486 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



placing it perfectly level, and pouring on it a thicker mixture 

 of plaster and water, until the layer is about four tenths to 

 five tenths of an inch thick. After fifteen to eighteen hours 

 it can be taken from the relief, by removing the thin edges 

 of the plaster with a knife, and gently pressing ; and it is 

 then fit for the type-founder, after being retouched, if de- 

 sired at any points, with a needle. 14 (7, 1874, CCXL, 318. 



CLEANING AND BLEACHING OLD COPPER-PLATE ENGRAVINGS. 



The brown tint in the paper of old engravings, as well as 

 any ink spots, insect stains, etc., may be removed by first 

 erasing with India rubber or bread-crumbs such stains as 

 yield to them, and then laying the engraving on gauze, 

 secured in a light frame, and moistening the spots with a so- 

 lution of one ounce of oxalic acid in one pound of water. 

 When these have disappeared, the engraving, on its gauze 

 support, is to be immersed in- lukewarm water, and occasion- 

 ally moved gently forward and backward for about twelve 

 hours. The dirty water is then to be replaced by fresh, and 

 chloride of lime solution dropped into it until the smell is 

 appreciable, in which the engraving is to remain immersed 

 for six hours, with occasional gentle agitation. Any brown- 

 ish tint remaining will bleach out immediately on the addi- 

 tion of a few drops of hydrochloric acid, and the lights and 

 shades appear in their original purity and sharpness. The 

 solution must then be replaced with clear water; and the 

 engraving, perfectly freed from the chloride bath, must be 

 dried in the air, and pressed in that condition. 5 (7, 1873, 

 xxxvi. , 2. 



Paul's new method of lithographic transfers. 



A new method has lately been announced by M. Paul for 

 transferring a photographic image upon stone by the process 

 of photo-lithography. The ordinary method consists in pro- 

 ducing a positive image on gelatinized paper treated with bi- 

 chromate of potash. After exposure, it is entirely covered 

 with a fatty ink, and then washed with warm water to re- 

 move the unaltered gelatine. The imasje remains with its 

 black coating of lithographic ink, and is next, by a well- 

 known process, transferred upon a stone suitably prepared. 

 Images thus obtained, however, lack sharpness of outline, 



