In the case of fabrics from which ‘inverted’ mantles are to be made, fixing is carried out as usual at one end, to a depth of about 1·5 cm. After drying, a strip of about 0·5 cm. width is bent over and sewn down, and through this double band an asbestos thread is drawn, by which the mantle is secured to a magnesia ring. The lower end is drawn together in the shape of a hemisphere, by means of threads drawn through the meshes of the fabric; an opening of 6-8 mm. is sometimes left, but in the more modern patterns the end is drawn almost completely together, and after cutting is pressed out on a wooden shaper by a wooden mallet.
The product is now ready for burning off; if it is to be marked, it is stamped at this stage with a solution of didymium nitrate and methylene blue; the former being only faintly coloured, the organic dye is added to give a definite impression. On ignition, the nitrate is converted into the oxide, which is deeply coloured, and, of course, permanent.
Burning off and Shaping.
—For the production of mantles of the best quality, these processes are usually carried out by skilled operators, each mantle being treated separately. Very frequently, however, mechanical arrangements are employed. The great objection to machine treatment of such a product lies in the fact that it must be identical for every mantle; whereas it is exceedingly difficult to ensure that the original fabric, and the processes of washing, impregnating, wringing, and drying have been absolutely uniform. The operation of shaping and hardening is a very delicate one, and on the care with which it is carried out, the quality of the mantle finally depends. Until quite recently, only the cheaper kinds of mantles were machine-treated; but as the uniformity of the fabric becomes more assured, and the earlier operations more exact, employment of machines at this stage will undoubtedly increase.
The prepared fabric is shaped on a suitable form, and removed by a holder, which supports it from the asbestos thread; a flame is then applied to the head. The burning-off proceeds readily, once started; when the upper half has been incinerated, the flame is removed. The weight of the unburnt portion prevents too rapid contraction taking place at first; when the flame is removed, the glow spreads slowly downwards, and the shrinking is thus kept as uniform as possible. The operation must be carried out under a ventilating hood. The organic material of the fabric is completely oxidised, and the nitrates are converted into oxides, which retain the exact shape of the original fibres. The skeleton now undergoes the process of shaping and hardening, for which a ‘radial’ blowpipe flame is used. The burnt-off product is placed over this; the gas is supplied at an initial pressure of only a few inches of water, which is increased towards the end of the operation. The process commences at the head, the mantle being slowly lifted and rotated so that it is shaped and hardened along the whole length. By this means the oxide skeleton is not only suitably shaped, but is rendered considerably more elastic and resistant. For inverted mantles, of course, specially shaped burners are required. The eyes of the operators must be protected from the glare by shades of green glass. Recently the processes of burning-off and hardening have been carried out by means of the same burner.
Where machines are employed, the prepared fabrics are burnt off on wire shapers, usually in rows of ten; mechanical arrangements for continuous ignition and motion and, in the hardening, for continuous elevation of the ash-skeleton, are in use, but the finished mantles maintain a uniform good quality only when the structure of the fabric and the earlier processes have been absolutely uniform.
Collodinisation.
—The burnt-off mantle is now ready for use, but is far too fragile for transport. A method has therefore to be found by which the finished product can be protected for a time without detriment to its use for illumination. Mantles of artificial silk, particularly those for use in high-pressure lamps, are sometimes sent out without having undergone the final processes of burning off and shaping, which, in this case, must be carried out on the consumer’s burner. ‘Inverted’ mantles also were formerly sent out after impregnation and drying. In this condition, of course, the mantles are readily packed and transported, and there is the additional advantage that the duty on the unburnt product is very much less than that on the finished mantle.
One of the earliest of Auer’s patents (vide supra, [p. 271]) protected the process of collodinisation, which is now extensively employed. The oxide skeleton is dipped into a solution of collodion (the mixed lower nitro-derivatives of cellulose, or cellulose nitrates) in a mixture of alcohol and ether, to which, to prevent shrinkage on drying, a little camphor is added. On account of the inflammability of the mixture, the ethyl alcohol and ether are occasionally replaced by a mixture of methyl alcohol and acetone, but with this less volatile mixture, drying of course is slower. After dipping, the solvents are removed in a current of air, leaving the mantle coated with an exceedingly thin film of collodion, which increases enormously its power of resisting shock and vibration. This film is not removed until the mantle is placed on the consumer’s burner, when on the application of a match it ignites instantly and burns away, leaving the oxide skeleton in the condition to which it was brought in the final stage of hardening and shaping in the factory. The process is now used for almost all kinds of mantles, having been successfully applied in Germany in recent years to those made from artificial silk. The addition of small quantities of various inorganic salts, e.g. nitrates of zirconium, magnesium, platinum, thorium, etc., to the collodion solution, has been proposed; these salts make the collodinised product extremely resistant, but have a very harmful effect on the oxide ash when the collodion has been burnt off.
The collodinised mantles are cut to length on a trimming machine, and are then ready for packing.