1919'\ IXDUSTRIAL ApPLICATIOXS OF ZiRCOXIUM 161 



Working tests show that it has much greater life duration as a 

 lining for furnaces than other refractories. In Germany, experiments 

 were carried out in a closed-hearth steel furnace, and it was found 

 that the zirconia lining was good for eight months use without re- 

 newal. This is several times longer than the usual life. Because of 

 the low thermal conductivity the thickness of the lining could be re- 

 duced one-half, a two-inch lining being equal to four inches of cha- 

 motte. Furthermore, there was a saving of one-half in maintenance 

 costs. In casting molds it shows a high resistance to steel, copper, 

 brass, and bronzes. 



It may be used as a protective coating for ordinary fire brick ex- 

 posed to the action of acids or slags. In such cases sodium silicate 

 serves as a binding material, also air-slaked lime may be added. If 

 it is desirable to increase the porosity and decrease the density, organic 

 substances or volatile salts may be added and burned out in the tiring. 

 It is, of course, detrimental to use a binder which may cause softening 

 at comparatively low temperatures. 



As a refractory it has also been used in making crucibles, muffles, 

 pyrometer tubes, and for a variety of chemical ware. Combustion 

 tubes made of it are said to be gas-tight up to 1.000^ C, Crucibles 

 and combustion tubes of zirconia have been used in the research lab- 

 oratory of the Royal Berlin Porcelain factory, as they possess great 

 strength and also conduct electricity. They withstand high tempera- 

 tures and sudden changes. Zirconia crucibles have been used for 

 determining the melting points of pure iron, tungsten alloys, and 

 platinum. Such ware can l)e plunged in water while red hot without 

 injury. 



Zirconia is also used as an opacifving agent in 



ENAMELS -11 



enamels and a clouding agent m glass as a sub- 

 stitute for the costly stannic oxide and the poisonous compounds of 

 antimony and arsenic. For this purpose it should be quite free from 

 iron, and a number of processes have been worked out and patented. 

 The increasing demand for tin for other purposes and the limited 

 supply may render this substitution necessary. According to some 

 authorities tlie zirconia has less covering power than stannic oxide. 

 For chea})er ware native* zirconia may ho w^r-oiX, or ground zircon which 



