This edited article about paint originally appeared in Look and Learn issue number 932 published on 1 December 1979.
The house painter at work
At one time it took 30 days to paint a coach and just as long to paint the first motor cars. The fact that it now takes less than four hours demonstrates how paint has responded to a changing world.
From its beginnings, 30,000 years ago, paint has been not only decorative but durable. Cave dwellers, Egyptians, Greeks and Romans all had a hand in making this remarkably versatile material and the equipment they used is still used today: clay pots for mixing, palettes, reed brushes and the all-important pigments.
With the coming of the Industrial Revolution, paint came to be regarded not merely as decoration, but as a vital form of protection.
Manufacturers experimented with new processes, with new lead and zinc-based paints to combat rust and corrosion. Paint had to cover enormous areas of iron and steel – and it had to be cheap.
As the 19th century advanced, however, it became clear that paint had its limitations. Its performance was no match for this new environment, nor for what was to follow – a revolution in the techniques of industrial production.
Mass production meant the end of paint-making as a traditional craft and the end of recipes derived from the Middle Ages. The craftsman had to give way to the scientist.
In today’s man-made world, paint plays a crucial role. Modern paints reduce corrosion while spray painting accelerates the speed of mass production. To achieve these ends, the raw materials of paint have changed drastically.
But what is paint? Basically, it has three main constituents: pigment, varnish or medium, and solvent. Pigment gives the paint its colour, covering power, and protective qualities; varnish gives the paint its flow and binds together the particles of pigment to form a tough film.
The ratio of varnish to pigment controls the degree of gloss of the paint. Varnishes are generally too thick for easy application and the paint is thinned with solvent so that it can be brushed or sprayed more easily. After the paint has been applied, the solvent evaporates and the film dries.
Raw materials are carefully weighed and measured, before being passed through one or more “milling” processes depending on the type of paint to be made. Milling disperses the pigment and other dry materials evenly through the varnish while breaking up any lumps into fine particles. The milling process also determines the gloss of the paint.
Four main methods are used in the manufacture of paint: the ball mill, the single roll mill, the sand mill, and a device called a high-speed disperser.
The ball mill is one of the oldest tools in the industry and is a cylindrical steel shell, either bare or lined with porcelain tiles, and filled to about 45 per cent of volume with either steel balls or beach pebbles. Porcelain ball mills use beach pebbles and are suitable for white and light colours, while the steel mills use steel balls and are reserved for the production of the darker shades. The ingredients are passed into the ball mill from a hatch on the floor above and, once filled, the mill rotates, allowing the balls to drop through the mixture in a regular cascade. The milling takes place by the action of the balls rubbing together. The process can take up to 20 hours, the time depending on the type of pigment.
Single roll mills are used for milling certain types of pigment. Here, the raw materials are first put into a “pug” mixer which mixes the ingredients but does not break up the pigment. The mixture is then fed into the roll mill, where it passes between a rotating steel roller and a bar which presses against it, rather like an old-fashioned laundry mangle.
One of the most modern techniques in paint manufacture involves the sand mill. In this machine, tiny particles of sand are circulated at a very high speed by spinning discs inside the cylinder.
High-speed dispersers distribute the pigment and, to a certain extent, mill it using specially designed blades rotating at high speeds. This type of machine is used for manufacturing undercoats and emulsions which require relatively little reduction of the pigment.
Whatever the milling process used, the resulting mixture, plus a proportion of solvent, is dropped into stationary tanks. Varnish and further solvent are then added, and the paint skilfully shaded (by eye) to the required colour. A sample of the paint is taken to the laboratory for rigorous tests for particle size, dry colour, viscosity, specific gravity, and flow characteristics.
The approved batches of paint are strained and passed down to the filling areas where the tins are labelled, filled, lidded and usually coded with a batch number – all in one operation.
The last stage of manufacture is reached when the tins are packed into cartons and sent to the warehouse before being dispatched to the customer.
Paint is older than the wheel. Its manufacture is now a great industry based on science – but it owes its existence to man’s desire to create and his age-old longing to decorate his dwelling and his possessions.