Mr. Couper on the Chemical Composition of Pottery. 175 



II. The colours used for printing are similar to those employed in 

 painting on waro, excepting that the colours for painting may not bo so 

 expensive as for print ini: ; 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, pearl ash, 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, and China 

 clay, which are mixed in various proportions, fused together at a high 

 ti ■! literature, then pounded and mixed with oil when it is ready for the 

 printer's use. 



For the following analysis of a blue cobalt calx, I am indebted to Mr. 

 John Adam — 



Silica, 1784 



Peroxide of cobalt, 19*42 



Peroxide of iron, 25*50 



Water, 841 



Carbonate of lime and magnesia, 28*45 



99*62 

 The oil that is used for mixing with the colours is made by boiling the 

 following substances together, viz., — linseed oil, rape oil, sweet oil, rosin, 

 common tar, and balsam copaiba in various proportions. 



III. It is but recently since a new method has been applied to cause 

 the colours to flow or spread over the surface of the ware. This object is 

 effected by washing the saggars in which the ware is placed previous to 

 its being fired in the glost kiln, with a mixture of — 



(1.) Lime, common salt, and clay slip. Dry flows are also used, which 

 answer equally well, the mixture being sprinkled on the bottom of the 

 saggar. The following are some of those flows: — 



(2.) Lime, sal ammoniac, and red lead. 



(3.) Lime, common salt, and soda. 



(4.) Whiting, lead, salt, and nitre. 



(5.) But there is a wash made of lime, clay slip, lead, in general 

 use for washing all the saggars employed in the glost kiln, which fuses 

 on the inner surface of the saggar, making it perfectly close and not 

 porous, otlurwix- tho gloss required on the surface of the ware could not 

 bo obtained. 



