—If the dried fabric, impregnated with the necessary salts, be finished and burnt off in the usual way, the oxide skeleton is extremely fragile, and soon falls to powder. The reason for this lies probably in the explosive decomposition of the nitrates, the weight of organic matter relative to the salts being very much less than in the case of ramie or cotton fabrics (vide [p. 295]). The additional ammonia bath advocated by Knöfler (vide [supra]) was therefore adopted by Plaisetty, and the nitrates in the dried impregnated fabric are converted into hydroxides by this treatment. For this process, to which the name ‘Fixing’ has been given by Böhm, numberless alternative proposals have been made. Plaisetty’s ammonia fixing gives a mantle which, after burning off, is exceedingly elastic and strong, but it is nevertheless open to serious objections. Thus the nitrates may be to some extent dissolved out by the fixing bath before precipitation of the hydroxides has occurred; to remedy this, the impregnated fabric must be very thoroughly dried before fixing, and in this case, apart from the trouble involved, the acid of the commercial nitrates will attack the fabric unless addition of thorium hydroxide has been made to the impregnating fluid. Again, the conversion into oxides is not complete, the outer layer first formed preventing free diffusion of the alkaline fluid. Finally, since ammonium nitrate is formed in the reaction, a very thorough washing is necessary to remove this salt.
It would be impossible to mention all of the numberless proposals which have been put forward for fixing; nor are the great majority worthy of mention.[538] One of the most important was that of Albrecht,[539] by which hydrogen peroxide is substituted for ammonia. This reagent, as is well known, precipitates from solutions of thorium salts ‘peroxide’ compounds (vide [p. 255]); from the fabric impregnated with the nitrate, free nitric acid is liberated in the reaction. Since the peroxide is soluble in nitric acid, two baths are used, the fabric being allowed to remain for a short time only in the first, which becomes strongly acid, and being then transferred to the second, in which the precipitation is completed. The burning off of the product so treated proceeds quite quietly, and leaves a very hard and elastic skeleton. The method, however, has the grave disadvantage that cerium salts are not precipitated under these conditions, but escape into the solution. To remedy this, arbitrary additions of cerium nitrate are made to the fixing bath, but the percentage of cerium, and therefore the lighting power (vide [p. 293]) of mantles fixed by this method is liable to fluctuate. A modification has been introduced[540] in which various salts are added to the hydrogen peroxide fixing baths to prevent the withdrawal of the cerium salt; these are chiefly acetates of the alkali metals and allied compounds. The fabric requires washing after this treatment.
[538] A large number of patents are mentioned by Böhm, J. Gasbel. 1909, 52, 855.
[539] D. R. P. 188427, September, 1907; E. 15295, 1907.
[540] Vide E. 2240, 1908. Cerofirm Gesellschaft, by Brit. Cerofirm Co.
A rather similar proposal[541] substitutes for ammonia an alkaline solution of hydrogen peroxide, obtained by dissolving sodium peroxide in water. After saturation for a minute or so, the fabric is wrung out and dried, there being no necessity, according to the patent, for any subsequent washing. The same company in an earlier patent[542] suggest a fixing bath of ‘an alkali or amine with an acid which can form insoluble basic double salts with the earth metals,’ the said salt to be precipitated on the fibre, whilst an alkali or amine nitrate goes into solution; acetic and benzoic acids and phenol are mentioned. Apparently this process did not give satisfactory results. The same may be said of the hypochlorite method of Visseaux.[543] Equally interesting, and doubtless equally practical is the proposal[544] to treat the dried impregnated mantle with ozone in a closed chamber, which possibly inspired the even more original suggestion[545] that the fabric be first washed in ozonised water, dried, then impregnated and dried, and finally treated with ozone. According to yet another French patent,[546] the impregnated and dried fabric is to be treated with an alcoholic solution of hydrofluoric acid, which will give a viscous insoluble mixture of thorium and cerium fluorides in the fabric, and at the same time will remove any impurities. Another patent[547] proposes the use of hydrazine and similar bases, cerium nitrate being added to the fixing solution.
[541] D. 247940, June, 1912; F. 430417, August, 1911.
[542] F. 403433, September, 1909, Bruno Co.
[543] F. 408807, February, 1910.
[544] F. 414700, June, 1910.