The Art of Glass-Blowing / Plain Instruction for the Making of Chemical and Philosophical Instruments Which are Formed of Glass

by T.-P. Danger

THE FLAME.

It is only by long habitude, and a species of routine, that workmen come to know, not only the kind of flame which is most proper for each object they wish to make, but the exact point of the jet where they ought to expose their glass. By analysing the flame, upon the knowledge of which depends the success of the work, we can immediately obtain results, which, without that, could only be the fruit of long experience.

Flame is a gaseous matter, of which a portion is heated to the point of becoming luminous; its form depends upon the mode of its disengagement, and upon the force and direction of the current of air which either supports its combustion or acts upon it mechanically. ([Pl. 1], fig. 1.)

The flame of a candle, burning freely in still air, presents in general the form of a pyramid, of which the base is supported on a hemisphere. It consists of four distinct parts: the immediate products of the decomposition of the combustible by the heat which is produced, occupy the centre, o, where they exist in the state of an obscure gaseous matter, circumscribed by a brilliant and very luminous envelope, s; the latter is nothing but the obscure matter itself, in the circumstances where, on coming into contact with the atmosphere, it combines with the oxygen which exists therein, and forms what is properly called flame.

The blueish light which characterises the inferior part of the flame, s, is produced by a current of cold air, which, passing from below upwards, hinders the combustion from taking place at the bottom of the flame, at the same temperature that exists in the parts of the flame not immediately subject to this influence.

Finally, on observing attentively, we perceive a fourth part, which is but slightly luminous, and exists as an envelope of all the other parts of the flame. The greatest thickness of this envelope corresponds with the summit of the flame. From this point it gradually becomes thinner, till it arrives at the lowest part of the blueish light, where it altogether disappears. It is in this last-described portion of the flame that the combustion of the gas is finished, and there it is that we find the seat of the most intense heat which the flame of the candle affords. If we compare the temperature of the different parts of the flame, we find that the maximum of heat forms a ring corresponding to the zone of insertion, A A; a point which is the limit of the superior extremity of the blueish light.

When the flame is acted upon by the blowpipe, it is subject to two principal modifications:—

1. If, by means of a blowpipe with a very fine orifice, you direct a current of air through the middle of the flame, you project a portion of the flame in the direction of the blast. The jet thus formed appears like a tongue of fire, blueish, cylindrical, straight, and very long; the current of air occupies its interior. This flame is enveloped on all sides by an almost invisible light, which, extending beyond the blue flame, forms a jet, A´ B, very little luminous, but possessing an extremely high temperature. It is at the point A´, which corresponds with the extremity of the blue flame, that the maximum of heat is found. The extreme point of the jet B possesses a less degree of heat. This flame is adapted for mineralogical assays, for soldering, for working enamels, and in general for all small objects.

2. When the orifice of the blowpipe is somewhat large, or when (the orifice being capillary) the current of air is very strong, or the beak is somewhat removed from the flame, the jet of fire, instead of being prolonged into a pointed tongue, is blown into a brush. It makes then a roaring noise, and spreads into an irregular figure, wherein the different parts of the flame are confounded beyond the possibility of discrimination. This flame is very proper for the working of glass, and particularly of glass tubes; it ought to be clear and very brilliant, and above all should not deposit soot upon cold bodies suddenly plunged into it. The maximum of temperature in this flame is not well marked; we can say, however, that in general it will be found at about two-thirds of the whole length of the jet. As this roaring flame contains a great quantity of carburetted hydrogen, and even of vapour of oil, escaped from combustion, it possesses a disoxidizing or reducing property in a very high degree.