~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. [Illustration: FIG. 11]