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The Drying Agent
Driers (also known as Dryers) are substances added to the mediums used in many paints and varnishes to enable the oil to absorb oxygen at a sufficient rate to dry within a reasonable time.
Without them, air-drying oil paints would, in most cases, take many days to dry, and ordinary painting processes would be quite unworkable.
The function of driers is to combine with oxygen and pass it on to the drying oil in the paint or varnish, thus accelerating the drying or oxidation of the film.
Once the latter has begun to harden, the absorption of oxygen is slowed down; if this were not the case and the oxidation continued at the initial rate, the film would become hard and brittle and would soon disintegrate; as it is, a certain amount of progressive oxidation goes on even after the film is hard because the driers, though less active, continue to exert their influence so long as further oxidation is possible.
The drying agents commonly used are mainly compounds of certain metals, notably lead, manganese, and cobalt.
Their properties vary in strength and character: lead compounds have a powerful action but are liable, in certain circumstances, to darken the oil to which they are added.
Manganese compounds do not bring about this discoloration but their
action, though energetic, is more erratic and less dependable than that of lead.
Cobalt has a stronger effect than either lead or manganese, but is also the most erratic, and for this reason is usually combined with lead.
It is common practice, in the making of paint, to combine two or more of these drying agents and thus to produce a more stable agent.
The compounds
referred to, and certain other drying agents, are used by manufacturers in the preparation of ready-mixed finishes made in bulk, but are too powerful to be employed by the painter in their original form. The driers which he buys are therefore substantially reduced.
They reach him in two main forms-paste (or patent) drier, and liquid drier.
Paste Drier
There are wide variations in the quality, efficiency, and formulation of paste driers, but in the main they consist of one or more of the drying agents referred to above, with the addition of inert pigments such asbarytes, Paris white, or similar pigments which have little opacity in oil and which can consequently be added to dark-coloured paints without making them noticeably lighter or paler.
The ingredients for paste driers are ground in oil, but they frequently contain small percentages of water which is added to dissolve the metallic drying salts.
For this reason, they are not compatible with gum and oil varnishes, and there is some risk in adding them to paints which contain the latter.
Paste driers are, however, widely used for decorators' paints and are popular with some grainers since they have the property of preventing the graining colour from flowing together after being combed, and thus act as a kind of megilp.
Liquid Driers
These include liquid oil driers, terebine, and gold size.Liquid oil driers uses linseed oil as the main reducing agent and is available in pale form to avoid discoloration of white or light-coloured paints.
It is not so powerful as terebine; this is prepared in various ways, usually by heating linseed oil, adding lead and manganese, and continuing the heat treatment until these are dissolved and the mixture very dark:
on cooling, it is thinned with turpentine and white spirit.
Pale terebine is also available.
Gold size, though not primarily a drier, is often used in that capacity.
It dries quickly to a hard film, without much gloss.