Making and Knowing
A minimal edition of BnF Ms Fr 640 in English Translation

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Sand from a mine

It wants to be well recooked for big metal objects. Some people burn it inside a furnace until it becomes quite black and they grind it on the porphyry. Other people burn it with asphalt, but when it is too burned, it does not mold so cleanly, because it lacks body, and is too thin. You can give body to the sand with tripoli or burned felt.