Unless you are a watchmaker, when you think of shellac you probably picture a finely finished piece of furniture, or maybe a fine guitar or violin. As a watchmaker I use it too, but in a different form.
Shellac is a resin secreted by insects of the lac family. Two common members of that family are Tachardia lacca and Kerria lacca. The resin is a secretion from the female deposited on the bark of trees. Most of the shellac harvested in the world comes from Thailand and Southeast Asia. Once harvested the shellac is purified into clean flakes known as seedlac.
One of the great properties of shellac is that it can be dissolved in some alkaline solutions as well as some organic compounds like alcohol. When dissolved in alcohol shellac can be applied to woods and forms a hard surface (lacquer). Shellac is a natural polymer considered a plastic. It softens when heated but returns to a very hard substance when cooled. Over time and after repeated heating and cooling the shellac breaks down and ceases to from strong bonds.
In watchmaking we take advantage of shellac’s bonding capabilities as well as its ability to be softened and re-hardened. Shellac is commonly used to hold pallet stones in place as well as roller jewels. In this application the stones are adjusted in their slots to provide the proper depth when interacting with the escape wheel teeth and then a small amount of shellac is melted to hold them in their position. If the pallet fork needs to be further adjusted the pallet fork can be heated and the stones can be moved, the shellac will then re-harden holding the stones securely in place. Since shellac dissolves in alcohol care should be taken when cleaning a watch not to use alcohol on the pallet fork or balance assembly.