Soiling in the form of soil from burial, dust from storage and as caked-on mud, grease, oil, dirt, sweat and blood from use, is detrimental to objects. Soiling associated with textiles, wood and paper acts as a food and moisture source for micro-organisms. Microbial activity, therefore, centres on the soiling resulting in subsequent attack of the substrate by microorganisms such as moulds. Soiling particles and microbial growths adsorb water and retain it in close contact with the substrate thus acting as a source for corrosion of metals and hydrolysis of organic materials. Soiling is usually acidic; the rates of reaction for corrosion and hydrolysis increase as the pH decreases. Soluble materials are drawn from the soiling into the object leaving staining and discolouration (e.g. iron staining). Dirt and dust obscure the surface of the object from view, making the object appear duller and darker than it actually is and obscuring decorative detail and other features. The extent to which the conservator removes soiling and decay products is a matter of careful judgement, balancing the loss of information which the soiling and decay products can contain against the benefit of improving the stability of the object and revealing more of its original visual form.