Powders having contact biocidal properties comprise a polysacharide carrying atomic / metallic silver. The preferred
polysaccharide is
chitin, although other polysaccharides including
chitosan, carboxymethyl celluloses and carrageenalls can be used. The
chitin may be obtained from deproteinated
crustacean shells without demineralisation, thus being admixed with
calcium carbonate and other naturally occurring minerals present in the shells, and may be
enzyme deacetylated. The powders of the invention can be used as biocidal
dusting powders, formulated into pastes, gels, hydrogels, creams, foams and
aerosol sprays for pharmaceutical applications, or dissolved to form solutions for
coating substrates such as
skin, fabrics, glass, leather and paper to give a bactericidal surface. A particular application of such a solution is as a protective, post-wash treatment for workwear in a laundering process. The powders of the invention may be prepared by slurrying a
polysaccharide, which is capable of interacting with silver ions and which is in
powder form, in a liquid in which the
polymer is insoluble, which liquid contains silver ions, filtering off the
powder, washing the
powder, reducing the silver ions which have interacted with the
polysaccharide to atomic / metallic silver, and
drying the powder. According to the polysaccharide chosen, the liquid is suitably water or
aqueous ethanol. The silver ions may derive from
silver nitrate. The reduction of the silver ions which have interacted with the polysaccharide to atomic / metallic silver can be effected photochemically through
exposure to light. To hasten the reduction, however, the washed powder is preferably slurried in a solution of an
alkali metal halide, irradiated under stirring with natural or
artificial light containing an
ultraviolet component, and again filtered off and washed, before
drying it.