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246
BOOK VII.
in the middle of it. As soon as the borax crackles and the lead-stone melts,
which soon occurs, remove the coal from the crucible, and the lead will settle
to the bottom of it ; weigh it out, and take account of that portion of it
which the fire has consumed. If you also wish to know what portion of silver
is contained in the lead, melt the lead in the cupel until all of it exhales.
Another way is to roast the lead ore, of whatsoever quality it be, wash
it, and put into the crucible one centumpondium of the concentrates, together
with three centumpondia of the powdered compound which melts ore, mixed
together, and place it in the iron hoop that it may melt ; when it has cooled,
cleanse it of its slag, and complete the test as I have already said. Another way is
to take two unciae of prepared ore, five drachmae of roasted copper, one uncia oi
glass, or glass-galls reduced to powder, a semi-uncia of salt, and mix them. Put
the mixture into the triangular crucible, and heat it over a gentle fire to
prevent it from breaking ; when the mixture has melted, blow the fire
vigorously with the bellows ; then take the crucible off the five coals and
let it cool in the open air ; do not pour water on it, lest the lead button being
acted upon by the excessive cold should become mixed with the slag, and the
assay in this way be erroneous. When the crucible has cooled, you will find
in the bottom of it the lead button. Another way is to take two unciae of
ore, a semi-uncia of litharge, two drachmae of Venetian glass and a semi-uncia
of saltpetre. If there is difficulty in melting the ore, add to it iron filings,
which, since they increase the heat, easily separate the waste from lead and
other metals. By the last way, lead ore properly prepared is placed in the
crucible, and there is added to it only the sand made from stones which easily
melt, or iron filings, and then the assay is completed as formerly.
You can assay tin ore by the following method. First roast it, then
crush, and afterward wash it ; the concentrates are again roasted, crushed,
and washed. Mix one and a half centumpondia of this with one centumpondium of the chrysocolla which they call borax ; from the mixture,
when it has been moistened with water, make a lump. Afterwards,
perforate a large round piece of charcoal, making this opening a palm deep,
three digits wide on the upper side and narrower on the lower side ; when
the charcoal is put in its place the latter should be on the bottom and the
former uppermost. Let it be placed in a crucible, and let glowing coal be
put round it on all sides ; when the perforated piece of coal begins to burn,
the lump is placed in the upper part of the opening, and it is covered with a
wide piece of glowing coal, and after many pieces of coal have been put round
it, a hot fire is blown up with the bellows, until all the tin has run out
of the lower opening of the charcoal into the crucible. Another way is to
take a large piece of charcoal, hollow it out, and smear it with lute, that the
ore may not leap out when white hot. Next, make a small hole through the
middle of it, then fill up the large opening with small charcoal, and put the
ore upon this ; put fire in the small hole and blow the fire with the nozzle of
a hand bellows ; place the piece of charcoal in a small crucible, smeared
with lute, in which, when the melting is finished, you will find a button
of tin.