As regards the glazes themselves they may either be free from lead or contain it.
Glazes free from lead are prepared by fritting and subse quently grinding the following constituents : Common salt, 4o parts ; saltpeter, 22 ; potash, 22; pow dered glass, i6. On account of its slight content of silica, this glaze is suitable for masses containing sand.
On the other hand, for a strongly clayey mass or one 'con taining marl, the following mass, ground and fritted, is very suitable on account of its high content of silica : Saltpeter, 20 parts ; potash, 2o; pulverized glass, 20; pul verized silex, zo ; common salt, o; clay, 10.
To those two formula;, taken from Bronguiart, we add two methods according to Chaptal : Dip the articles to be glazed in a dilution of Marviel-earth and, after drying, repeat the manipulation in water containing porphyrized glass. In burning the constituents fuse to an earthy alkaline colorless glaze.
According to the second method, dip the well-dried articles in a very strong solution of common salt, and burn. The silico alkaline glaze formed requires a mass containing silica for the formation of the silicate with the common salt.
In the following table we give glazes containing lead, the bases being lead monosulphide or galena (aIquifour) and min ium or red lead.
The cheaper galena, as compared with the more expensive minium, has the disadvantage of yielding, with the same weight, one-half less glaze, and hence is in the end as costly ; on the other hand, it is not nearly so dangerous as regards the poison ing of the workmen.
The manufacture of colored glaze tiles, as carried on in Munich and its vicinity, where it is very skillfully done, is much imitated in France. A brief sketch of the Munich process may, therefore, be of interest.
The mass prepared with great care consists of one volume of potter's clay, one-half volume of red clay, and one volume quartz sand, or one volume marl and one volume quartz sand, the following composition of glaze or enamel being especially suitable for this mixture. Great attention is also paid to the working of the glaze composition. The separate constituents are very finely ground dry and then treated with water. The composition consists of twelve parts by weight of lead-ashes, four of lithargyrum, three of quartz sand, four of white alumina, two of common salt, three of powdered glass and one of saltpetre.
Flat tiles having been carefully dried and strongly burned are, previous to glazing, placed for two days in water to free the surface from impurities acquired in burning and to slake the lime which may be contained in them.
For the purpose of glazing, the constituent parts of the glaze are sifted, thoroughly mixed and vitrified in glass pots. The glass thus obtained is finely ground, with the admission of water, in a mill, and in this state employed. By adding, prior to the preparation of the lead-ashes, twenty to twenty-five pounds of tin to every wo pounds, the whiteness is considera bly increased.
For the production of colored enamel to be applied with the brush, add to every ioo pounds of enamel mass the following substances, sifted and finely ground, whereby the respective colors are obtained. By greater or smaller deviations, all imaginary shades can be produced. For Golden yellow 5.12 lbs. of antimony.
Pale blue 0.305 lbs. red ox. of cobalt.
Violet .... 2.500 lbs. red manganese.
Deep brown violet . 5.000 lbs. red manganese.
Red 3.75o lbs. red manganese. Green . 2.500 lbs. copper ash. Black 0.100 lbs. lithargyrum.
0.050 lbs. manganese.
In the manufacture of enameled tiles, lead-poisoning some times occurs. The great principles to be observed in all works where lead is used, are the inculcation of cleanliness ; avoiding eating with unwashed hands, or in working-clothes, or in work-shops ; moist grinding; free ventilation ; precautions against dust raising, or wearing of flannel respirators when dust is unavoidable ; and occasional doses of sulphate of magnesia acidulated with sulphuric acid. Sulphuric acid lemonade has been recommended as a drink.
In acme lead-poisoning from any cause, the stomach must be emptied by the stomach-pump, or by emetics—of which sul phate of zinc is to be preferred. Solutions of the alkaline or earthy sulphates—of which the best is sulphate of magnesia— are indicated, with the view of forming the comparatively in soluble sulphate of lead, and expelling it from the intestines.
Workmen who begin to show signs of lead-poisoning should at once give up the work, and take to some other employment.