Stone Consolidation - PAYE
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Stone Consolidation

Stone Consolidation

Consolidate – verbcon-sol-i-dateto unite or combine, to become solid of firm

Natural stone by its very nature, will erode and weather, although some stones are subject to this process at a much higher rate than others. This is somewhat exacerbated by the (generally) wet, cold climate of The United Kingdom. As such, stone choice and its proposed location and use are fundamental to its longevity.

As a result of the climate, the process of stone consolidation as a conservation technique has been used for centuries. During this time there has been much effort applied to developing suitable methods to ‘halt decay and prolong life’ in line with conservation principles. Despite the ongoing research and continued academic writing surrounding the subject, issues concerning stone conservation remain under debate. Examples of previous, sometimes disastrous surface treatments used historically have included wax, paraffin, linseed oil, shellac, paint and varnishes. Any material applied to the stone surface that traps moisture will actually only exacerbate the problem.

Consolidation is not a preservative treatment. Preservatives aim to fix something in its current state indefinitely. Consolidants however, should allow a material to continue to weather at a ‘normal’ rate, but slow down and control the rate of decay. A consolidation treatment can be considered to be successful if it stabilises the material and does not lead to damage resulting in either material or aesthetic change.

Technologies are always moving forwards and works within the heritage sector are certainly no exception. Innovative products and methods are constantly being touted as the answer to age old problems. It is obviously sensible to approach these ‘solutions’ with an open mind, but also I would suggest a healthy level of cynicism.

A lesson learnt at the very beginning of my career was in relation to the (well meant) application of silanes to friable stonework from a decade or so earlier. During the late 1970’s and throughout the 1980’s silane applications were championed as a technique to consolidate and protect decayed stone, which still allowed full permeability. Alkoxysilanes and alkyl alkoxysilanes or ‘silanes’ for short, were developed.

The first of these products to be used extensively were products by Wacker and Brethane; a product developed by The BRE (Building Research Establishment). Over the forthcoming decades, these products were proven to be problematic in certain usage, as they did not permeate the stone evenly or to a sufficient depth. Conversely, what was actually found to have happened was that this process created a hardened layer below the stone surface, which actually led to an exacerbated breakdown of material behind it, often causing a much more catastrophic level of failure such as was found at Bolsover Castle, Derbyshire.

The advice given by many specialists and organisations including SPAB and English Heritage these days is therefore that consolidation with these kind of materials should only be considered as an absolute last resort as they may lead in the longer term to greater loss.

Nano-limes are the current ‘new breed’ of stone and plaster consolidants that are being trialled to reported good effect. In principle, they are a suspension of extremely fine particles of Calcium Hydroxide in alcohol. This size of particle is much smaller than that of Calcium Hydroxide Ca(OH)2 in water (or limewater). Whilst limewater has traditionally been used as a consolidant on friable plaster and stone, it requires many saturations to get sufficient lime into the substrate and its consolidation effect can only really be detected after an excess of 150-200 applications – in reality, making its use limited.

Nano limes in contrast, claim to be able to carry 30 times more lime than in limewater, which in theory would allow equivalent deposition of calcium hydroxide into a porous material with only a thirtieth of the number of applications. Whilst in principal, I understand the chemistry and logic behind this technology, I fear that as a young product it needs far greater trialling and standardising, before we see it as the go to answer to all stone and plaster issues. Similar can be said of the use of ‘hot-limes’ which have become so fashionable in recent times. I think the industry understands the science of approach in both cases, but some sceptically would like to see a far more standardised approach to useage and a longer-term track record proven for both.

As we know, technology cannot be halted and should actually be actively encouraged. I am certainly open to the development of new techniques and innovations which may offer solutions to age old problems, but in many regards, when working with traditional materials in a heritage context, the application of ‘tried and tested’ techniques are in my mind a safe approach. We know their limitations, we understand the full implications of their introduction and they are normally sacrificial in nature, allowing for a fully reversible solution. The key to this ‘mindset’ of approach is maintenance. I personally consider it is far better to use a surface dwelling consolidant such as a lime based sheltercoat and replace it regularly, than to use a ‘foreign’ material that penetrates the substrate, changing it on a physical level and proves impossible to undo.

If a consolidant is proposed of course, then trials should always be advocated. Ideally these are completed in an area which is unobtrusive, but still remains reflective of overall condition. We would also always recommend that individual stones are treated in the own right and not entire facades. Each case must be evaluated on its own merits and considered as to whether intervention is justified or whether the rate of decay can be tolerated and accepted until a differing solution can be championed (…ironically, possibly through the use of a new technology currently unknown to us!)