The problems in cleaning stone and other porous surfaces are numerous notwithstanding the very nature of the stone specie and the type and extent of its damage. The general nature and source of dirt on the stone must be determined in order to remove it in the most effective, yet least harmful, manner. Types of ”dirt” include soot, smoke, leached salts, rust and organic stains, products of chemical attack from air pollution, micro-organisms and various applied coatings of paint, wax, etc. Stubborn dirt can be treated in a variety of ways including chemical cleaners and abrasives. Each method has its advantages and drawbacks. The safest of all stone cleaning methods is poulticing.
All stone cleaning procedures generally attempt to maximize cleaning power while minimizing chemical and physical attack that can result in further stone degradation. The use of inert natural clay poultices, or mud packs, is recognized as one of the safest methods for removing dirt deposits without affecting the stone’s integrity or surface detail. Poultices act to draw out hidden dirt, salts and microbes and is the least harmful of all techniques.
Fuller's earth is a mineral that differs from attapulgite in that it is composed of a three layer sheet crystal which forms a flake-like particle. The principal mineral of fuller's earth is montmorillonite. Fuller's earth swells when added to water. Its expansion is as much as 15 times the original volume. Swelling is reversed when the water is removed by drying, and shrinks back to its original dry volume. Our fuller's earth has an extremely fine particle size of less than 0.5 micrometer in diameter with a light grey to white color. It has a pH value of 8.5 – 10.5.