Varnish is a clear, pale solution of a resinous substance dissolved in either oil, turpentine or alcohol. The resinous substances are amber, common resin, copal, lac or shellac. The solution spread on the surface after drying forms a hard, transparent and glossy film on the varnished surface.
12.3.1 Functions of Varnishes
In general, varnish is applied on wooden surfaces of doors, windows, etc. to decorate the surface without hiding the beautiful grains of wood and at the same time to protect the surface from atmospheric effects. Sometimes painted surfaces are also varnished to enhance the appearance of the paint and also to increase the durability of the painted surface.
As the preparation of varnish is tough, readymade varnishes are used.
12.3.2 Constituents of Varnish
The three ingredients of a varnish are: resins or resinous substances, solvents and driers.
1. Resins
As stated earlier, the commonly used resins are copal, lac or shellac and resin. Copal is available from the earth at places where pine trees existed in the past. It is a hard substance in a variety of forms. Lac or shellac is obtained by the exudation of some insects. Resin is obtained from pine trees.
2. Solvents
Based on the type of resin, the type of solvent is decided. Table 12.1 shows the solvent for different resins.
Table 12.1 Solvents for different resins
3. Driers
To accelerate the process of drying, driers are added to the varnish. Common driers used in varnishes are litharge, white copper and lead acetate.
12.3.3 Properties of an Ideal Varnish
The properties of an ideal varnish are as follows:
- It should always provide a glossy surface and be pleasing in appearance.
- It should dry fast and provide a uniform finished surface.
- When exposed to the atmosphere, the colour of the varnish should not fade.
- The developed thin film on the surface should be tough, hard and durable.
- During or after drying it should not show cracks or shrink.
12.3.4 Types of Varnishes
Based on the solvent used, varnishes may be classified as given below:
1. Oil Varnishes
Oil varnishes are prepared from the hardest resin or gums, such as amba, copal, etc., by heating and dissolving them in linseed oil with turpentine. Heat is added to increase the workability. Although it dries slowly, it is the hardest and most durable of all varnishes. In general, it can be used both for interior and exterior works. It is preferred for joinery and fitting purposes and for all outside works.
2. Turpentine Varnishes
These varnishes are prepared from soft resins such as gum dammar, mastic, common resins, etc., by dissolving them in pure spirits or turpentine. These varnishes are flexible, lighter in colour and quick in drying. Compared to oil varnishes they are weak.
3. Spirit Varnishes
These varnishes are also called lacquers. These are prepared from soft resins such as lac or shellac dissolved in methylated spirit. They are harder, dries quickly and brilliant in appearance. These are easily affected by weathering actions.
4. Water Varnishes
These varnishes are prepared using shellac, which is dissolved in hot water and mixed with an adequate quantity of ammonia or borax, potash or soda such that the shellac gets dissolved. Water varnishes are used to varnish wallpapers, maps, pictures, etc.
5. French Polish
This is a refined spirit varnish. It is prepared by dissolving 0.15 kg of black or light brown shellac in one litre of methylated spirit without heat. The polish is obtained by adding a suitable colour pigment. This polish dries out quickly within a few minutes. It gives a fine glossy surface. It is mostly used for superior wood works like furniture, hand rails, etc.
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