Cast iron is more employed in artificial fire than forged iron or steel, at least in the preparation of some, as gerbes, white fountains, and Chinese fire.
The filings of iron and steel may be sifted through sieves. A fine hair sieve will answer for common purposes. Their fineness depends, in the first instance, on the file, which is used. Steel or iron filings are more commonly employed in the compositions for brilliant fire.
The sparks produced by cast-iron are very brilliant; but the reduction of the iron to powder, or to a degree of fineness sufficient for use, is a difficult operation. It is of too hard a nature to be cut by a file.
This operation is generally performed in the following manner: Procure from an iron foundry, some thin pieces of cast iron, such as generally run over the mould at the time of casting, and pound them on a block, made of cast iron, with an iron hammer of four pounds weight, putting, under the block, a cloth to catch the pieces of iron, which fly off. They are beaten with the hammer in this manner, until the whole is reduced to grains, which are more or less small. It is then thrown into a sieve, which should be fine, and the dust separated. This is used, in the place of steel dust, in small cases of brilliant fire. The remainder is then put into a sieve, a little coarser, and again sifted. This portion is preserved separately. The same operation is repeated, but with sieves of different sizes, till the iron passes through about the bigness of small bird shot.
The pulverization may be effected in an iron mortar, with a steel pestle, having the mortar covered in the usual manner, to prevent the escape of the finer particles of the iron.
According to a writer in the Dictionnaire de l'Industrie, vol. iii, p. 34, the Chinese prepare their iron-sand for fire-works by igniting iron, and plunging it in cold water. They then pulverize the scales thus formed, and pass the powder obtained, through different sized sieves, which is then called No. 1, 2, 3, 4, &c. as it is very fine or coarse. This cannot be a good method, and we doubt whether it is at present employed; because it is obvious, that the scales, in this case, consist of the metal in the state of protoxide. D'Incarville, a missionary at Pekin, obtained the process for making Chinese fire; and observes, that the pulverized cast iron they employ is called iron-sand, of which they have six numbers or varieties.
As the goodness of iron or steel dust, in fire-works, depends greatly on its being dry, and not oxidized or rusted; its preservation must be accordingly attended to. The usual preservative is to put it in a box, lined with oiled paper, and covered with the same, or in tin cannisters, with their mouths well closed.
When it is to be used, it is taken according to its size, and in proportion to the cases, for which the charge is intended. Large gerbes, of 6 or 8 lbs. require only the coarse sort.
As the brilliancy of the sparks, produced by the iron and steel dust, is a desideratum in the formation of some fire-works, and as this brilliancy depends upon the nature and quality of the metal, it may not be improper to offer some remarks on these subjects.
That iron, when finely divided is capable of producing sparks of fire, is a well known fact; and we see it daily in the operations of the smith, when ignited iron is hammered on the anvil. The scintillation produced by the steel, when struck with a flint, is of the same character. In the latter case, the metal is actually fused, and, when caught on a paper, and examined with a microscope, will appear globular, and partly oxidized. Hence it is, that gunpowder is inflamed by this spark, which is nothing more than highly ignited, and inflamed iron, possessing a temperature more than sufficient to inflame gunpowder.