A wood stain consists of colourants dissolved and/or suspended in a ' vehicle ' or solvent. Vehicle is the preferred term, as the contents of a stain may not be truly 'dissolved' in the vehicle, but rather 'suspended', and thus the vehicle may not be a true 'solvent'. The vehicle often may be water, alcohol, a petroleum distillate, or a finishing agent such as shellac, lacquer, varnish and polyurethane. Coloured or 'stained' finishes do not typically deeply penetrate the pores of the wood and may largely disappear when the finish deteriorates or is removed.
Pigments and/or dyes are largely used as colourants in most stains. The difference between the two is in the solubility and in the size of the particles. While dyes are molecules that dissolve into the vehicle, pigments are larger than molecules and are temporarily suspended in the vehicle, usually settling out over time. Stains with primarily dye content are said to be 'transparent', while stains with more pigment in them are said to be 'solid' (opaque); some stains may be called 'semi-solid' or 'semi-transparent', and these may be interchangeable terms, and the relative transparency or opaqueness may fall somewhere between the two extremes. Typically, dyes will color very fine-grained woods (such as cherry or maple ) while pigments will not color woods such as these as well. Fine-grained woods generally have pores that are too small for the pigments to settle into.
