CHAPTER XIX

THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE

The fabric chosen for the manufacture of the original Welsbach mantles was a specially selected cotton, woven from threads of a specified thickness. The oxide skeleton left after burning off the impregnated fabric, however, showed many serious defects. Gradual shrinkage occurred during use, so that the mantle was gradually withdrawn from the hottest zone of the flame; the contraction also resulted in crumpling, which caused the fragile fabric to fall to pieces. The light-giving power showed a gradual but continuous diminution, so that after a hundred hours, the decrease sometimes amounted to thirty per cent. of the original intensity. Lastly, owing to the fragility due to the torsion introduced by the twisting together of so many short fibres in the spinning of the fabric, the life of these mantles was very short, and their susceptibility to shock very great.

With the introduction of ramie, many of these defects disappeared. Mantles made from this fibre do not shrink continuously, nor to any considerable extent, so that crumpling does not occur; the decrease in light-giving power is very much less than with the cotton mantles, the life is much longer, and the resistance to shock very much greater. Microscopic examination of the fibres, and of the ash left after burning off, shows that these differences are traceable to differences in the mechanical structure of the two fabrics. The cotton thread is spun from a very large number of very short fibres, which are twisted together, whereas the separate fibres of ramie are of much greater length, and the torsion in the thread correspondingly less. In the case of artificial silk, continuous filaments are spun into threads, and this simple fact accounts for the very much greater durability and elasticity of mantles made from this fabric.

The introduction of ramie for the manufacture of mantles effected no great alterations in the processes employed in the case of cotton; the treatment of artificial silk, however, requires a profoundly modified method, and in consequence the preparation of this fabric and the manufacture of mantles from it, are considered separately in another chapter. Though the manufacture of artificial silk was first commenced in France, it is in Germany that its adaptation to the incandescent mantle industry has been successfully effected. Its adoption, however, has not yet become general, and by far the greatest number of mantles are to-day made from ramie. In the United States, where the early working of the monazite deposits gave a great impetus to the industry, the manufacturers still cling largely to the older methods, so that even now quite a considerable number of mantles are made from cotton.

In the present chapter, a short account will be given of the methods employed in the manufacture of mantles from ramie and cotton. The general methods of preparation of cotton fabrics are well known. Ramie is prepared from the tschuma plant, which is found in India, China, and other parts of Asia. The fibre is obtained from the inner side of the bark; this is dried, pressed into bales, and exported. After removal of gum and resin, by heating with sodium hydroxide solution under pressure, the fibre is bleached, dried and cleaned, and then combed and spun in the usual manner.[517]

[517] For the preparation and technical uses of Ramie, or Rhea, as it was formerly called, _vide_ Cross, ‘The Industrial Uses of Cellulose,’ in the _Cantor Lectures of the Society of Arts_, 1897, ~vi~, p. 20.

~Washing.~--The influence of even small quantities of impurities on the intensity of the light emitted by a mantle is remarkably great, and the manufacture requires a degree of care and attention far beyond that needed in ordinary technical processes. Washing of the fabric in particular must be very thorough and careful, if a mantle of reasonable quality is required. If the mineral content of the fabric, _i.e._ the ash left on incineration, amount to more than 0·03 per cent. of the total weight, the quality of the mantle is seriously affected. Particularly is this the case if even the smallest traces of iron come in contact with the fabric; on this account wooden implements must be used as far as possible in the washing house, and all iron parts must be carefully protected so that no water can drip from them on to the material.

The fabric is used in the form of a long cylindrical tube usually of about twice the diameter required for the base of the finished mantle. Before the mineral impurities can be removed, this must be entirely freed from grease. It is therefore washed thoroughly with a warm solution of sodium carbonate, which removes all the hydrolysable fats. If paraffin or other non-hydrolysable grease is present, the alkaline wash must be followed by a soap wash. The fabric is now cleansed from alkali and soap by running water, and mineral impurities are removed by treatment with dilute hydrochloric acid (1-3 per cent.) at a temperature of 50°-60°C.; it is finally washed free from acid with distilled water. So susceptible is the mantle to traces of impurity that the use of ordinary tap water, or even of a distilled water which has become contaminated to the smallest degree, for this final washing, will considerably lower its efficiency.

