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Sand

The sand to be used for casting should be chosen such that it is not too dry for it won’t hold together, nor should it be too fat. And although you find some in nature, however, it is not everywhere. And if you are in a place where it is not found, you can put it together yourself but not from fat earth, because the sand should contain none at all, since it makes it porous. But you can make it bind by mixing it with brick well ground on marble, or plaster or calcinated alabaster or something similar, or the burned marrow of ox horn or burned asphalt. If you grind it quite finely on porphyry, it binds better & then you can burn it with asphalt or mix it with a quarter of tripoli. Make sure no bread falls into your sand because this makes it porous.

at left middle margin of folio 041r

Try mixing in soot black.