Not every parchment will take a single tempering of colour or glair, for this reason, that there are several sorts of parchment, namely, calf, sheep, goat. But when sheep or calf parchment is uniform in colour, that is, all white, and smooth and handsome, like that of Flanders and Normandy, for instance, the tempering described above is suitable. That is to say, you temper the vermilion with glair as you have found above; and when the glair is mixed with the vermilion and straightway laid on the parchment, it will be very fine in colour.
But when the parchment is rough, and mottled, and thin, and very ugly, and uneven in color, gray and black and white, like the sheep parchment of Burgundy, which is very seldom found well painted by anyone, you conduct the tempering of the glair as follows.
Put the yolk of an egg, separated from the white, upon a platter, and adding water, so that the yolk may lose its thickness in the water, and become lighter, you beat it with the little whisk almost like the other glair, until it is all whipped. Then you strain it through a medium cloth, and adding it to the red color you may use it at once. And I do not believe that you will find any parchment so bad that through the use of this yolk glair the olor will not shine, and give almost the effect of the most prized purple. True, this yolk glair is slippery stuff, and inseparable from its froth; whereas the other, which is produced during the beating of the white, dries up and sticks to the platter.