Historic finishes and coatings, as well as object substrate matrices can become compromised, losing cohesion, as a result of chemical loss of binder, repeated swelling and shrinkage with moisture exchange, cyclic expansion and contraction with thermal fluctuations, freeze thaw cycles, ongoing crystallization and dissolution of salts with varying humidity levels, and metal corrosion. The obvious tell tale signs are coating loss, friable surfaces and loss of the parent material. Differing materials show unique characteristic of damage to the various causes. Plaster surfaces can become chalky to the touch, historic marble gravestones came loose cohesion between the crystals and be “sugary”, and sandstones, particularly Connecticut Valley Sandstones, can separate in complex layers.
Consolidants can be applied that form bonds between the weakened particles and strengthen the bonds of the surface coating with the parent material or strengthen the structure of the object itself. The selection process for consolidation materials and application procedures evaluates the physical and chemical properties of the objects, the severity of damage, the environmental conditions, and the structural function of the compromised region. Potential need for reversible or re-treatable measures are balanced with practical concerns for longevity of the object and public safety, particularly in regards to large-scale objects and architectural features.
Consolidation application can be a laborious process and the cure times for consolidation can be substantial, as is the case with Alkoxysilanes, which can require 4-8 weeks for appropriate chemical stabilization. For successful conservation treatment, the consolidation process requires appropriate phasing with other needed treatments, such as cleaning, injections, and loss compensation.