The raw material for the process is cellulose, usually in the form of cotton. Treatment of this with a suitable mixture of concentrated sulphuric and nitric acids replaces some of the hydroxyl groups by the ‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being formed, in which the so-called tetra-, penta-, and hexa-nitrates predominate.[525] The product, cellulose nitrate or collodion, very closely resembles the original cellulose in appearance and structure. It is washed thoroughly to free it from traces of acid--which render it liable to explode spontaneously--and after drying, dissolved in the minimum quantity of the mixed solvents;[526] the solution is filtered from insoluble impurities through wads of cotton, pressures of thirty to sixty atmospheres being required. This filtration purifies and thoroughly mixes the solution, so that perfect uniformity is obtained in the product. The glass jets through which the solution is now forced, under a pressure of forty to fifty atmospheres, have a diameter of 0·08 mm., but the threads obtained contract on the removal of the solvents, so that fibres of 0·01-0·02 mm. are formed. [525] The cellulose esters are usually named as if they were derived from a compound C₁₂H₂₀O₁₀, the formula for cellulose being (C₆H₁₀O₅)_{_n_}. Thus the formation of the ‘hexa-nitrate’ would be represented-- C₁₂H₂₀O₁₀ + 6HNO₃ = C₁₂H₁₄O₄(NO₃)₆ + 6H₂O. [526] In the Lehner process, in which collodion is also used, larger quantities of solvent are employed, so that much more dilute solutions are obtained; these require low pressures to form the thread, which is then hardened chemically. Chardonnet probably began his work about 1885. It is interesting to observe that an Englishman, Swan, had proposed in 1883 to use a solution of collodion in acetic acid, fabrics prepared by his process being shown at the London Exhibition of 1884.[527] [527] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. There is no account of this process in the English patent literature. ~The Pauly or Cuprammonium Process.~[528]--It has long been known that a solution of copper hydroxide in ammonia solution--Schweitzer’s reagent--will dissolve cellulose. The use of this solvent for the production of artificial silk was proposed about 1900, and the method has become a serious rival of the older Chardonnet process. The solvent is prepared on a large scale by passing air through an ammonia solution to which copper turnings have been added. After addition of the cellulose, and filtration, the solution is forced through tiny jets into a bath of dilute acid, which removes the copper and precipitates the cellulose again. [528] A full account of this and of the other processes employed in the manufacture of artificial silk will be found in the work of Piest, _Die Zellulose_, Stuttgart, 1910.