rumours of Wallerius retirement had reached Bergman in advance, but he still had
very little time to acquire the required chemical experience. It was probably to
Rinman, whom he had likely befriended during the studies of tourmalines
(Sect. 5.2), that he turned for advice [33]. Rinman was at the time developing an
alum extraction plant in Garphyttan, and improved methods for the production of
alum became Bergman’s introduction to chemistry. The results of his studies were
published in a paper in the Transactions of the Royal Academy of Sciences [34].
The topic was well-chosen, as it was of great practical importance, well in line with
what the government expected from a chemistry professor in eighteenth-century
Sweden.
Alum (aluminium potassium sulphate dodecahydrate, KAl(SO 4 ) 2 Á 12H 2 O;
Fig. 9.6) was an important chemical product used in large quantities, for example,
not only as a fixative in dyeing textiles but also found use in medicine. In Sweden,
it was produced by burning alum shale and extracting the residue with water.
Unfortunately, the quality of the alum was often poor, and the product contaminated
with iron and a fatty substance (oily organic residues from the shale). It had been
suggested that iron could be precipitated with alkali, but Bergman showed that
alkali actually had a negative effect, decomposing alum and increasing the solubility of the fatty substances. Bergman found that, in modern terms, aluminium
actually precipitates before iron(II) on adding alkali to a solution of alum and
iron(II) sulphate (green vitriol).
7 Bergman instead suggested fractional crystallisation. He also found that the mother liquor contained an excess of sulphuric acid
(probably originating from the combustion of sulphurous compounds in the shale),
and this acid prevented crystallisation of alum.
8 Bergman’s solution to this problem
was to add lime-free clay, which neutralised the acid giving alum, and absorbing the
fatty substance. Bergman’s opinion that alum consists of pure clay and sulphuric
acid (vitriolic acid) is not correct and suggests that Bergman did not discriminate
between alum and aluminium sulphate. In fact, neutralisation of any excess sulphuric acid with potassium carbonate generates potassium sulphate which gives
alum with aluminium sulphate. It is a fine balance, however, since a too high pH
leads to precipitation of aluminium hydroxide and aluminium carbonate. Bergman’s
paper in the Transactions was followed by papers by Jacob Faggot and Anton von
Swab. Faggot, who was more oriented towards economics, was pleased that
Bergman had paid attention to a problem of economic importance, but was sceptic
to Bergman’s proposed method of purifying alum by adding clay. The criticism was
probably justified, as the outcome of Bergman’s method would have been highly
dependent on the quality of the clay. Whether it would have worked or not is not
easy to determine, since it would be virtually impossible to recreate the conditions
of Bergman’s experiments. The author of this book only noticed a small degree of
absorption of Fe
2+ by a native Swedish clay, and no absorption at all by pure
7
The situation is rather complicated with several competing acid–base and solubility equilibria, but
upon adding sodium carbonate solution to a solution containing of aluminium sulphate laced with
iron(II) sulphate, I found Fe
2+ in both the precipitate and the remaining solution, thus proving
Bergman’s criticism to be justified.
8
I found no significant difference in solubility of alum in water compared to 0.1 M sulphuric acid.
9.2 Bergman’s First Chemical Study
119
very little time to acquire the required chemical experience. It was probably to
Rinman, whom he had likely befriended during the studies of tourmalines
(Sect. 5.2), that he turned for advice [33]. Rinman was at the time developing an
alum extraction plant in Garphyttan, and improved methods for the production of
alum became Bergman’s introduction to chemistry. The results of his studies were
published in a paper in the Transactions of the Royal Academy of Sciences [34].
The topic was well-chosen, as it was of great practical importance, well in line with
what the government expected from a chemistry professor in eighteenth-century
Sweden.
Alum (aluminium potassium sulphate dodecahydrate, KAl(SO 4 ) 2 Á 12H 2 O;
Fig. 9.6) was an important chemical product used in large quantities, for example,
not only as a fixative in dyeing textiles but also found use in medicine. In Sweden,
it was produced by burning alum shale and extracting the residue with water.
Unfortunately, the quality of the alum was often poor, and the product contaminated
with iron and a fatty substance (oily organic residues from the shale). It had been
suggested that iron could be precipitated with alkali, but Bergman showed that
alkali actually had a negative effect, decomposing alum and increasing the solubility of the fatty substances. Bergman found that, in modern terms, aluminium
actually precipitates before iron(II) on adding alkali to a solution of alum and
iron(II) sulphate (green vitriol).
7 Bergman instead suggested fractional crystallisation. He also found that the mother liquor contained an excess of sulphuric acid
(probably originating from the combustion of sulphurous compounds in the shale),
and this acid prevented crystallisation of alum.
8 Bergman’s solution to this problem
was to add lime-free clay, which neutralised the acid giving alum, and absorbing the
fatty substance. Bergman’s opinion that alum consists of pure clay and sulphuric
acid (vitriolic acid) is not correct and suggests that Bergman did not discriminate
between alum and aluminium sulphate. In fact, neutralisation of any excess sulphuric acid with potassium carbonate generates potassium sulphate which gives
alum with aluminium sulphate. It is a fine balance, however, since a too high pH
leads to precipitation of aluminium hydroxide and aluminium carbonate. Bergman’s
paper in the Transactions was followed by papers by Jacob Faggot and Anton von
Swab. Faggot, who was more oriented towards economics, was pleased that
Bergman had paid attention to a problem of economic importance, but was sceptic
to Bergman’s proposed method of purifying alum by adding clay. The criticism was
probably justified, as the outcome of Bergman’s method would have been highly
dependent on the quality of the clay. Whether it would have worked or not is not
easy to determine, since it would be virtually impossible to recreate the conditions
of Bergman’s experiments. The author of this book only noticed a small degree of
absorption of Fe
2+ by a native Swedish clay, and no absorption at all by pure
7
The situation is rather complicated with several competing acid–base and solubility equilibria, but
upon adding sodium carbonate solution to a solution containing of aluminium sulphate laced with
iron(II) sulphate, I found Fe
2+ in both the precipitate and the remaining solution, thus proving
Bergman’s criticism to be justified.
8
I found no significant difference in solubility of alum in water compared to 0.1 M sulphuric acid.
9.2 Bergman’s First Chemical Study
119