It is sometimes convenient and ornamental that the cases of some fireworks should burn or flame with the fire; in others it produces a bad effect, or is Hurtful. In the fire wheels it adds to the ornament, by producing or increasing the interior white flame. In speckies it is injurious, because the colour of the flame of the paper interferes with that of the composition. In this case, the paper should be impregnated with alum, by which its flaming is prevented ; and this is easily done by soaking it in a saturated solution of that salt.
On the General Principles of Composition for Fireworks.
IT will conduce to perspicuity to point out the gene ral urinciplcs of the compositions which are used in fireworks, as it will also enable us hereafter to class them, in a great degree, in a certain order, according to their affinities and objects; and thus to clear our essay, not only from the confusion that exists in all the com mon books on pyrotechny, but of many superfluous and absurd receipts and repetitions of the same composition under different names, in which they all abound, There are four principal objects in all the composi tions of fireworks, under which some varieties are in cluded, and some of which are also necessarily com bined. The simplest requisite in these is explosion, or the mere temporary blast, designed either for noise, or for the purpose of throwing burning bodies to a dis tance, or, lastly, for conducting fire from one place to another, through the parts of a simple or complicated piece. For all these purposes, the only composition is common gunpowder, either entire or mealed, according to the particular object that it is intended for.
Next to this are those compositions which are re quired to produce motion, on the principle of recoil and not of projection, where the piece, instead of the ball, if we may use such an expression, is to be fired away. The sky-rocket is the first of these in which the object is to produce the greatest possible recoil, con sistently with the safety of the piece itself ; since, if the charge were to go beyond this point, it would burst. In the same class are serpents, together with tourbillons, and the whole tribe of wheels, of whatever construction these may be. For such works force is required, although in different degrees, and therefore, the compositions approximate in their nature to gun powder. In all of them, the basis is a mixture of salt petre, sulphur, and charcoal; and, as far as mere mo tion is concerned, the objects are to be obtained by this species of composition alone. Where the motion
is required to be most rapid, as in sky-rockets and line-rockets, the proportions of the ingredients must approach nearest to those used for gunpowder ; where less so, as in some wheels, and other similar move ments, there the proportions depart more from that. This is done by increasing the quantities of the sulphur and the charcoal, or, what is the same thing, by dimi nishing the proportion of nitre. At the same time, the appearance of the fire may. be altered while the force continues the same; as if, while the proportion of nitre remains the same, the sulphur chiefly is increased, the effect will be to produce more flame; but if, on the con trary, it is the charcoal which is augmented, red sparks will predominate.
But, without materially changing the proportions of these ingredients as used in gunpowder, the force of explosion may be diminished by using an imperfect mixture. Hence, when these three ingredients are merely mixed in the way practised by artificers in fire works, instead of being ground together in the powder mill, they will only a manageable recoil, when, if they had been more perfectly united, they would have exploded. In the same manner, the mere act of condensation, by driving, will cause even the most perfect gunpowder composition to burn slowly. Coarseness of ingredients and imperfect mixture pro duce also, in these cases, another advantage; as the charcoal, which would otherwise give nothing but flame, gives rise, in consequence of its imperfect pulve rization, to a torrent of sparks, producing one of the or namental effects which are desired in fireworks. It is plain, therefore, that the ingredients of gunpowder, which form the basis of all fireworks, arc moderated in violence so as to answer the desired end, of a controlla ble force, by three methods : imperfect proportions, im perfect pulverization, or mixture, and hard driving. Thus, any one of these can, in some measure, be made to compensate the want of the other two, and consequent ly, the same effects are to be produced by compositions of different proportions, or different ones by composi tions of the same nature. As ornament in the appear ance of the fire is, in most of these cases, no less import ant than force, so the fundamental composition is modified for that end by various additions. These we shall con sider now, in describing the compositions of which the sole purpose is ornament, or in which force is, at least, only a secondary consideration.