At the Royal Powder Works at Spandau, Prussia, frequent ignition of the powder at a certain stage of the process led to an examination of the
machinery, when it was found that where, at certain parts, bronze pieces which were soldered were in constant contact with the moist powder, the
solder was much corroded and in part entirely destroyed, and that in the joints had collected a substance which, on being scraped out with a chisel,
exploded with emission of sparks. It was suspected that the formation of this explosive material was in some way connected with the corrosion of the
solder, and the subject was referred for investigation to Rudolph Weber, of the School of Technology in Berlin. The main results of his investigation
appeared in Journal für Praktische Chemie are given below:
The explosive properties of the substance indicated a probable nitro-compound of one of the solder metals (tin and lead), and as the lead salts are
more stable and better understood than those of tin, it was resolved to investigate the latter, in hope of obtaining a similar explosive compound.
Experiments on the action of moist potassium nitrate on pure tin led to no result, as no explosive body was formed. Stannous nitrate, Sn(NO3)2, formed
by the action of dilute nitric acid on tin, has long been known, but only in solution, as it is decomposed on evaporating. By adding freshly
precipitated moist brown stannous oxide to cool nitric acid of specific gravity 1.20, as long as solution occurred, and then cooling the solution to
-20°, Weber obtained an abundance of crystals of the composition Sn(NO3)2•(20)H2O. They resemble crystals of potassium chlorate. They cannot be
kept, as they liquefy at ordinary temperatures. An insoluble basic salt was obtained by digesting an excess of moist stannous oxide in solution of
stannous nitrate, or by adding to a solution of stannous nitrate gradually in small portions, with constant stirring, a quantity of sodium carbonate
solution insufficient for complete precipitation.
Thus obtained, the basic salt is a snow-white crystalline powder, which is partially decomposed by water, and slowly oxidized by long exposure to the
air, or by heating to 100°. By rapid heating to a higher temperature, as well as by percussion and friction, it explodes violently, giving off a
shower of sparks. This compound is also formed when a fine spray of nitric acid (sp. gr. 1.20) is thrown upon a surface of tin or solder. It is also
formed when tin or solder is exposed to the action of a solution of copper nitrate, and thus formed presents the properties already described.
In this, then, we have a probable cause of the explosions occurring in the powder works; but the explanation of the formation of the substance is
wanting, as potassium nitrate was shown not to give an explosive substance with tin. A thin layer of a mixture of sulphur and potassium nitrate was
placed between sheets of tin and copper foil, and allowed to stand, being kept constantly moist. After a time the copper was found to have become
coated with sulphide, while the tin was largely converted into the explosive basic nitrate. The conditions are obviously the same as those found in
the powder machinery, where bronze and tin solder are constantly in contact with moist gunpowder. The chemical action is probably this: the sulphur of
the powder forms, with the copper of the bronze, copper sulphide; this is oxidized to sulphate, which reacts with the niter of the powder, forming
potassium sulphate and copper nitrate; the latter, as shown above, then forms with the tin of the solder the explosive basic nitrate, which, being
insoluble, gradually collects in the joints, and finally leads to an explosion.
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