Superimpose several layers of paint with it, it is better not to abuse these superimposition's, and it is recommended not to color with the jelly any portions of the painting but those that are to be repainted.
The jelly itself makes an ideal vehicle for glazing, as described in the account of Rubens' method of working. The artist using this jelly may find that although it will dry, it may sometimes (particularly in hot weather) remain sticky for several weeks. This may be due to the fact that our oils today are subjected to such high degree of pressing that they differ in quality from the oil of the old masters and so produce different reactions. Or it could be due to the turpentine which today is so highly rectified that it dissolves too much of the mastic. We do not know yet, but pending the solution of this problem, the artist is cautioned to exercise restraint in the superimposition in his painting.
In the, connection it is interesting to read, in a letter written to, Phillip 1V of Spain, by the Cardinal Infante, Ferdinand, ruler of the Netherlands during Rubens' latter years, The following comments: "I have a little dispute with Rubens because he says that although all the work is finished, we artists wait until it is dry or the pictures will be spoiled when they are rolled. He thinks it will take twenty days or a month, as the sun only coming out now as by a miracle, they will not dry in less time. I tried to argue with him as much as possible, but as he knows more than I do, I had to give in." From this we can take comfort that such technical an annoyances are not a monopoly of our own time.
Van Dyck, being a pupil of Rubens, used essentially the same technique and materials as his master but, as has been shown in the discussion of the differences between his medium and that of Rubens, visible under the raking light, Van Dyck had a much lighter black oil. This was prepared in the same way as Rubens prepared his, but it is definitely known to have included a small percentage of lead, and to have been cooked less.