If you don’t add enough alum de plume or crocus your mold will easily crack when heated. But if your sand is mixed as it ought to be, the mold will sustain itself. You’ll know it’s of good quality when, while still hot after casting, it immoderately soaks up the water when dipped and breaks easily, because the alum and the crocus render it spongy. To the contrary, bad sand, which is composed of plaster and brick and is not well mixed with alum, breaks easily in fire and hardens in water. If possible, cast all at once.