We discussed the depth that the consolidant needs to penetrate to to be successful, which according to Snethlage and Wendler (specific reference unknown) should be 'deeper than the mean moisture penetration depth.' This is essential because otherwise if the average rainfall penetrates the stone further than the deepest part of the consolidation you will create a stress zone which will be subject to enhanced weathering/decay. I fear that in an exposed Scottish sandstone building where there is an open porosity the rain penetrates so deep into the stone that it may be difficult to achieve this depth with a consolidant. Gottfried has recommended that I read the chapter in the book referenced yesterday 'Stone in Architecture' related to this topic.

We were shown the various techniques that can be employed for applying the consolidant to the stone including; 'flooding' which requires the capillarity of the stone to draw the consolidant in, 'poulticing' where a stone is wrapped in cellulose paper and the consolidant continuously applied to the paper so that it drains into the stone, 'immersion' which is only possible for movable elements where it is fully immersed into a bath of consolidant and finally 'vacuum impregnation' which is a new technique requiring the object to be fully sealed at its surface and then the consolidant added under a vacuum. Gottfried emphasised the need for good protection of your stone before, during and after consolidation to create the required environment for the consolidation process to be appropriate to the technique. It is possible to have some 'remigration' of the consolidant to the surface of the stone if the environment is not carefully controlled.

George Wheeler finished the day with a case study of 'Consolidating Deteriorated Marble with Alkoxysilanes at St. Trophime Cloister, Arles, France'. A wide range of consolidants were trialled on samples of a deteriorated stone similar to that found at the site and then a rigorous testing program was carried out including capillarity, water vapour transmission, micro-abrasion, ultrasonic velocity, biaxial flexure and colorimetry. The same tests were carried out on a second set of samples following time in a weathering chamber set at a low 'intensity'. There was no clear result in favour of a particular consolidant and the success of each consolidant varied with the different tests. I believe more tests will be carried out before a consolidation regime is decided upon.

Note to reader: This is an entirely new topic to me and I would particularly welcome comments or corrections on any of the notes I have made.