Oil has the tendency to turn dark with time, thus turning the color dark also. The only way to reduce this tendency is to clarify the oil by long exposure to the sunlight. The early German painters used oil so clarified, and their pictures are the best preserved as to color of any that we have. But the drying is even slower with purified oil than with the ordinary oil.
It would be best, then, to use oil as little as may be in painting, and if you need a dryer, use it only as you actually need it in bad drying colors, and then very little of it.
The essences of turpentine and of petroleum may be used to thin the paint, and are preferable to oil, because they have less darkening tendency. They do not, however, bind the color so well, and the paint should not be put on too thinly with them. Usually there is enough oil ground with the pigment as it comes in the tubes to overcome any probability of the paint scaling or rubbing when thinned with turpentine, but in the slow-drying, transparent colors there will be a liability to crack. Moderation in the use of any and all vehicles is the best means of avoiding difficulty. Use vehicles only when you need them, not habitually, and then only as much as there is real need of. If you use oil, use the lighter oils, and expect some darkening in time. Prefer turpentine to oil, and expect your color to dry rather "dead," or without gloss, by its use. If you intend to varnish, this is all right. If you do not intend to varnish the picture, keep the color as near the pure tones as you can. The grayer the color, the more the "dead" or "flat" drying will make it look colorless.
=Varnishes.=--When the picture is done, after it is dry, varnishes are used to bring out the freshness of color, and to preserve the surface from outside influences of all sorts. A picture must be well dried before it is varnished, or it is likely to crack; six months is not too long to be safe. If you are in a hurry to varnish, use a temporary or retouching varnish.
The best varnish is necessary for use on pictures. Never use any except a varnish especially made for the purpose by a reliable colorman. Those made by Winsor and Newton may all be depended upon. Pay a good price for it, and don't use too much.