A centrifugal machine removes most of the water, and the drying is completed by passage over wooden rollers, through a small chamber, of which the air is kept at 30°-40°C. The dried fabric is now cut into lengths ready for the next process.

~Impregnation.~--The composition of the ‘Lighting Fluid,’ as the solution of salts used for impregnation is technically termed, varies slightly according to the nature of the mantle required, and the conditions of washing. It is of the greatest importance that the ratio of thoria to ceria should be constant and definite; the usual proportions are chosen so that the ratio of the oxides is 99 : 1. Fig. 11 shows at a glance to what a remarkable extent small variations in the percentage of ceria affect the luminosity of the finished product.[518] The thorium nitrate is made up with distilled water to a solution of 25-35 per cent. strength, and the calculated quantity of a standard solution of cerium nitrate is added. It is usual to add to the mixture a small quantity of another nitrate, which on ignition will leave an oxide of which the function is to strengthen the skeleton of ash. Beryllium, zirconium, magnesium, or aluminium nitrate is usually employed, in quantity calculated to leave an amount of oxide constituting about 0·5 per cent. of the total oxides; for ramie fabrics, beryllium nitrate is generally chosen.

[518] Numberless theories have been advanced to account for the extraordinarily high light-emitting power of this particular mixture of thoria and ceria. An account of these would be beyond the province of the present work; the reader who desires to pursue the subject should consult the interesting work of Dr. H. W. Fischer, _Der Auerstrumpf_, Ahren’s _Sammlung_, 1906, vol. xi. _Vide_ also Lévy, _L’Éclairage à l’incandescence par le gaz_, Paris, 1910, Ch. II; and Foix, _Thèse présentée à la Faculté des Sciences de Paris_, Paris, Gauthier-Villars, 1910.

The diagram is after Drossbach, _J. Gasbel_. 1898, 352.

After having been immersed for two to five minutes in the solution, the separate lengths are freed from excess of the lighting fluid by means of a small wringing machine. The pressure between the rollers must be regulated very exactly, since on the amount of solution taken up by the fabric will depend the mass of the oxide skeleton. The weight of oxides left after ignition should be 0·5-0·6 gm. for a ‘normal’ upright mantle of 9·5 cm. length, corresponding to 1·0-1·2 gms. of the nitrates, or, for a 30 per cent. solution, to 3·3-4·0 gms. of solution. The weight of the fabric before impregnation is approximately 5 gms. for cotton, 3 gms. for ramie, and 1·5 gms. for artificial silk. A cotton mantle-fabric, therefore, must be allowed to retain rather less, a ramie fabric rather more, than its own weight of solution, whilst an artificial silk fabric must take up 2-2¹⁄₂ times its own weight of the fluid. The weight of the oxide ash left from these quantities has been found by experience to be most suitable; if the mass is greater than this, the light-emission is diminished without a compensating gain in strength; if it is less, the light-emission is indeed greater, but the mantle becomes too fragile.

The impregnated fabric-lengths, after passing through the wringing machine, are drawn singly on to glass forms which are arranged on stands, and freed from moisture in a drying room by hot air, a temperature of about 30°C. being maintained. Three to four hours are required, under these conditions; if the drying be too rapid, considerable shrinkage occurs, and the mantles obtained are then extremely fragile.

