524 ALUMINUM. 



mixed with a little clay is used as the glaze for this porcelain. 

 Elsewhere a mixture of sulphate of lirne, ground porcelain and 

 flint is sometimes used as a glaze.- In painting porcelain, the 

 same metallic oxides are employed as in staining glass. They 

 are combined with a verifiable material, generally made thin 

 with oil of turpentine, and applied to the pottery sometimes 

 under, and sometimes above the glaze. To fuse the latter co- 

 lours, the porcelain must be exposed a third time to heat, in 

 the enamel furnace. 



Stoneware. The principal varieties of clay used here, accord- 

 ing to Mr. Brande, are the following: \. Marly clay, which, 

 with silica and alumina, contains a portion of carbonate of lime ; 

 it is much used in making pale bricks, and as a manure ; and 

 when highly heated enters into fusion. 2. Pipe clay, which is 

 very plastic and tenacious, and requires a higher temperature 

 than the preceding for fusion ; when burned it is of a cream 

 colour, and is used for tobacco-pipes and white pottery. 

 3. Potters' clay is of a reddish or grey colour, and becomes red 

 when heated ; it fuses at a bright red heat ; mixed with sand it 

 is manufactured into red bricks and tiles, and is also used for 

 coarse pottery (Manual of Chemistry, p. 861). The glaze is ap- 

 plied to articles of ordinary pottery after they are fired, and in 

 the condition of biscuit ware. They are dipt into a mixture of 

 about 60 parts of red lead, 10 of clay, and 20 of ground flint 

 diffused in water to a creamy consistence, and when taken out 

 enough adheres to the piece to give a uniform glazing when 

 again heated. To cover the red colour, which iron gives to the 

 common clays when burnt, the body of the ware is sometimes 

 coloured uniformly of a dull green, by an admixture of oxide 

 of chromium, or made black by oxides of manganese and iron ; 

 or oxide of tin is added to the materials of the glaze, to render 

 it white and opaque. The patterns on ordinary earthenware 

 are generally first printed upon tissue paper, in an oily compo- 

 sition, from an engraved plate of copper, and afterwards trans- 

 ferred by applying the paper to the surface of the biscuit ware, 

 to which the colour adheres. The paper is afterwards removed 

 by a wet sponge. The fusion of the colouring matters takes 

 place with that of the glaze, which is subsequently applied, in 

 the second firing. The prevailing colours of these patterns 

 are blue trom oxide of cobalt, green from oxide of chromium, 

 and pink irom that compound of oxide of tin, lime, and a small 

 quantity of oxide of chromium, known as pink colour. 



