1086 VITRIFIABLE PIGMENTS 



oxide of cobalt, 1 part oxide of zinc, 1 part lead-glass (prepared by fusing together 

 2 parts of minium and 1 of white sand), are well mixed and fused in a porcelain 

 crucible, for at least 3 hours, at a red heat: then poured out, reduced to powder, and 

 ground upon the glass. When this pigment cools slowly, it solidifies to a mass of 

 acicular crystals. Long-continued fusion, at not too high a temperature, is requisite 

 to obtain a beautiful tint ; this is best attained by fusing it, during one of the bakings, 

 in the second floor of the porcelain furnace ; this is also the cheapest and best way of 

 fusing the lead-glasses. 



Light Blue. 1 part oxide of cobalt, 2 parts oxide of zinc, 6 parts lead-glass (pre- 

 pared by fusing together 2 parts of minium and 1 of white sand, and 1 part lead-glass 

 (prepared by fusing together 2 parts of minium, 1 part white sand, and 1 part calcined 

 borax), are well mixed and fused, as directed for the dark blue. 



Blue for Shading. 10 parts oxide of cobalt, 9 parts oxide of zinc, 25 parts of lead- 

 glass (obtained by fusing 2 parts of minium and 1 of white sand), and 5 parts of lead- 

 glass (prepared by fusing together 2 parts of minium, 1 part of white sand, and 

 1 part of calcined borax), are mixed and fused, as directed for the dark blue. The 

 colour is only used for shading, or to be applied upon or beneath the two preceding 

 blue pigments, for which purpose it is admirably suited, from its being very difficult 

 of fusion. 



Sky Blue. 2 parts of dark blue, 1 part oxide of zinc, and 4 parts of lead-glass 

 (prepared by fusing 4 parts minium with 1 of white sand), are intimately mixed and 

 ground upon the glass plate. This pigment is employed, either alone, or mixed with 

 other colours, only for painting the sky in landscape. 



The blue pigments described likewise appear under the microscope, after having 

 been burnt-in upon the porcelain, not to be homogeneous blue glasses, but mixtures 

 of a transparent blue substance (silicate of cobalt and zinc ?) and a colourless glass. 



Turquoise Blue. 3 parts of chemically pure oxide of cobalt, and 1 part of pure 

 oxide of zinc, are dissolved together in sulphuric acid ; then an aqueous solution of 

 40 parts ammonia-alum added, the mixed solutions evaporated to dryness, and the 

 residue heated to expel the whole of the water ; then reduced to a powder, and exposed 

 in a crucible to an intense red heat for several hours. The colour is most beautiful, 

 when it has been exposed, during one firing, to the heat of the porcelain furnace. It 

 is a combination of nearly 4 equivs. alumina, 3 equivs. oxide of cobalt, and 1 equiv. 

 oxide of zinc, and is of a beautiful turquoise-blue colour. When the oxides are mixed 

 in other proportions th;m those above given, they do not furnish such beautiful 

 coloured compounds. To impart to it a slightly greenish tint, a little moist recently- 

 precipitated protochromato of mercury is mixed with the above-described solution of 

 ammonia, alum, zinc, and cobalt ; with the above quantities, ^tli part of the chromate, 

 calculated in the dry state, suffices. 



The turqiioi so-blue vitrifiable pigment is prepared by mixing one part of the 

 compound of alumina-oxide of zinc and cobiilt with 2 parts of bismuth glass (prepared 

 by fusing 5 parts of oxide of bismuth with 1 part of crystallised boracic acid). 



The receipt for the preparation of the turquoise-blue pigment, communicated in the 

 Traite des Arts Ceramiqites by Brongniart, is incorrect ; for a lead-glass of the com- 

 position there given (3 parts minium, 1 part sand, 1 part boracic acid) destroys the 

 turquoise-blue pigment entirely on fusion, and only a dirty bluish-grey colour is pro- 

 duced. On examining under the microscope the turquoise-blue pigment burnt-in upon 

 porcelain, it appears to be a mixture of a transparent blue substance and a colourless 

 glass. The transparent blue substance in all probability is the above-described 

 compound of oxide of cobalt and alumina, which is of itself transparent under the 

 microscope, but the transparency of which is increased by the surrounding fused glass 

 of bismuth, just like the fibres of paper by oil. This is probably the case also with 

 the microscopic blue constituent of the other blue vitrifiable pigments, and which is 

 probably silicate of zinc and cobalt ; for this, when prepared separately, forms a pure 

 blue transparent powder. 



Black and Grey Colours for fainting upon Porcelain. Iridium Black. Iridium, as 

 obtained in commerce from Russia in the state of a fine grey powder, is mixed with an 

 equal weight of calcined chloride of sodium, and heated to a faint red in a porcelain 

 tube, through which a current of chlorine is passed. In this manner a portion of the 

 indium is converted into the bichloride of iridium and sodium, which is dissolved out 

 with water from the ignited mass. The aqueous solution of the double salt is eva- 

 porated to dryness with carbonate of soda, and thon extracted with water, which 

 furnishes black sesquioxide of iridium. This is dried and mixed with twice its weight 

 3f lead-glass (prepared by fusing together 12 parts of minium, 3 parts of white sand, 

 and 1 part of calcined borax), and ground upon a plate of glass. The iridium, which 

 remained undecomposed in the first treatment with sea-salt and chlorine, is agaiu 

 submitted to the same treatment. 



