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. Composition Stain is composed of the same three primary ingredients as paint (pigment, solvent (or vehicle), and binder) but is predominantly vehicle, then pigment and/or dye, and lastly a small amount of binder. Much like the dyeing or staining of fabric, wood stain is designed to add colour to the substrate (wood and other materials) while leaving some of the substrate still visible. Transparent varnishes or surface films are applied afterwards. In principle, stains do not provide a durable surface coating or film. However, because the binders are from the same class of film-forming binders that are used in paints and varnishes, some build-up of film occurs. Comparison with paint and varnish The initial application of any paint or varnish is similarly absorbed into the substrate, but because stains contain lower amounts of binder, the binder from a stain resides mainly below the surface while the pigment remains near the top or at the surface. Stains that employ metallic pigments such as iron oxides usually are more opaque; first because metallic pigments are opaque by nature, but also because the particles of which they consist are much larger than organic pigments and therefore do not penetrate as well. Most wood stains for interior uses (e.g. floors and furniture) require a secondary application of varnish or finish for longer-term protection of the wood, and also to adjust for matter or gloss effects. Stains are differentiated from varnishes in that the latter usually has no significant added colour or pigment and is designed primarily to form a protective surface film. Some products are marketed as a combination of stain and varnish. Siding stain Siding stain is one variety of wood stain with very high viscosity (others can be quite thin). Effectively, siding stains are paints that do not cover as well and do not form a hard film. They are designed to penetrate better and contain binders that