VITRIFIABLE PIGMENTS 1083 



tion of tin is poured into a glass vessel, and gradually mixed with 10 itres of distilled 

 water. It must still contain just so much acid that no turbidness results from the 

 separation of oxide of tin ; this may be ascertained previously by taking a drop of the 

 concentrated solution of tin upon a glass rod, and mixing it in a watch-glass with 

 distilled water. A clear solution of 0'5 gramme gold in nitromuriatic acid, which 

 must be as neutral as possible, is poured into the solution of tin diluted with 10 litres 

 of water, constantly agitating the whole time. The gold-solution should have been 

 previously evaporated nearly to dryness in the water-bath, then diluted with water, 

 and filtered in the dark. 



On adding the gold-solution, the whole liquid acquires a deep red colour, without, 

 however, any precipitate being formed ; this instantly separates upon the addition of 

 50 grammes of solution of ammonia. But if no precipitate should result, which may 

 happen if the amount of ammonia was too great in proportion to the acid contained 

 in the liquid, and in which case the liquid forms a deep red solution, the precipitate 

 immediately results upon the addition of a few drops of concentrated sulphuric acid. 

 It subsides verj quickly. The supernatant liquid should be poured off from it as 

 soon as possible, and replaced 5 or 6 times successively by an equal quantity of fresh 

 spring water. When the precipitate has been thus sufficiently washed, it is collected 

 upon a filter : and as soon as the water has drained off completely, removed while 

 still moist with a silver spatula, and mixed intimately iipon a ground plate of a glass 

 by means of a spatula and grinder with 20 grammes of lead-glass, previously ground 

 very fine upon the same plate with water. The lead-glass is obtained by fusing 

 together 2 parts of minium with 1 part of quartz-sand and 1 part of calcined borax. 



The intimate mixture of gold-purple and lead-glass is slowly dried upon the same 

 glass plate upon which it had been mixed in a moderately warm room, carefully pro 

 tected from dust, and, when dry, rubbed to a fine powder, and mixed with three 

 grammes of carbonate of silver. 



In this manner we obtain 33 grammes of light purple pigments from 0'5 gramme 

 of gold. 



The above proportion of lead-glass and carbonate of silver to the gold precipitate 

 holds good only for a certain temperature, at which the colour must be burnt-in upon 

 the porcelain, and which is situated very near the fusing-point of silver. 



To obtain the colour with a less degree of heat, the amount of lead-glass added to 

 the gold must be greater, but that of the carbonate of silver less. The same holds 

 good with respect to the preparation of the purple pigment for glass-painting. 



The best purple may be spoiled in the baking in the muffle. When this is done at 

 too low a temperature, the colour remains brown and dull ; but if the right degree of 

 temperature has been exceeded, it appears pale and bluish. Reducing, and especially 

 acid vapours, vapours of oxide of bismuth, &c., have likewise an injurious effect 

 upon it. 



Dark Purple. The clear and neutral solution of - 5 gramme gold in nitromuriatic 

 acid is diluted in a glass vessel with 10 litres of distilled water, and mixed under con- 

 stant agitation with 7'5 grammes of the solution of protochloride of tin of 1'700 sp. gr. 

 prepared in the manner described above. The liquid is coloured of a dark brownish- 

 red ; but the precipitate is only deposited on the addition of a few drops of concen- 

 trated sulphuric acid. The siipernatant liquid is poured off, and replaced five or six 

 times successively with an equal amount of spring water. The precipitate, which is 

 sufficiently washed, is collected on a filter ; and after the excess of water is (brained 

 off, removed while still moist with a spatula, and mixed, exactly as described for the 

 light purple, upon a glass plate with 10 grammes of the above lead -glass, dried, then 

 reduced to n, fine powder, and mixed with 0'5 gramme carbonate of silver ; it fur- 

 nishes about 13 grammes of dark purple pigment. The stated proportion of lead-glass 

 and carbonate of silver to the gold is for the same temperature of firing as given 

 for the mixture of light purple ; for a lower temperature, and also for painting 

 upon glass, the quantity of lead-glass must be increased, and that of the silver salt 

 diminished. 



Bed Violet. The gold precipitate from 0'5 gramme gold is prepared in the same 

 manner as for the dark purple, and whilst moist taken from the filter, and mixed inti- 

 mately upon the plate of glass with 12 grammes of a lead-glass prepared by fusing 4 

 parts of minium with 2 parts of quartz-sand and 1 part calcined borax ; it is then 

 dried as above, and reduced to a fine powder upon a plate of glass, but without any 

 addition of silver. The proportion of lead-glass to gold applies likewise for the same 

 degree of temperature as in the case of the light and dark purple pigments ; a lower 

 temperature requires a larger proportion of lead-glass. A slight addition of silver to 

 this pigment converts the red violet into a dark purple : and when employed alone 

 for painting upon glass, it gives a very excellent purple. 



Blue Violet. This same gold precipitate of 0'5 gramme gold is mixed, while still 



