The primary object of varnishing an oil painting is to protect it, much in the same way as glass is put over a water-colour drawing; in fact, valuable or delicately painted oil pictures are often protected by glass, and a lot of future trouble saved. Mastic varnish is used for oil paintings because a thin coat is generally sufficient to bring out all the detail in the dark parts without giving a vulgar gloss. It has very little colour, and can be easily Amoved when necessary, which is not the case when a "durable" varnish, that is, one made from hard gums and drying oil, is used. An oil painting from the artist's studio should be carefully hung up to lean forward slightly, so as not to catch any dust, etc., certainly not over a fireplace or near a gas burner. At the end perhaps of about three years the surface dirt, fly spots, etc., should be removed with a clean wet cloth (not flannel) and a coat of varnish applied. This will protect the surface of the picture from future atmospheric influences; in fact, all dirt, etc.will be on the varnish instead of on the picture. Mastic varnish will sometimes "bloom," that is, the picture will be covered with a slight opalescent film.

This can be removed by breathing on a small portion at a time and gently rubbing in small circular strokes with a tuft of cotton wadding. Never partially varnish a picture, because even mastic will turn yellow with age, and show an objectionable distinction between what is varnished and what is not.

To finish oil paintings that have not been varnished, they should not, as a rule, be entirely coated with varnish, as this will tend to make them objectionably glossy. When a painting has become thoroughly dry, certain parts of it will be much duller than others, and these parts may be brightened by applying a little raw linseed oil with a hog's-hair brush. If the whole picture is dull and requires varnishing, a thin coat only of varnish may be put on. Both varnish and oil should be bought from an artists' colourmau.