BLEACHING 245 



bones), it has been found not only more convenient but also 

 more effective to use an aqueous solution. This is mainly 

 because it is possible to attain a greater adsorption in a 

 liquor. Unfortunately, however, sulphurous acid is not a 

 very soluble gas, and although 8-10 per cent, solutions may 

 be, with great care, obtained, they are really supersaturated 

 and readily yield the gas, even with slight mechanical 

 agitation. Solutions even of 2 to 3 per cent, strength are 

 also liable to this, and the general experience is that i to 2 

 per cent, solutions are most economical and convenient for 

 practical purposes. As the freight on weak solutions is 

 prohibitive, the manufacturer using sulphurous acid is 

 faced with the necessity either of purchasing cylinders of 

 sulphur dioxide liquefied by pressure or making the gas 

 and solution himself. The former is the most convenient 

 course when only small amounts are required, but the 

 latter preferable for a gelatine factory of any size. Sul- 

 phurous acid is easily manufactured by burning sulphur 

 and leading the fumes by induced draught up a scrubber 

 down which water slowly trickles. Forced draught may 

 also be used, as in the Sachsenburg plant. 



Of the other reducing agents which have been used, 

 sodium hydrosulphite (Na 2 SO 2 ) deserves mention. It is 

 a very powerful reducing agent, and has been found most 

 useful when employed as an assistant to sulphurous acid. 

 This reagent is usually added to the sol, after evaporation 

 and before gelation. It is sold as a white powder, usually 

 under trade names. Sometimes a mixture of bisulphite 

 and powdered zinc replaces it, but this is objectionable for 

 pure food gelatines. Its use also involves an impurity in 

 the finished article, and a greater amount of " inorganic ash." 



Bleaching by Oxidation. Many oxidizing agents 

 have been suggested for bleaching gelatine, but most of 

 them have some practical disadvantage. Most of them 

 contradict the maxim (previously noted, pp. 222-223) that 

 it is desirable to avoid adding any soluble substance, as this 

 involves a permanent impurity, possibly concentrated to 

 supersaturation in the finishing processes, and possibly 



