Oil drying accelerants

The speed of drying of the paint layer depends largely on the type of oil used, but also on external conditions such as heat and light, the presence of which greatly accelerates drying.
Even under the most favorable external conditions, the drying speed of most oils is quite slow. (For pure linseed oil, the drying time under average conditions is several weeks.)
With the introduction of special catalytically accelerating substances into the oil, the drying time, such as linseed oil, can be reduced to 12 hours.
Substances that catalytically accelerate the drying of oils are called siccatives.
Metal salts, mainly lead, manganese and cobalt, are sometimes used as siccatives.
Linseed oil containing the addition of an appropriate amount of such a siccative is called linseed varnish and is widely used in painting and varnishing techniques.
The amounts of siccative necessary for sufficient drying of the oil depend on the type of oil and the siccative.
Linseed oil achieves maximum drying rate with 0.1% cobalt or 0.5% lead or 0.12% manganese.
In the case of tung oil, twice the amount of siccative is sufficient.
Additions of siccatives larger than the specified standard no longer affect the acceleration of drying, and this standard must not be exceeded, since oils containing excessive amounts of siccatives give a very unstable coating.
Sometimes, too, oil coatings containing an excess of siccatives also exhibit, over time, the so-called "secondary de-bonding", i.e., repeated partial softening, after a certain period of time, of the paint film due to the reversal of the colloidal coagulation process of the oxidized and polymerized oil.

Cobalt siccative

Dries quickly. Causes the oil film to dry from the top.
It gives a painting film that is soft and malleable, quite sensitive to water and alkali. Instead, it is very durable in solutions, not prone to release from the varnish even when exposed to moisture.

Lead siccative

Dries quite slowly. It gives hard and durable coatings, evenly hardened throughout.
It is quite unstable in oil solutions. Under the action of moisture, frost or contaminants in the oil, it is irretrievably precipitated out of solution and loses its effectiveness.

Manganese siccative

It is not sometimes used on its own, because at higher humidity it dries very slowly and clumps the fresh paint film into droplets.
Manganese siccative is used together with lead siccative. The resulting films are hard, flexible, fast-drying.
Per weight unit of oil, 0.4% lead and 0.1% manganese are used.
To improve lead and lead-manganese siccatives, an addition of about 0.05% calcium is used.
Lead, manganese or cobalt is introduced into the oil in the form of oil-soluble salts, mainly resinates, linolenates, naphthanates or metallic ricinoleates.
These siccatives are obtained by dissolving the appropriate amount of salts or metal oxides hot in molten rosin, linseed oil or castor oil, or in naphthenic acids.
Lead is introduced into the siccatives most often in the form of lead gley PbO. Manganese in the form of the mineral braunstein MnO2.
Cobalt is used in the form of acetate or oxide.
Calcium in the form of so-called hydrated slaked lime.
Dissolution is carried out at 220 ÷ 260°C with continuous stirring.
In addition to siccatives obtained by dissolving metal compounds directly in hot rosin, oil or naphthenic acids, so-called precipitated siccatives are sometimes used. They are obtained by acting aqueous solutions of soluble salts of relevant metals (lead, cobalt, manganese) on solutions of sodium resin, linseed, castor or naphthenic soaps.
By draining off the precipitates, washing and drying, finished siccatives easily soluble in oils are obtained.

Linseed varnish

It is obtained by directly dissolving in linseed oil the appropriate amount of melted or precipitated siccatives.
The temperature of the oil at the time of dissolving the siccative should be at least 150°C. This is a way of preparing hot varnish.
It is possible to dissolve drying preparations, such as linoleates and resinates (resinates), in turpentine or gasoline and add a certain amount of this substance to the oil.
In this way, a less colored oil is obtained.

French turpentine

Is an agent that accelerates the drying of oil by oxidation (it pulls large amounts of oxygen from the air) and at the same time bleaches the oil.
Larger amounts of French turpentine introduced into the paint film make it dull.
A paint film with siccatives is less durable.