438 Mr. R. A. Couper on the Chemical Composition of 



Silica 66-16 



Alumina 22*54 



Protoxide of iron . . . . 5*31 



Lime 1*42 



Magnesia trace 



Water 3'14 



98-57 



8. Flint as used in potteries is first calcined, then water- 

 ground, in which state it is used for mixing with clays, and is 

 called slop flint; but for glazes it is evaporated to dryness, 

 and used in the dry state with other articles which constitute 

 the glaze. 



9. Cornish stone or granite is water-ground, then evapo- 

 rated to dryness for mixing in glazes, and is used in the slop 

 state for mixing with clays. 



10. Plaster of Paris or gypsum, which is employed in form- 

 ing the moulds in which certain kinds of pottery are cast, is a 

 native sulphate of lime. It is a very important article to the 

 manufacturer of earthenware, owing to its singular property 

 of parting easily with the clay by the application of a slight 

 heat. Plaster of Paris requires to be dried at a high tempe- 

 rature before using it; but if it is over-dried, it will not again 

 set for making moulds ; the drier the stucco the harder are 

 the moulds that are made of it, and they will stand more 

 readily a greater degree of wear. Plaster of Paris casts, as 

 commonly prepared, cannot again be used for the same purpose. 



11. The colours used for printing and painting on ware are 

 similar to one another, excepting that the colours for painting 

 may not be so expensive as for printing ; both however form 

 an important and extensive part of the materials of a pottery. 

 The manufacturers of earthenware are much occupied with 

 the improvement of the variety and beauty of the colours, as 

 well as of the patterns or styles that are produced, and hence 

 a great emulation exists among those employed in the trade. 



1. The blue colour in printing is produced from cobalt, 

 which is used with flint, ground glass, pearlash, white lead, 

 barytes, china clay, and oxide of tin in reducing its strength. 



2. The brown colour by ochre, manganese, and cobalt. 



3. The black colour by chromate of iron, nickel, ironstone, 

 and cobalt. 



4. The green colour by chrome, oxide of copper, lead, flint, 

 and ground glass. 



5. The pink colour by chrome, oxide of tin, whiting, flint 

 ground glass, and china clay, which are mixed in various pro- 

 portions, fused together at a high temperature, then pounded 

 and mixed with oil, when it is ready for the printer's use. 



