Why do Paints dry?

Manufacturer and builder 7, 1869

A reply to this question, from a source which unquestionably commands considerable respect, has recently appeared in one of our public journals. The question is an interesting one, and, as we believe the opinions advanced in the article to which we have referred are not in all respects correct, we offer no apology for giving our views on the subject.

It was proved long ago, by De Saussure, that linseedoil, when exposed to the air, became covered with a hard crust, and that thin crust is produced by the absorption of oxygen. Paint made from oil and coloring matter alone does not dry because it parts with any thing or because it gives off any vapor, but because it becomes hard by the action of the atmosphere. It is stated in a recent work on paints that oil does not form even the basis of a paint. This is more technical than judicious. Oil alone, if laid on in thin, successive coats, becomes very hard and forms a durable and impervious varnish, which protects the wood beneath almost as well as paint would do, at least so far as moisture and air are concerned. Against the sun's rays, however, it is a poor defense. But as it has been found impossible to apply a sufficient coat of this varnish in any thing like a reasonable time, the oil has in general been mixed with various colored powders or pigments, which thicken it and thus enable us to lay on a heavier coat. Many of these pigments have no action on the oil, and it is always best , that they should have no action. Compounds of lead, which are known, to form chemical compounds with the oil, are amongst the very poorest paints. White lead is confessedly one of the least efficient of all our preservative agents, the authority referred to to the contrary notwithstanding.

When paint is applied to a fresh surface of wood, it often appears to dry in a very short time. In this case, however, it will be found that the paint has not really dried, but that the oil has been absorbed by the wood; and in this case the. pigment is often left in the form of a friable powder, loosely adherent to the surface to which it was applied. It rubs off very easily. This occurs to a less extent with white lead than with any other paint, simply because the lead combines with the oil and holds it on the surface, thus preventing its sinking in. We are inclined to regard this feature as one which confers no advantages upon white lead. It is probably better for the wood that as much oil as possible should soak into it, and it is no great disadvantage that the paint of the first coat should not adhere strongly. Where economy is an object, the absorption of the 'oil is prevented by first applying a coat of cheap sizing. The size fills up the pores of the wood, and prevents the sinking in of the oil. For in-door work, this answers very well, but for out-door purposes it is objectionable.

But all paints do not dry in the manner that we have mentioned. For in-door work, where it is desirable that the paint should dry rapidly and have a dead or non-reflecting surface, paint is generally mixed with turpentine. This is a volatile oil, which passes off rapidly when exposed to the air, and thus leaves the paint behind as a thin crust. The ability of the paint to resist atmospheric influences is thereby lessened; but this, for in-door work, is a matter of no conse-quence. It would be wrong, perhaps, to say that the oil of turpentine passes off entirely by evaporation, as a small portion probably becomes oxidated and re-mains behind. The amount so retained is, however, very small.

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