examine. Rinman reported these tests without making any conclusions about the
constituents of tourmalines. If we compare Bergman’s methods with those of
Rinman’s, the magnitude of Bergman’s contribution to mineral analysis becomes
evident.
Bergman had now reached the point where a handbook in mineral analysis by
wet methods could be written. This work, essentially equivalent to the thesis on
analysis of waters defended by Scharenberg, was also presented as a thesis,
defended publicly in June 1780 by Peter Castorin [38].
It was natural, Bergman concluded, that the first attempts to analyse ores were to
simply carry out the reduction on smaller scale and determine how much metal a
certain ore yielded. Traditional assaying by reduction with charcoal had its limitations, however, and Bergman listed three requirements for a successful outcome:
the reduction must be quantitative, the metal must assemble in a single lump rather
than small grains, and once reduced, the metal must be chemically inert to air.
Bergman complained that the process was notoriously difficult to control. There
was no way of measuring high temperatures, and as the reduction had to be performed in a closed crucible, the process could not be monitored. Estimation of
losses was difficult. There were no methods to analyse minerals by using solution
methods exclusively: “Chemistry has at length begun to examine the composition
of ores by means of various menstrua; yet it must be confessed, that the fragments
of the humid art of assaying, which have hitherto been published, are rather to be
considered as instances of a mixed method, in which the mineral analysis is
accomplished partly by the dry, partly by the humid method. The metallic part is
indeed extracted by a menstruum, but is afterwards reduced by fire. In the following
pages, however, I shall endeavour to point out means by which the end may be
answered in the humid way alone, with out calcination or fusion” [39].
The following section of the thesis, which must be the result of a tremendous
amount of laboratory work, is essentially a manual for the analysis of ores of gold,
platinum, silver, mercury, lead, copper, iron, tin, bismuth, nickel, arsenic, cobalt,
zinc, antimony and manganese. His methods will be illustrated by taking the silver
ores as an example.
Native silver frequently contained gold and/or copper. To determine the composition of native silver, it was treated with nitric acid, which dissolved silver and
copper, leaving solid gold. Copper was precipitated from solution with iron or alkali
carbonate, but instructions for the isolation of silver are missing. One can assume
that it was performed with metallic copper or sodium chloride. The next class of
minerals were the silver sulphide ores (“silver mineralised by sulphur”) which were
boiled with dilute nitric acid, which left sulphur as a precipitate, mixed with any
traces of gold. The loss of sulphur as H 2 S was probably severe. Due to the difference in density, sulphur and gold were easily separated. Sodium chloride was
added to the solution, which precipitated silver chloride. Bergman used stoichiometry to calculate the amount of silver, rather than reducing the silver chloride
to metal and weighing it. The silver content was calculated from 100a/129, where
a is the weight of the silver chloride. This corresponds to a silver content of 77.5%
in silver chloride, the true value being 75.3%. The sum of the silver and sulphur
320
23 Bergman as an Analytical Chemist
constituents of tourmalines. If we compare Bergman’s methods with those of
Rinman’s, the magnitude of Bergman’s contribution to mineral analysis becomes
evident.
Bergman had now reached the point where a handbook in mineral analysis by
wet methods could be written. This work, essentially equivalent to the thesis on
analysis of waters defended by Scharenberg, was also presented as a thesis,
defended publicly in June 1780 by Peter Castorin [38].
It was natural, Bergman concluded, that the first attempts to analyse ores were to
simply carry out the reduction on smaller scale and determine how much metal a
certain ore yielded. Traditional assaying by reduction with charcoal had its limitations, however, and Bergman listed three requirements for a successful outcome:
the reduction must be quantitative, the metal must assemble in a single lump rather
than small grains, and once reduced, the metal must be chemically inert to air.
Bergman complained that the process was notoriously difficult to control. There
was no way of measuring high temperatures, and as the reduction had to be performed in a closed crucible, the process could not be monitored. Estimation of
losses was difficult. There were no methods to analyse minerals by using solution
methods exclusively: “Chemistry has at length begun to examine the composition
of ores by means of various menstrua; yet it must be confessed, that the fragments
of the humid art of assaying, which have hitherto been published, are rather to be
considered as instances of a mixed method, in which the mineral analysis is
accomplished partly by the dry, partly by the humid method. The metallic part is
indeed extracted by a menstruum, but is afterwards reduced by fire. In the following
pages, however, I shall endeavour to point out means by which the end may be
answered in the humid way alone, with out calcination or fusion” [39].
The following section of the thesis, which must be the result of a tremendous
amount of laboratory work, is essentially a manual for the analysis of ores of gold,
platinum, silver, mercury, lead, copper, iron, tin, bismuth, nickel, arsenic, cobalt,
zinc, antimony and manganese. His methods will be illustrated by taking the silver
ores as an example.
Native silver frequently contained gold and/or copper. To determine the composition of native silver, it was treated with nitric acid, which dissolved silver and
copper, leaving solid gold. Copper was precipitated from solution with iron or alkali
carbonate, but instructions for the isolation of silver are missing. One can assume
that it was performed with metallic copper or sodium chloride. The next class of
minerals were the silver sulphide ores (“silver mineralised by sulphur”) which were
boiled with dilute nitric acid, which left sulphur as a precipitate, mixed with any
traces of gold. The loss of sulphur as H 2 S was probably severe. Due to the difference in density, sulphur and gold were easily separated. Sodium chloride was
added to the solution, which precipitated silver chloride. Bergman used stoichiometry to calculate the amount of silver, rather than reducing the silver chloride
to metal and weighing it. The silver content was calculated from 100a/129, where
a is the weight of the silver chloride. This corresponds to a silver content of 77.5%
in silver chloride, the true value being 75.3%. The sum of the silver and sulphur
320
23 Bergman as an Analytical Chemist