When evaluating and caring for antiques, an important factor to consider is the type of finish that has been applied to the wood. The finish can be a clue to the age of the piece and will dictate how the piece should be cared for.
Types of finishes:
Wood furniture is finished to protect the wood and/or to decorate it. Wood is a porous material and unfinished wood will absorb dirt and moisture. The finish is intended to create a barrier against dirt and moisture. The finish can also decorate the wood by adding color, if a stain is added, or shine.
Furniture finishes fall into two categories: oil and film. Oil finishes penetrate and form a barrier just below the surface. They also leave a very thin film on the wood. Oil finishes include straight oil, such as linseed oil.
Oil finishes do not offer much protection for the wood, because the film on the surface is very thin. Linseed oil was used long ago because it was inexpensive and readily available. Oil finishes are still used today on new and antique furniture, because they are very easy to apply and give a very natural look the the finished wood.
Film finishes create a film on the surface of the wood that can be built up to the desired thickness by applying successive coats of finish. Film finishes offer better protection because they leave a thicker film on the surface of the wood, which protects against water and scratches. Varnish and water base are common film finishes.
Most of the better 18th century furniture and almost all of the 19th century furniture is finished with shellac or varnish. Water base finish is a development of the 20th century and resulted from concerns over pollution, so it would not be used on antique furniture by enthusiasts.
Shellac was the favored finish throughout the 19th and early 20th century. Shellac is a natural resin secreted by lac bugs. Shellac is easy to apply. In the early 19th century, a technique was invented for rubbing shellac to a very high shine, called French Polishing. Despite it's potentially beautiful appearance, shellac is very susceptable to damage by alcohol, water and heat, and is easily scratched.
Varnish is the most durable of the common film finishes. It is made from oil and has been used as a finish since the 19th century, but the resins changed from natural ones to synthetic ones in the 20th century. There are now several kinds of varnish sold - all from different resins. Polyurethane is probably the best known
Finally, although wax is usually applied over a finish as a polish, it is sometimes used as a finish, because it has the least affect on the wood's appearance. It offers virtually no protection to the wood against moisture and is primarily decorative.
Source: Understanding Wood Finishing, by Bob Flexner