BOOK XII.
565
This kind of earth having first been dug up in such quantity as would
make three hundred wheelbarrow loads, is thrown into two tanks ; then the
water is turned into them, and if it (the earth) contains vitriol it must be
diluted with urine. The workmen must many times a day sur the
ore with long, thick sticks in order that the water and urine may be
mixed with it ; then the plugs having been taken out of both tanks, the
solution is drawn off into a trough, which is carved out of one or two trees.
If the locality is supplied with an abundance of such ore, it should not
immediately be thrown into the tanks, but first conveyed into open spaces
and heaped up, for the longer it is exposed to the air and the rain, the better it
is ; after some months, during which the ore has been heaped up in open
spaces into mounds, there are generated veinlets of far better quality than
the ore. Then it is conveyed into six or more tanks, nine feet in length
and breadth and five in depth, and afterward water is drawn into them
of similar solution. After this, when the water has absorbed the alum, the
plugs are pulled out, and the solution escapes into a round reservoir forty
feet wide and three feet deep. Then the ore is thrown out of the tanks
into other tanks, and water again being run into the latter and the urine
added and stirred by means of poles, the plugs are withdrawn and
the solution is run off into the same reservoir. A few days afterward,
the reservoirs containing the solution are emptied through a small launder,
and run into rectangular lead caldrons; it is boiled in them until the
from this description exactly how they were separated. In a condensed solution allowed to
cool, the alum would precipitate out as "alum meal," and the vitriol would "float on top "—
in solution. The refereree to " meal " may represent this phenomenon, and the re-boiling
referred to would be the normal method of purification by crystallization. The " asbestos "
and gypsum deposited in the caldrons were no doubt feathery and mealy calcium sulphate.
The alum produced would, in any event, be mostly ammonia alum.
The second process is certainly the manufacture from " alum rock " or " alunite "
(the hydros sulphate of aluminium and potassium), such as that mined at La Tolfa in the
Papal States, where the process has been for centuries identical with that here described. The
alum there produced is the double basic potassium alum, and crystallizes into cubes instead of
octrahedra, i.e., the Roman alum of commerce. The presence of much ferric oxide gives the
rose colour referred to by Agricola. This account is almost identical with that of Biringuccio
(II., 4), and it appears from similarity of details that Agricola, as stated in his preface, must
have "refreshed his mind " from this description ; it would also appear from the preface that
he had himself visited the locality.
The third process is essentially the same as the first, except that the decomposition
of the pyrites was hastened by roasting. The following obscure statement of some interest
occurs in Agricola's De Natura Fossilium, p. 209 :—". . . . alum is made from vitriol,
" for when oil is made from the latter, alum is distilled out (expirât). This absorbs the clay
" which is used in cementing glass, and when the operation is complete the clay is macerated
" with pure water, and the alum is soon afterward deposited in the shape of small cubes."
Assuming the oil of vitriol to be sulphuric acid and the clay " used in cementing glass " to
be kaolin, we have here the first suggestion of a method for producing alum which came into
use long after.
" Burnt alum " (alumen coctum).—Agricola frequently uses this expression, and on p.
568, describes the operation, and the substance is apparently the same as modern dehydrated
alum, often referred to as "burnt alum."
Historical Notes.—Whether the Ancients knew of alum in the modern sense is a
most vexed question. The Greeks refer to a certain substance as stypteria, and the Romans
refer to this same substance as alumen. There can be no question as to their knowledge and
common use of vitriol, nor that substances which they believed were entirely different from
vitriol were comprised under the above names. Beckmann (Hist, of Inventions, Vol. 1.,
p. 181) seems to have been the founder of the doctrine that the ancient alumen was vitriol,
and scores of authorities seem to have adopted his arguments without inquiry, until that belief
