﻿420 cox. 



In most places, the supply of clay for commercial uses is obtained 

 from surface openings or clay banks and, with the exception of Numbers 

 8 and 29, this is the case with the specimens given in Table I. Some com- 

 panies are operating underground works and so obtain a more uniform 

 and purer product. 



It is exceptional for clays to be used in their natural state for other 

 than common work, because of the numerous bits of rock which are 

 mingled with their mass. In almost every parent rock there are very 

 hard mineral particles which resist the weathering and remain behind, 

 contaminating the clay. A certain amount of sorting is always necessary. 

 Herein is the justification for printing analyses of the unpurified speci- 

 mens. It will at once be evident that the deposits will not through 

 sorting become poorer than the published results. For the above reason, 

 a chemical analysis of a residuary clay may not always be reliable as a 

 means of judging its usefulness from the standpoint of the ceramic 

 industries, as it may contain impurities which render it useless in the 

 rough state, but by grinding and decantation it may be separated from its 

 impurities. Eies s in summing up the facts obtainable from the ultimate 

 analysis of a clay, gives the following main points : 



1. The purity of the clay, showing the proportions of silica, alumina, combined 

 water, and fluxing impurities. High-grade clays show a percentage of silica, 

 alumina and water, approaching quite closely to those of kaolinite. 



2. The refractoriness of the clay for, other things being equal, the greater the 

 total sum of fluxing impurities, the more fusible the clay. 



3. The color to which the clay burns. This may be judged approximately, for 

 clays with several per cent or more of ferric oxide will burn red, provided the 

 iron is evenly and finely distributed in the clay, and there is no excess of lime. 

 The above conditions will be affected by a reducing atmosphere in burning, or the 

 presence of sulphur in the fire gases. 



4. The quantity of water. Clays with a large amount of chemically com- 

 bined water sometimes exhibit a tendency to crack in burning, and may also show 

 high shrinkage. If kaolinite is the only mineral present containing chemically 

 combined water, the percentage of the latter will be approximately one-third that 

 of the percentage of alumina, but if the clay contains much limonite or hydrous 

 silica the percentage of chemically combined water may be much higher. 



5. Excess of silica. A large excess of silica indicates a sandy clay, and if 

 much is present in the analysis of a fire clay, it indicates low refractoriness. 



6. The quantity of organic matter. If this is determined separately and it 

 is present to the extent of several per cent, it will require slow burning if the 

 clay is dense. 



7. The presence of several per cent of both lime (CaO) and carbon dioxide 

 (C0 2 ) in the clay indicates that it is quite calcareous. 



Several of the above analyses do not to any large degree indicate acci- 

 dental impurities. A few show minute traces of sulphuric anhydride, 

 probably accounted for by crystals of gypsum which probably could be 

 removed by washing. The iron in most cases is in all likelihood present 



"Wis. Geol. and Nat. Hist. Sur. (190G), 15, Econ. Ser. 10, 12. 



