Wood stain is a type of paint used to colour wood and 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 colour very fine-grained woods (such as cherry or maple) while pigments will not colour woods such as these as well. Fine-grained woods generally have pores that are too small for the pigments to settle into. As a result, usually pigment-containing stains will also include a small amount of a 'binder' which helps to adhere the pigments to the wood. A common binder would be a drying oil such as linseed oil.
Most commercial stains contain both dyes and pigments, though often in varying colours and ratios (to each other) depending on the desired colouring effect. Additionally, to some extent, the degree to which a stain will colour a particular wood may be dependent on the length of time it is left on the wood, with longer exposure times accomplishing deeper colouration. Typically a 'transparent' stain will accentuate the wood grain (as it is transparent), while a more 'solid' stain will tend to obscure the wood grain (as it is more opaque, akin to what we would usually call 'paint'). Pigments, regardless of the suspension agent, will not give much colour to very dense woods but will deeply colour woods with large pores (e.g. pine).
Gel stains are a late 20th century innovation in stain manufacturing, in that they are high-viscosity liquids and do not 'flow'. This property allows more control during application, particularly when the wood is in a vertical position, which can often cause traditional liquid stains to run, drip, or pool. Gel stains often have limited penetrating ability, as they are thixotropic (a liquid that nevertheless does not 'flow').A combination of coffee and vinegar or water is a natural alternative to synthetic wood stain.
