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.