Alum is a substance which considerably mystified the ancient chemists,
who knew the salt but did not understand its composition. Ancient
writers like Pliny and Dioscorides were acquainted with a product which
the former called alumen and which is evidently the same as had been
described by Dioscorides under the name of Stypteria. Pliny says there
were several varieties of this mineral used in dyeing, and it is clear
from his account that his alumen was sometimes sulphate of iron and
sometimes a mixture of sulphate of iron with an aluminous earth. It is
the fact that where the various vitriols are found they are generally
associated with aluminous earth.
Alum as we know it was first prepared in the East and used for dyeing
purposes. Alum works were in existence some time subsequent to the
twelfth century at a place named Rocca in Syria, which may have been a
town of that name on the Euphrates, or more probably was Edessa, which
was originally known as Roccha. It has been supposed that it was the
manufacture of alum at this place which bequeathed to us the name of
Rock or Rocha alum, but the Historical English Dictionary says this
derivation is “evidently unfounded.”
The alchemists were familiar with alum and knew it to be a combination
of sulphuric acid with an unknown earth. Van Helmont was the first to
employ alum as a styptic in uterine hæmorrhage, and Helvetius made a
great reputation for a styptic he recommended for similar cases. His
pills were composed of alum 10 parts, dragon’s blood 3 parts, honey
of roses q.s., made into 4 grain pills, of which six were to be taken
daily. Alum and nutmeg equal parts were given in agues. Paris says the
addition of nutmeg to alum corrects its tendency to disturb the bowels.
It has also been advocated in cancer and typhoid, but these internal
uses have been generally abandoned. Spirit of Alum is occasionally met
with in alchemical writings. It was water charged with sulphuric acid
obtained by the distillation of alum over a naked fire.
Until the fifteenth century the only alum factories from which Europe
was supplied were at Constantinople, Smyrna, and Trebizonde. Beckman
relates that an alum factory was founded in the Isle of Ischia, on
the coast of Tuscany, by a Genoese merchant named Bartholomew Perdix,
who had learnt the art at Rocca. Very soon afterwards John de Castro,
a Paduan who had been engaged in cloth dyeing at Constantinople but
had lost all his property when that city was captured by Mahomet II
in 1453, was appointed to an office in the Treasury of the Apostolic
Chamber, and in the course of his duties found what he believed to be
an aluminous rock at Tolfa, near Civita Vecchia. He asked the Pope,
Pius II, to allow him to experiment, but it was some years before the
necessary permission was granted. When at last the truth of Castro’s
surmise was established the Pope was greatly interested. He looked
upon the discovery as a great Christian victory over the Turks, and
handsomely rewarded de Castro, to whom, besides, a monument was erected
in Padua inscribed “Joanni de Castro, Aluminis inventor.” The factory
brought in a splendid revenue to the Apostolic exchequer, and the Pope
did his utmost to retain the monopoly, for when in consequence of
the extravagant prices to which the Tolfa alum was raised merchants
began again to buy the Eastern product his Holiness issued a decree
prohibiting Christians from purchasing from the infidels under pain
of excommunication. Later, when, in Charles I’s reign, Sir Thomas
Challoner discovered an aluminous deposit near his home at Guisborough
in Yorkshire, and persuaded some of the Pope’s workmen to come there to
work the schist, he and those whom he had tempted away were solemnly
and most vigorously “cursed.”
Meanwhile the nature of the earth with which the sulphuric acid was
combined remained unknown to chemists. Stahl worked at the problem and
came to the conclusion that it was lime. The younger Geoffroy, a famous
pharmacist of Paris, ascertained (1728) that the earth of alum was
identical with that of argillaceous earth and Alumina was for some time
called Argile. Marggraf observed that he could not get alum crystals
from a combination of argile and sulphuric acid, but noting that in
the old factories it had been the custom to add putrid urine to the
solution, for which carbonate of potash was subsequently substituted,
went so far as to make the salt, but did not appreciate that it was
actually a double salt. The name alumina which the earth now bears
was given to it by Morveau. It was Vauquelin (another pharmacist) who
clearly proved the composition of alum, and Lavoisier first suggested
that alumina was the oxide of a metal. Sir Humphry Davy agreed with
this view but failed to isolate the metal. Oersted was the first to
actually extract aluminium from the oxide, but his process was an
impracticable one, but in 1828 Woehler, and in 1858 Deville, found
means of producing the metal in sufficient abundance to make it a
valuable article of industry.