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Red Pigments
A useful range of red pigments is made from natural or artificially prepared oxides of iron. When properly prepared, they have great staining power, durability, resistance to light, acids, and alkalis, opacity and spreading power.Red Oxide of Iron.-Natural deposits of oxides, containing varying proportions of oxide of iron, are found in many parts of the world, including this country.
Most of them have a high degree of purity and thus require a minimum of treatment before grinding.
The pigment has long found extensive use in the making of protective coatings for iron and steelwork,
although it is now generally recognised that, by itself, it possesses little, if any, rust-inhibitive power: mixed with a pigment such as red lead or zinc chromate it gives a valuable anti-corrosive paint.
Other natural oxides of iron, as chocolate, purple-brown, etc.. serve equally well for a similar purpose.Indian Red.-The true Indian red is made from a natural earth found in India, but nowadays this red is more commonly manufactured by the calcination of yellow ochres and ferrous sulphate. It varies in colour but is usually of a somewhat darkish tone, often with a slight purplish hue.
Venetian Red.- This is also mainly artificially prepared on a principle similar to that followed for Indian red. It is rather brighter and lighter than the latter, and is widely used for tinting and also for distemper colours.
Turkey Red.-This has properties similar to those of Venetian red, but it is not so bright as the latter.
Burnt Sienna.- This valuable earth colour, is obtained by exposing raw sienna to a moderate heat.
The resulting product is of a reddishorange colour, though its shade depends on the source of the raw sienna, as well as the temperature at which it is calcined and the length of time for which the process is carried on.
It is extremely transparent in oil and thus finds extensive use as a glazing colour.
Red Lead.- This pigment, one of the oldest in the history of painting, is prepared from oxide of lead in two distinct stages.
In the first, the metal is converted in the furnace into yellow monoxide of lead, or massicot;
in the second, the massicot, on being further heated, takes up more oxygen and forms the familiar bright, strong red powder.
Probably the most important use of red lead in paint is in priming coats for iron and steelwork, for which. its anti-corrosive properties make it especially valuable.
A small proportion of red lead is usually added to white lead paint employed as a first coating on woodwork, to make the traditional" pink primer," still widely used in the trade; the presence of the red lead helps to produce a harder film and to assist the oxidation of the oil which has penetrated within the structure of the wood.
Red lead exerts a powerful drying influence on linseed oil and paints made with it set hard within a short time. In spite of its bright colour,
it is seldom used for purely decorative purposes, as it does not work well under the brush and is inclined to separate from the oil.
An extremely heavy pigment, it tends to settle in the container; for this reason, paint containing any considerable proportion of it must be constantly stirred when in use and should be made up only a short time before it is to be applied.
This difficulty has, however, been overcome in recent years by the introduction of what is known as " non-setting" red lead, which contains more oxygen and in which the pigment remains better in suspension in the medium.Orange Lead.-This pigment, the name of which describes its colour, is of much the same composition as red lead and has similar properties. It is employed as a rust-inhibiting primer on ferrous metalwork and also in the manufacture of vermilionette and other lakes.
Vermilion.- The use of this pigment dates from very ancient times and probably originated in China. It is a sulphide of mercury and provides a red of extreme brilliance.
It has great opacity and tinting strength and is normally fast to light, though occasionally it will blacken on exposure.
The conditions which bring about this discoloration are not yet completely understood but the change occurs only very rarely, and not at all when the paint has a protective film of varnish.
Its high cost and, to a minor extent, its rather coarse texture and heaviness greatly restrict its use and the introduction of permanent lake reds has made it more or less obsolete, so far as the decorator is concerned.