Wood is rubbed or coated with a finish to slow the movement of moisture between the air and the treated wood. Finishes also protect wood from stains and physical damage, such as scratches. There are two common ways of finishing wood. The first is to use in-surface finish, such as tung oil, and the second is to use on-surface finish, which includes polishes like urethane and shellac.
Our preferred in-surface finish is tung oil. It was more commonly used in the past than it is today: tung oil was rubbed into the Great Wall of China. It is a natural oil that brings out the colour of wood, which deepens as it ages. As an oil, it prevents moisture from accumulating in the wood. When you touch a piece of furniture that is finished with an in-surface finish, you are touching the wood, not a finish.
So, we normally finish tables and other hard-use surfaces with both tung oil and an on-surface finish. The latter protects the wood’s surface, but on its own tends to trap moisture inside the wood. To prevent this, we rub or even soak the wood in tung oil before applying an on-surface finish.
The most common on-surface finish is urethane (modern varnish). Urethane is a plastic, which dries in air. It is more water, alcohol, abrasion, and heat resistant than shellac. Shellac is secreted by the Lac bug, and gathered by melting the sticky substance off bushes (this is done in the Indian sub-continent). It is environmentally friendly, and easy to make. It also brings out and enriches the colour of the wood. Looking at a shellac-finished surface, it seems like you can see deep into the wood.
Methyl alcohol is the solvent for shellac. Once applied, the alcohol evaporates, and leaves the shellac on the surface of the wood. As it is a solvent, you must be careful to wipe wine or liquors away if they are spilled on one of these surfaces. The on-surface finishes, are easily repaired if scratched or discolored.
Right use of finishes reveals the beauty of the wood, and makes it endure throughout the life of the piece by preventing damage to its surface. See the Oak Lectern unfinished, and with an oil stain and urethane finish.