PURPLE DYEING, ANCIENT AND MODERN. 391 



GERMAN METHODS. 



I'cvrts. 



I. White potter's clay, free from water 100 



Glauber salt, free from \vat(;r S.5 to 100 



Coal 17 



II. White potter's clay, free from water 100 



Glauber salt, free from water 41 



Soda, free from water 41 



Sulphur 13 



Coal ; ; ; ;; n 



FRENCH METHOD. 



White plotter's clay, free from water ] 00 



Soda, free from water 100 



Sulphur GO 



Coal 11 



The next operation to be performed is that of what is called the over-glow- 

 ing of this mixture. It is placed in melting pots of potter's clay, formed to 

 withstand intense heat, and slowly dried till burnt. Absolute exclusion of air 

 being indispensable, it is especially requisite that the meltiug pots be so tempered 

 that they will neither burst nor become softemid in the intense heat requisite 

 to burn the mass within them. They may vary from 4 to 12 inches in height, 

 with the like diameter. When filled up they are packed one on the other in a 

 furnace resembling in form a flattened brickkiln. They occupy the whole 

 centre of the surface, while the space on each side of them is used for the burn- 

 ing of similar pots. The furnace being properly filled, the mouth is walled up, 

 and the firing commences. The burning continues during from seven to eight 

 hours up to three days, according to the size, construction, and contents of the 

 furnace. Fuel must be added till the mass is thoroughly incorporated and 

 begins to melt. Upon this operation everything depends. If it be not prop- 

 erly conducted, the best and most accurately proportioned raw material will 

 not yi'eld a profitable result. The temperature must be of a certain height, 

 which is to be ascertained beforehand by trials in a small testing oven. It 

 approaches a bright red or incipient white heat, and must be kept at the same 

 point during a specified time; and it msist be mule to heat the whole mass as 

 thoroughly as possible. When the furnace is cooled, the glowed mass is taken- 

 out and cooled with water, and then repeatedly washed and drained to renu)v 3 

 any salt still remaining. The now dried and spongy mass is next removed to 

 the mill and broken and pulverized to the utmost possible degree of fineness ; 

 the powder is repeatedly washed with water, and after being thoroughly dried, 

 again ground and nicely sifted. It lias now reached the first stage of ultra- 

 marine, or what is called green ultramarine, and is ready either for sale or for 

 transmutation into the blue colored or proper ultramarine. Hitherto, however, 

 the green ultramarine has been in no very great request, as compared with- the 

 blue. It varies through several shades, from apple green to blue green; and ia 

 beauty it is far excelled by the copper color and even by the cobalt. Its chief, 

 if not its only recommendations are its cheapness and its innoxiousness ; and 

 those qualities, important as they undoubtedly are, seem insufficient to counter- 

 balance its want of brilliancy. 



The next important operation is the transmutation of the green into the blu-^ 

 color. Here there is but one cause for anxiety. To obtain a perfectly beautiful 

 blue, we must previously have a perfectly beautiful green. The latter is roasted 

 with sulphur, air being freely admitted during the process. It sometimes hap- 

 pens that the change of color takes place without any interference. The sul- 

 phuric natrium contained in the mass causes spontaneous ignition on the admis- 



