Fattening of Colours, is a term employed by painters, and signifies a coagulation of the oil, which is occasioned by mixing it with several kinds of pigments : hence, when it has been kept for a considerable time, it becomes so viscid and glutinous, as to be wholly incapable of being worked, either with the brash or pencil. In this state, a due proportion of fresh oil should be added.

Colours will also fatten, after they have been laid on the proper ground ; so that one part of the oil will run off in small streams or drops, while the other adheres to the canvas with the colours, but without- drying. - This defect, we conceive, may lie remedied by evaporating the waterv parts of the oil, and grinding the colours more carefully, so as to prevent their precipitation.

Oils will likewise fatten, when they have been too long kept, or exposed to the sun and air.