Shellac is a type of resin mainly used for restoration and surface treatment of wooden furniture.
It is formed from the excretions of the lac bug (Kerria lacca), an insect of the average size of an apple seed. During its reproductive phase, it sucks sap from the leaves of the trees. This liquid is excreted as an amber-colored resinous substance and forms a protective cocoon for egg laying, which is the base material for shellac.
The natural is great for sealing smooth surfaces: Several countries of southeast Asia, especially India and Thailand, used to extract shellac in large quantities. It was the first industrially used resin in the world and was used in the paint and varnish industry. Shellac became known primarily as a binder in records and also as their surface protection. In addition, it is still used today in the food and pharmaceutical industries. Shellac seals well and is harmless when used in food.
Shades and forms
After several processing steps, the yellowish-brown shellac is sold in small, thin, sheet-like fragments. They are translucent, glossy and solid. The product is also called raw shellac. Other forms are called flake shellac and button shellac. There are several color variants, for example, the blood-red ruby shellac. Orange coloration is also common. The varnish for furniture restoration can be achieved with and without wax. On a natural basis, the wax content is three to six percent. Bleached shellac reaches different levels and can even be colorless.
Properties under the influence of heat
At room temperature as well as below, shellac is brittle, hard and fragile. Under these conditions, it has no odor or taste. At 65 to 85 °C it begins to melt. During this process, it develops a pleasant odor. When shellac burns, it has a bright, luminous flame. It does not dissolve in water, but swells. Shellac is soluble in ethanol and ammonia, for example. However, the wax it contains remains behind. If it is kneaded and rolled out for restoration, shellac acquires a silky sheen. Because of its special properties, shellac is very well suited as a varnish coating on violins and other plucked instruments. On woodwind instruments, it also serves as an adhesive for cork or other coverings on metal parts.
Application and use as shellac polish
The surface treatment or restoration of antique pieces of furniture is done with a shellac polish. For this purpose, the varnish is mixed with ethanol. Shellac polish experienced its high time in the 19th century. Therefore, the repair or restoration of this furniture cannot be done with varnishes commonly used today. Even care of shellac polished surfaces with modern agents is not recommended, because they can easily attack the resin layer and thus the special surface gloss. For these reasons alone, a well-trained furniture restorer should always perform such work. A non-expert usually does not possess the necessary craftsmanship for a shellac polish and, on top of that, can cause irreparable damage to the valuable piece of furniture.
Pre-treatment for polishing
For a shellac polish, the wood surface must first be primed. This closes the wood pores. Then, especially on old furniture, the existing shellac layer is dissolved. The new layer is applied over this. Foreign varnish must be carefully softened beforehand and removed with a spatula. The wood in question is then carefully cleaned, usually with the help of alcohol. Any damage can be carefully burned out with a little shellac. Fine steel wool and suitable bronze brushes are also practical tools for pre-treatment of the shellac polish. Pumice powder is often used as a pore filler to close wood pores. After a resting period of about 24 hours, the treated wood surface can then be polished out. During this process, any oil residues are removed from the surface at the same time.
Working steps - Woolen polishing bales
During the entire shellac polishing process, the furniture restorer uses special bales made of pure wool. However, these polishing bales differ depending on their purpose. Different bales should also be stored separately. For example, residual pumice powder in the bale for base polishing could damage or scratch later layers of varnish. The polishing bales are soaked in a little oil and moved evenly over the entire area in circular motions. Every good professional will find the right balance here so that the already finished paint layer is not torn open again, for example if the bale is too dry. It is also important to work all corners and edges precisely with the polishing bale. This is the only way to achieve an even gloss over the entire surface in the end.