~The Mantle Head.~--The normal upright mantle is supported from a central rod of compressed magnesia--fused quartz has recently been suggested[519]--by means of an asbestos thread. The thread in the older patterns was supported by simply doubling over the fabric at the end which was to become the head; more generally, however, a strip of tulle or gauze is sewn to the head end before the impregnation. In order to strengthen the head, it is treated before ‘finishing’ with a hardening or ‘fixing’ fluid, which usually consists of a mixture of magnesium and aluminium nitrates in aqueous solution; the following may be cited as a typical mixture: Aluminium nitrate, 300 parts; magnesium nitrate, 300 parts; chromium nitrate, 3 parts; borax, 5 parts; distilled water, 1500 parts. In order to secure that this fluid is applied to the head only, a little organic colouring matter is generally added, so that it may be clearly seen. The solution is soaked on to the head from mechanically held felt pads, which are kept at a convenient degree of saturation with the fluid by means of an ingenious compressed-air device. The mantle is then rapidly dried in a hot-air chamber.

[519] _Vide_ _D. R. P._ 244959, March, 1912.

After the fixing and drying processes, the head is ‘finished.’ The ordinary upright mantle is sewn together, at the end which has been treated, with carefully selected asbestos threads, an opening of some ten millimetres being left, and the asbestos is threaded diametrically across this opening--these diametrical threads support the mantle on its rod during use. These operations were formerly done by hand, when mantles of good quality were required, but machine treatment is gradually coming into extended use. Several mantles now on the market are supported at the head by metal rings, made from thin sheets of iron which have been plated with aluminium. In petroleum lamps, the mantle is usually supported from both sides by means of asbestos threads.

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.

* * * * *

The present chapter may be concluded with a bare mention of a few disconnected details, selected from the great mass of proposals, suggestions, and developments which have sprung up round the incandescent mantle industry.[520]

[520] For a complete account of the mechanical developments, the reader is referred to the monograph ‘Beleuchtung und Lichtmessung,’ by Dr. Börnstein, in Dammer’s _Chemische Technologie der Neuzeit_, Stuttgart, 1910-11, ii. 243-266.

With regard to the composition of mantles, numerous proposals have been made. It is stated that thoria with 0·25 per cent. of uranic oxide, UO₃, gives a light almost equal to that of the Auer mantle. Zirconia with 0·40 per cent. of vanadium, in the form of the pentoxide, is said to give a splendid white light; the vanadium oxide slowly volatilises, but addition of an equivalent proportion of silica is said to prevent this. Langhans claims to have obtained a product equal in light-giving power to the Auer mantle, by using as impregnating fluid a solution of colloidal silica, obtained by the addition of nitric acid to a solution of sodium silicate, to which suitable quantities of rare earth nitrates are added. Bodies obtained by the use of very similar solutions give skeletons which are coming into extended employment for gas radiators. The ‘Sunlight’ mantles use a mixture of thoria (50 per cent.), alumina (40 per cent.), and chromium sesquioxide (10 per cent.).

A direction of development in which some success has been attained is the introduction of self-lighting devices. The catalytic action of finely divided metals has been proposed in innumerable patents,[521] but these devices are unreliable, and it seems doubtful if chemical methods will ever be successfully applied to the problem. For the lighting of streets, shops, etc., the ‘by-pass’ system is employed; a tiny jet of gas burns continuously from a pin-hole nozzle, which is momentarily increased, when the main supply is turned on, to such an extent that the gas issuing from the burner is ignited.[522]

[521] _Vide_, _e.g._ _D. R. P._ 158974 and 253550; _F._ 417934.

[522] For automatic regulators for self-lighting, _vide_ _J. Gasbel_. 1910, ~53~, 490.

An account of the innumerable forms of lamps and burners which have been introduced in the last twenty years would fill several volumes. The theoretical grounds on which improvements in this direction are based are outlined in an able article by Dr. H. Bunte, a recognised authority on incandescent lighting, which appeared recently;[523] for an account of some of the lamps which have been successfully applied, the reader is referred to a recent French publication.[524]

[523] _J. Gasbel._ 1911, ~54~, 469; _vide_ also Pickering, _J. Gaslighting_, 1911, ~113~, 156.

[524] _L’Éclairage à l’incandescence par le gaz_, Lévy, Part I. Ch. III.