Gums had been reduced practically to shellac alone (the use of gum arabic as an adhesive is quite distinct), carbons to lampblack and charcoal, and these with sulphur and sulphides of antimony and arsenic practically completed the list.
Of the metals the use of pure zinc, copper, and brass has been discontinued, and the two almost revolutionary additions of magnesium and aluminium made, the former about 1865 and the latter in 1894.
The date of the introduction of these metals marks almost as great advances in the art as did the introduction of chlorate of potash. Not only are they used as spark-producing metals in the same way as are steel and iron, but they are also used as “burnables,” that is, they are consumed inside the case; and many of the present-day firework compositions owe their brilliance to one or other of these metals.
It is, however, in colour compositions that the tendency towards simplification is most strongly exhibited. In Kentish’s book colour compositions containing as many as seven or eight ingredients are common, whereas to-day formulæ containing over four are the exception rather than the rule.
Roman Candles. This untouched photograph illustrates the extraordinary brilliance of aluminium compositions. Each “plume” (200 ft. high) is produced by a single star about ¾ in. diameter and ¾ in. long. Each line is a microscopic particle of aluminium.
The reason for this complexity is not easy to follow, but it may have been in some measure due to the difficulty of obtaining sufficiently finely ground chemicals before the days of machine grinding; in some cases it was found that by melting two of the ingredients together and allowing the mass to cool they could be ground with greater ease. Chertier went so far as to melt shellac and salt together, grind them and remove the salt by dissolving in water. Also by adding a finely ground chemical of similar action to one only coarsely ground a better result was obtained.
Whatever may have been the reason, there can be no doubt that, except for secondary shades, the fewer the chemicals used the more brilliant will be the resulting fire.