Surface Treatments

The idea of stone consolidation treatments is to strengthen stonework that has decayed to the point of being severely weakened, so that it is less prone to further erosion and collapse.

Stone consolidation is aimed at strengthening the internal binding of decayed stone as well as its binding to the deeper, stronger layers of the substrate.

To be effective as a stone consolidant, a substance must be able to penetrate the stone by capillary absorption, and then set in place.

In theory this can be achieved by using heated substances that are in liquid form when at higher temperatures but solid when cooled. There is a risk of failure if unusual natural heat exposure occurs at a later stage. For example, direct sunlight on an exceptionally warm day, causing the substance to revert to liquid form and seep out of the pores it was supposed to be firming up.

Another method is to dissolve a substance with consolidant properties in a solvent that is then left to evaporate by itself.

A final method of introducing a stone consolidant is to depend on a chemical in a liquid state that naturally reacts with ingredients in the substrate to produce a solid result.

Depending on the method used and on the material state and properties of the substance used, it may be more useful to apply the consolidant by immersing the substrate in it, spraying it on, using a pipette, or brushing it on.

Substances commonly used as stone consolidants include: calcium hydroxide, barium hydroxide, and a wide range of organic polymers, including alkooxysilanes, epoxy resins and acrylics.

The use of calcium hydroxide (lime) as a consolidant, where chosen, works by its reaction with carbon dioxide to set as calcium carbonate. However, careful protocols need to be followed to ensure that this is deposited in a way that serves effectively to strengthen the stone.

Where barium hydroxide is used, it is recommended by some researchers to precede the application with one of ammonium carbonate for better results.

Alkooxysilanes, notably methlytrimethoxysilane and tetra-oxysilane, are collectively the most popular group of stone consolidants used in conservation today. but they require careful pairing with the right types of stone and are not ideal for stones that are rich in clay.

Where stone has been damaged, it can be carefully repaired using a specialised stone repair mortar. Restorative Techniques supplies the powder and liquid components needed to produce a high-quality stone repair mortar. See the Technical information sheet for more details.

It is possible to treat porous and soluble stone with coatings that resist water in order to protect it from damage and from hygric expansion, which is a form of swelling caused by the presence of moisture.

However, many conservators take the view that it is necessary to allow the stone to remain water-permeable in order to prevent a hazardous build-up of water and soluble salts at the juncture between the treated and untreated portions of the stone.

Alkoxysilanes, fluoropolymers and silicones are the main groups of chemicals used in water-repellent surface treatments where they are used.

As the name suggests, anti-graffiti coatings are designed to resist the unauthorised application of graffiti by human passers-by and intruders.

Because most anti-graffiti agents used in modern urban environments (such as on concrete blocks) tend to severely inhibit the passage of water, they are a risk factor for the conservation of historic masonry. Highly specialised formulae are needed instead, such as those supplied by Restorative Techniques and found in our chemicals catalogue.