SYNTHETIC TANNING MATERIALS 189 



This view receives some support from the other types of 

 method of manufacture. 



With the use of other condensing reagents the procedure 

 may be as in the process of the B.A.S.F. (Fr. pat. 451,875-6), 

 thus : 225 parts of o-cresol-sulphonic acid are heated to 

 60 C. for 4 hours with 262*5 parts of phosphorus oxychloride. 

 The excess of oxychloride is removed by distillation under 

 reduced pressure and the residue washed with dilute hydro- 

 chloric acid. 



Condensation by heat alone is illustrated by the method 

 given in the same patents, thus : phenol-p-sulphonic acid 

 is heated to 130 C. for 24 hours under a pressure of 20 mm. 

 or in a current of dry air at atmospheric pressure. The 

 product may be used direct or may be purified by dissolving 

 in water, neutralizing with caustic soda, filtering and evapo- 

 rating to dryness. A white powder is obtained which tans 

 when its solution is acidified. An alternative is to mix 

 phenol with sulphuric acid and heat the mixture to 140 C. 

 for 72 hours under 20 mm. pressure and purify as before. 



Methods of Use. The synthetic tanning materials may 

 be put to many uses. When well manufactured they make 

 practically a white leather, and this fact makes a valuable 

 opening for their use in connection with light leather tan- 

 nages and the dressing of rugs. It is also claimed that they 

 improve the colour usually obtained in the ordinary vege- 

 table tannages. If used in the suspenders to the extent of 

 5-10 per cent, they are said to brighten the colour through- 

 out the tannage. If used in bleaching and finishing they 

 are said to lighten the colour of the finished leather. About 

 5 per cent, on the weight of the goods may be added to the 

 bleach or vat liquors ; they may be also mixed with sumac 

 during finishing, and in effect act as a sumac substitute ; 

 solutions are also brushed over the grain before oiling, with 

 a view to obtaining good colour. It is also claimed that 

 their use prevents vegetable-tanned leather from becoming 

 red under the action of sunlight. The syntans are also 

 used to lighten the colour of chrome leather, even of chrome 

 sole leather after it has been dipped. 



