Whatever the garnish, the quality of eggs in aspic depends absolutely on the quality of the jelly. And, however sapid the jelly, all of its delicacy depends on the precise degree of natural gelatin contained in it: The jelly must be only just firm enough to hold its form easily, once unmolded—and it should tremble. No matter how rich and pure the flavor, an excess of gelatin will produce a rubbery and distinctly unpleasant material. A trembling jelly melts luxuriously on the tongue; a rubbery jelly smears it with gum.
A jelly may be prepared exactly like a stock with the addition of a calf’s foot (first covered with cold water, brought to a boil for 5 minutes, drained and rinsed before being added to the other ingredients), but the most succulent of jellies is essentially a by-product of several different preparations. A plentiful veal stock (the meat from which may be removed when done and put into a tomato sauce, sorrel sauce, blanquette, etc., or simply accompanied by boiled vegetables, like a pot-au-feu) can be subsequently used to moisten a pot-au-feu (including gelatinous cuts—shank, oxtail, shoulder) and, still again, to moisten a poule-au-pot (or chicken parts may be thrown into the pot from time to time during the other preparations. For each subsequent preparation, the stock, completely cleared of all fat when cold, should be warmed only enough to melt before adding the meat; vegetables should be added to each new preparation, but the head of garlic and the cloves should be eliminated. You will judge from tasting whether a new bouquet garni should be added each time and what and how much it should contain. Water may have to be added to provide sufficient moistening. I often keep a stock of this sort for weeks on end, using it, supplementing it, and, when flavor and gelatinous content coincide in the correct degree of concentration (the jelly quite firm, inasmuch as it will be somewhat lengthened by an addition of wine, the flavor rich but not so concentrated as to resemble a demi-glace), profiting to prepare some eggs in jelly. Obviously, one need not piously await the ideal moment; reduction can bring it about. A stock that is to be kept, unused, for some time should be reboiled every 2 or 3 days, simmered for about 15 minutes with regular skimming, returned to a non-metallic (unless it be stainless steel) container, cooled, and kept uncovered in the refrigerator. Submitted two or three times to this purifying process, the stock rapidly bypasses the respectable meat-jelly stage, becoming a demi-glace, invaluable for enriching sauces but too rich and too gelatinous to serve as a jelly. (To prepare a sauce demi-glace, which is nothing more than an honest Espagnole, it should be thickened slightly with a flour and butter roux before arriving at the demi-glace stage, simmered, and skimmed—or “skinned”—for a good hour; a bit of tomato is often added to enrich the color and to hasten the purification. Reduced to a meat glaze [glace de viande] by progressive transferrals to smaller saucepans and continued reduction to the state of a thick syrup [cooled, it is the consistency of fairly hard rubber; it can be kept, refrigerated, almost indefinitely and is one of the sauce-enrichening standbys of classical cuisine]. It should not be imagined that it undergoes no physical change other than that of concentration: dilution cannot transform it back into what it once was.)