when it is used, a great increase of it is apt to appear some time afterwards when the oil is not good. There are three changes which oils of the kind proper for painting are liable to suffer in their nature, and which affect them as vehicles, that are mentioned by painters under one term, that of fattening; notwithstanding, these several changes are brought about by very different means, and relate to very different properties in the oils.
The first is a coagulation by the mixture of the oil with some pigment improperly prepared. This, indeed, is called the fattening of the colors, but the real change is in the oils and the pigments are only the means of producing it. This change is generally a separation of the oil into two different substances, the one a viscid body which remains combined with the pigments, the other a thin fluid matter which divides itself from the color and thicker part.
This last appears in very various proportions under different circumstances, and, in some cases, it is not found where the pigments happen to be of a more earthy and alkaline nature, for then only a thick clammy substance that can scarcely be squeezed out of the bladder, if it is put up in one, is the result of the fattening. This fattening not only happens when oil and pigments are mixed together in bladders or vessels, but sometimes, after they have been laid on the proper ground for them, instead of drying, the separation will ensue, and one part of the oil will run off in small drops or streams, while the other will remain with the color, without showing the least tendency to dry.
The second is the change that takes place in oils from long keeping. This, if it could be afforded by the oil-manufacturer or the painter, is by far the best method of purifying linseed and other oils, as, by thus keeping, they become lighter colored and acquire a more unctuous consistence; and, though they are said to become too fat, they are in a very different state from that before mentioned, which is caused by unsuitable pigments.
