| and transparent color of coffee and loses the muddy look it had earlier in the process.. If there is no means of gaging the heat, as with a thermometer, it is important to see that the oil smokes only very slightly. Too long and too hot a cooking (as high as 280 degrees) will lend to a sort of rubberiness in the oil and the product will be unfit for use in painting.. It is difficult to give precise instructions as to the length of time this oil should be cooked. The masters always spoke of a "low fire," whatever they meant by that, and they spoke of also of two hours of cooking. When the oil has become a dark brown and the foam which partly covers it has taken on a the golden tone, the lead should be entirely combined with the oil and the operation might be considered as finished. But with this is a matter of individual taste and another artist might have a preference for an oil that has been cooked for a longer or shorter time. Over a low fire the cooking might be allowed to last for from two to three hours to obtain a thicker and more brilliant product. After the cooking is finished, the temperature of the oil must come down to 75 degrees centigrade before it can be poured into a bottle.. Whether the black oil is made with litharge or with ordinary white lead, the results are almost identical to the brush, but the oil made with litharge is stronger and more siccative. The cooking transforms the oil into a kind of soap in which the glycerin has remained incorporated. When rubbed by itself onto the canvas the oil will produce alight foam or lather, but after a few seconds this will disappear. When it has completely cooled, the bottle must be closed as tightly as possible to prevent oxidation and, in consequence, the thickening of the oil. Watin specified that this oil should not be used until the lead, which it contained in suspension, was all deposited in the bottom. And de Mayerne, in one of the receipts which he gave, recommended the addition of a little essence of spike, when the oil was cool, to ease and facilitate the deposit of the lead. In oils that have not been cooked much, this deposit takes place in two or three days, but it takes longer for oils that have been cooked longer. It is definitely recommended never. |