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196
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DE NATURA FOSSILIUM
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Foam
is produced now, as in former times, in many places. The best is made
in Misena and Bohemia. Dioscorides gives preference to that from Greece
which was produced at Mt. Laurium and called lauriotis. He places the Spanish second and the Puteolian that was made from sheets of lead, third.
Galen
believed that foam of silver was moderately drying but obviously it
neither warms nor cools. It is moderately cleansing and astringent.
Among the ores it has merits of a moderate order and is often mixed
with other substances that are either strongly astringent or biting. By
itself it is only used for diseases of the thighs. That made from
copper and lead, such as is produced in smelters where silver is parted
from copper is moderately warming. Although this variety, being of an
intermediate composition, tends to be warming plumbago is said
to be cooling according to the Latins, although the two do not differ
in other ways. When produced from copper and lead it is of uniform
composition.
The material the Latins call plumbago the Greeks call μολίβδαινα, each
word having been correctly derived from the words for lead. It is
produced from boiling lead when the upper part of the crucible takes up
the lead itself in the manner already described. It is not of uniform
color. The upper portion approaches the color of foam of silver, the
lower portion a gray color and the intermediate portion a mixture of
the two. The upper portion is the best, the intermediate, the poorest.
Both the intermediate and upper portions have a certain luster and a
reddish brown color when pulverized. When boiled in olive oil the color
changes to liver-brown. The lower portion with a lead-gray color is
impure.
Foam
of silver is sometimes melted and colored with pigments and then used
by potters to glaze the inside of their jars and by sculptors to glaze
the outside of their works. At one time the warming furnaces of the
Germans were made in this same manner. They also use the powder for
letter sand. Chemists make a powder from the gray portion of plumbago and call it plumbarius cinis. So much concerning foam of silver and lead.
Aerugo (verdigris), caeruleum (blue minerals), and cerussa (white
lead) are produced by treating metals with acid. The various kinds of
verdigris are made from copper. One variety is smooth, another full of
holes. One variety is called santerna since it is used in
soldering gold. The smooth variety is made in many ways. Sometimes
strong vinegar is poured into a dolium or similar jar and a copper
vessel placed over the top. It is better to use an arched vessel but if
none is available a flat bottomed vessel will serve. It should be clean
and without holes. After ten days the cover is removed and the
verdigris scraped off. It is also made by suspending strips of copper
in the dolium above the acid. These are left for ten days before the
verdigris is scraped from them. It is sometimes produced by covering
masses of copper or copper strips with wine that has gone sour. New
wine will not serve. Sometimes copper filings or thin sheets are mixed
with gold foil and then sprinkled with vinegar and stirred with a
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