678 



Repoet of the State Geologist. 



present, they may form a white coating on the surface of the burned ware. 

 The decomposition of sulphates is also accompanied by the disengagement 

 of sulphuric acid, which, if it escapes with violence, may cause blistering. 



Lime is a common detrimental impurity. It is abundant in some mem- 

 bers of the New York shales, especially those of the Salina group in central 

 New York. When present to the extent of only two to three per cent, it is 

 practically harmless. The common source of lime in shales is feldspar, which 

 is a double silicate of lime and soda ; or calcite, which is carbonate of lime. 

 The latter may be detected by the effervescence produced on the addition of 

 muriatic acid. 



Lime in the condition of silicate may serve as a useful flux, if present to 

 the extent of four or five per cent. 



Carbonate of lime may render a shale very fusible and materially lower 

 the difference in temperature between the points of incipient fusion and 

 viscosity. Lime also diminishes the shrinkage of a clay in burning ; in fact, 

 an excess, say twelve to fifteen per cent., may cause the clay to swell slightly. 



An excess of lime over iron will counteract the reddening effect of the 

 latter by the formation of a silicate of lime, iron and alumina. 



Seger found that if the lime exceeded the iron in the proportions of three 

 to one, the product is a good buff. lie also found that a good brick could, 

 with care, be made from a clay containing twenty to twenty-five per cent, of 

 carbonate of lime, but it is not safe to make a vitrified one from such clay. 

 Sulphate of lime is not uncommon in some New York shales, and its action 

 on heating may be the same as described under sulphates of the alkalies. 

 Some of the marly shales of central New York may be found more suitable 

 for the manufacture of Portland cement than for clay products. 



Magnesia is not an abundant element in most shales, although in some of 

 the Salina shales it is common. Magnesia is derived from the same classes of 

 compounds as lime, and, as far as known, exerts the same influence. 



Iron. This element acts not only as a flux but also as a powerful color- 

 ing agent of clay materials, both in their green and burned conditions. It 

 may exist in clays in a variety of forms, according to the mineral of which it 

 is a component element. These mineral compounds may be silicates, carbon- 

 ates, oxides or sulphates. In all these combinations it exists in the condition 

 of either a ferrous or ferric salt ; but in burning, the former generally become 

 oxidized to the latter, unless the tire is reducing in its action. When pyrite 

 occurs in the shale, it will, if in grains or lumps, produce fused spots in 

 burning. It acts as a strong flux. 



