An oil paint essential consists of the following ingredients:
(1) a base,
(2) a vehicle or carrier,
(3) a drier,
(4) a colouring pigment, and
(5) a solvent.
(1) Bases: A base is a solid substance in a fine state of division and it forms the bulk of a paint. It determines the character of the paint and imparts durability to the surface which is painted. It reduces shrinkage cracks formed on drying and it also forms an opaque layer to obscure the surface of material to be painted.
(2) Vehicles: The vehicles are the liquid substances which hold the ingredients of a paint in liquid suspension. They are required mainly for two reasons:
(i) to make it possible to spread the paint evenly and uniformly on the surface in the form of a thin layer; and
(ii) to provide a binder for the ingredients of a paint so that they may stick or adhere to the surface.
(3) Driers: These substances accelerate the process of drying. A drier absorbs oxygen from the air and transfers it to the linseed oil, which in turn, gets hardened.
The various patented driers are available in the market. They may be either in the form of soluble driers or paste driers. The former driers are compounds of metals such as cobalt, lead, manganese, etc. Dissolved in linseed oil or some to her volatile liquid. The latter driers are compounds of the same metal. But they are mixed with inert fillers such as barites, whiting, etc. and then weight of inert filler in a paint should be not exceed one-fourth the weight or base. They are used for the following purposes:
(i) to bring down the cost of paint;
(ii) to improve the durability of paint;
(iii) to modify the weight of paint; and
(iv) too prevent shrinkage and cracking.
The litharge, red lead and sulphate of manganese can also be sued as driers. The litharge is the most commonly used drier, the proportion being 1.25 N to 5 liters of oil. The red lead is less effective than litharge and it is to be used when its addition does not interfere with the tint of the paint. The sulphate of manganese is used with zinc paints so as to eliminate the risk of discolorat
The turpentine is inflammable, evaporates rapidly and dries the oil consequently. The use of a thinner in paint reduces the protective value of the coating, flattens colour and lessens the gloss of the linseed oil as the spirits evaporate leaving an excess of colour not mixed with the oil.
The turpentine is a transparent volatile liquid and it is obtained by distilling the resinous exudation of some varieties of pine trees. It has a pungent odour and it often adultered with mineral oils and some of them have higher penetrating values but are otherwise inferior. The benzene and naphtha are used as substitutes.
A solvent or thinner is not generally used in finishing coats on the exposed surfaces as it has a tendency to impair or damage or injure the firmness of the paint. But if the surface is to be exposed to the sun, the turpentine is added to reduce the possibility of the paint blistering.
Following are the simple tests for ascertaining the purity of turpentine:
(i) On evaporation, it should not leave any residue.
(ii) The paper coated with turpentine and left to dry should remain unstained and should then take ink freely.
(iii) When shaken vigorously, it should not forth i.e. form a mass of bubbles.
(iv) When warmed gently, it should not smell of resin or coal tar.