Alum found at archaeological sites The western desert of Egypt was a major source of alum substitutes in antiquity. These
evaporites were mainly , , , , and . The
Ancient Greek Herodotus mentions Egyptian alum as a valuable commodity in his
Histories. The production of potassium alum from
alunite is archaeologically attested on the island
Lesbos. The site was abandoned during the 7th century CE, but dates back at least to the 2nd century CE. Native
alumen from the island of
Melos appears to have been a mixture mainly of alunogen () with potassium alum and other minor sulfates.
Alumen in Pliny and Dioscorides A detailed description of a substance termed
alumen occurs in the
Roman Pliny the Elder's
Natural History. By comparing Pliny's description with the account of
stypteria (στυπτηρία) given by
Dioscorides, it is obvious the two are identical. Pliny informs us that a form of
alumen was found naturally in the earth, and terms it
salsugoterrae. Pliny wrote that different substances were distinguished by the name of
alumen, but they were all characterised by a certain degree of
astringency, and were all employed for dyeing and medicine. Pliny wrote that there is another kind of alum that the ancient
Greeks term
schiston, and which "splits into filaments of a whitish colour". From the name
schiston and the mode of formation, it seems that this kind was the salt that forms spontaneously on certain salty minerals, as alum
slate and bituminous
shale, and consists mainly of sulfates of iron and aluminium. One kind of
alumen was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to
pomegranate juice. This property seems to characterize a
solution of
iron sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors. Pliny describes several other types of alumen but it is not clear as to what these minerals are. The
alumen of the ancients, then, was not always potassium alum, not even an alkali aluminum sulfate.
Alum described in medieval texts Alum and
green vitriol (iron sulfate) both have sweetish and astringent taste, and they had overlapping uses. Therefore, through the
Middle Ages, alchemists and other writers do not seem to have distinguished the two salts accurately. In the writings of the
alchemists we find the words
misy,
sory, and
chalcanthum applied to either compound; and the name
atramentum sutorium, which one might expect to belong exclusively to green vitriol, applied indiscriminately to both. Alum was the most common
mordant (substance used to set dyes on fabrics) used by the dye industry, especially in Islamic countries, during the
middle ages. It was the main export of the
Chad region, from where it was transported to the markets of
Egypt and
Morocco, and then to
Europe. Less significant sources were found in Egypt and
Yemen.
Modern understanding of the alums During the early 1700s,
G. E. Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum. The error was soon corrected by
Johann Heinrich Pott and
Andreas Sigismund Marggraf, who showed that the
precipitate obtained when an alkali is poured into a solution of alum, namely
alumina, is quite different from
lime and
chalk, and is one of the ingredients in common
clay. Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in
sulfuric acid and adding
potash or
ammonia to the concentrated solution. In 1767,
Torbern Bergman observed the need for potassium or ammonium sulfates to convert
aluminium sulfate into alum, while sodium or calcium would not work. The composition of common alum was determined finally by
Louis Vauquelin in 1797. As soon as
Martin Klaproth discovered the presence of potassium in
leucite and
lepidolite,
Vauquelin demonstrated that common alum is a
double salt, composed of sulfuric acid, alumina, and potash. In the same journal volume,
Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and an alum manufactured by himself, confirming
Vauquelin's result. ==Production==