C O N T E X T 1 3 2 : N O V E M B E R 2 0 1 3 37 There is also a problem with the nature of academic publishing. Peer-reviewed papers in learned journals are too expensive and too hard to access to reach many specifiers.There is an extensive academic literature on stone consolidation, but it is almost never read by architects, surveyors, conservation officers or conservators. Stone consolidation experiments rarely yield obvious outcomes.The time-scales of decay mean that apparent success at contract completion may become failure 10 years later. All that can realistically be hoped for is a retarded rate of decay over the medium term. This matters hugely. Commercial survival as a conservation contractor depends on having a reputation for success. If success is undefined and open to hostile challenge, there is almost nothing to motivate conservators to disseminate their results. Because we do not have a reliable system of ascribing success and failure without also attaching blame, we are condemned to run a secretive system of unacknowledged experiment. If we do not make our mistakes public, they will continue to be replicated. Without well-defined detailed aims, stone consolidation is unlikely to succeed. It is not sufficient to have a generalised aim, such as ‘preventing future loss of detail’, because the decay mechanisms behind loss of detail are complex and may require multiple stone consolidation strategies. Our understanding of stone decay is primitive. Knowledge of the basic processes (such as sulphation) is sound enough2, but the modifying effects of local variations in detailing, use, stone composition and weather are poorly understood. On top of this, perception of the need for intervention tends to be based on short-term observation of the rate of decay. Short-term snapshotting may sometimes be the best that can be done, but the information gained is highly likely to misrepresent the true rate of decay. One-off condition surveys look at the change of condition over a short time period and project the observed rate of decay forward.The choice of observation snapshot yields widely varying predictions of future condition. The true rate of decay is often non-linear but marked by sudden, catastrophic step declines in condition.The structure and surface of a stone may gradually and evenly dissolve, or turn to dust and fall away. But in many decay scenarios periods of no loss will be separated by catastrophic loss events.The non-linear nature of decay is hard to grasp and harder to observe. The common response is to ignore its complexities, note condition at two random points in time (usually from memory or by means of poor-quality photographs), and to project that observed decay forward. This is not likely to provide a rational basis for any decision to consolidate. In short, we do not understand decay well, we diagnose it badly and we predict it very badly.There could hardly be a worse starting point for experimental design. Even if aims were well-defined and decay well understood, the process of contracting presents a further difficulty. Conservation contracts rarely provide adequate resource for preliminary site investigations. Contractors are generally required to survey, report on and estimate for proposed work at their own expense. Some experiments translate relatively easily between laboratory conditions and application on a site scaffold. Silane-based stone consolidation does not. It relies on the consolidant penetrating deeply into the stone. Shallow penetration will produce thin skins of consolidated material that may flake under thermal, mechanical and hydrostatic stress. Deep penetration is not an issue in the laboratory. Conditions in and around the stone sample (independent variables) can be controlled and unwanted variation eliminated. On the scaffold this is impossible. The interior structure of the stone is unknown. There may be large or numerous sub-surface cracks and voids.Will the consolidant be capable of filling and crossing any cracks or voids? The moisture content of the stone can only be guessed at. If a water resisting consolidant is being used, how will it penetrate deeply into wet stone? If a hydrophilic consolidant is used, howwill it penetrate effectively into stone where the pore spaces are already partly water filled? How will the consolidant be applied? If it is sprayed, how much will actually reach the surface of the stone? How can this be measured? Of the consolidant that does reach the surface of the stone, how much will penetrate the surface, and how deeply will it go in? On a laboratory test sample, this can be assessed by trial drilling. On the scaffold, it is hardly ethical to drill holes in the object of the conservation. Non-destructive testing techniques, which would allow proper pre-consolidation understanding of stone condition, are being developed.They are being refined The south door of St Nicholas Church, Barfrestone. Early proponents of silane consolidation noted that consolidation is often not applied until the state of decay is hopeless and that one should consolidate before it is too late if you want to stave off decay. This appears to be borne out by the silane consolidation of the Barfrestone door. Unlike the carving at Steetley, the carved stone at Barfrestone did not appear to be seriously decayed. More than 30 years after treatment it remains in good condition. Because of poor experimental technique we can not judge whether its current condition is in spite of, or because of, consolidation.The silane consolidant seems to have promoted a change in biological colonisation and given the stone a grey colour. A south door capital at All Saints Church, Steetley.This photo was taken in 1993, around five years after consolidation. The eroded surface has developed a thin skin which is detaching. Because none of the criteria for good experiment have been met, it is impossible to say what might have happened if there had been no consolidation.
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