Finishes are applied to furniture to provide moisture resistance (see last weeks post on how moisture damages timber), improve colour, add sheen and to bring out the natural beauty in the timber grain.
There is a lot of mystique surrounding timber finishes. Much of this is a result of marketing by the companies that produce and sell the products. Labels on finishes vary wildly and seem to be based more on how much wine was consumed by the marketing department rather than the ingredients inside the tin. There are finishes labelled "Tung Oil" that contain no tung oil whatsoever and the label is apparently meant to imply the finish will look like tung oil after it is applied. Have a look at the labels on finishes at your local hardware store and you will quickly see what I mean.
The following five categories give a quick overview on the main finishes used on furniture.
1. Stains - Contain a colour pigment or dye and a thinner (e.g. water or mineral turpentine). Pigments also require a binder to stick the pigment particles to the wood. Ground earth is an example of a pigment. Dyes have the colour dissolved in liquid. Tea and coffee are weak dyes. Beetroot juice is a very strong dye, especially when dribbled onto a white shirt...
Image - Earth Pigment
2. Oils - The two common ones are tung oil and boiled linseed oil (which is not actually boiled these days). Both tung oil and boiled linseed oil have been used for centuries and come from the seeds of a tung oil tree and the flax plant.
Image - Tung Oil Tree
3. Varnishes - Come in a variety of flavours including alkyd, spar and the most commonly found polyurethane variety. Varnishes can be purchased as oil based or water based (easily identified by clean up method on the tin; oil based will say clean up with mineral turpentine/spirits). Water based polyurethane has the advantage that it does not colour the timber like oil based polyurethane which leaves a slight yellow/orange tinge to surface. Varnish is often combined with boiled linseed or tung oil and a thinner to produce a danish oil that is used on furniture.
4. Lacquer - Originally produced from the resin in trees, lacquer now uses nitrocellulose as the resin. Each coat partially dissolves the previous layer to form a hard and slightly flexible finish. It is often sprayed onto the timber and gives a similar appearance to polyurethane when dried. Lacquer is fast drying and needs many thin layers to build a suitable finish. The solvent used in lacquer is toxic and flammable.
5. Shellac - Obtained from the resin secreted from the female lac bug that is scraped off tree bark. Shellac has been used for thousands of years and is still used today. I use shellac as a finish on the insides of drawers, as it is one of the few finishes that does not give off gas and create an odour inside the closed drawer cavity.
Image - Lac Bug
Stains and oils get absorbed into the timber and do not build up a protective film on the surface. Varnish, lacquer and shellac all provide a protective coating on the surface of the timber. The more coats that are applied, the thicker the coating becomes. Thicker coatings mean better protection from moisture.
There are many other finishes used for special situations, but those listed above cover the main ones found in the paint section of your hardware store. Each type of finish has advantages and disadvantages with the exception of beetroot juice which has only disadvantages. If you want to read more on the subject, Box Flexner is a well regarded expert and has written some good books on finishing. I often read one if I am having trouble getting to sleep at night - seems to do the trick.