SILICATE PAINT (EXTERNAL)
Silicate paints were first patented in Germany in the late 19th century. King Ludwig I wanted to copy Italian frescos but found that lime plaster couldn’t withstand the harsh climate. He asked his scientists to develop a paint that looked like lime, but was more durable.
Like limewash, silicate paint works by soaking into the underlying material. The silica binds chemically with the mineral substrate, forming an insoluble crystalline bond. This is likely to be a far more compatible treatment than painting walls with a surface film of synthetic masonry paint.
Apart from being very durable, silicate masonry paint is exceptionally vapour-permeable (breathable). It is also unaffected by UV light, resistant to acid rain, mould growth and fire. In regions where fire is a threat , such as earthquake zones, sodium silicate is sometimes sprayed onto wooden buildings as a cheap method of fireproofing.
You can also use silicate paint internally on areas where other paints might not work as well, such as damp internal stone walls or basements.
Limewash is a very ancient paint made from limestone which has been crushed, burnt and slaked with water to make lime putty. The lime putty is matured for several months before being thinned with water to make limewash.
Limewash is naturally white and forms a complex crystalline matrix which has a matt, slightly chalky appearance. It is coloured with pigments and can be used internally or externally where it works best on porous surfaces such as a traditional lime plaster, lime render, stone and brick. Unlike modern barrier paints, limewash works by sinking into the surface. It hardens by absorbing carbon dioxide from the atmosphere to form crystals of calcium carbonate which give the limewash its deep matt appearance and protective qualities. When used on porous surfaces, the colour will deepen if there is any dampness in the background material, hence its attractive shading.
Tallow (animal fat) or raw linseed oil are traditional additives which help to improve its water-shedding qualities when used externally (we use linseed oil as standard).
As it is a water-based paint, limewash isn’t easily absorbed into less porous surfaces such as cement renders or hard gypsum plasters, and therefore it won’t wear nearly as well on these. Additives such as casein (skimmed milk) can be added to help it bond to these less porous materials.