516 NEW YOEK STATE MUSEUM 



The iron oxids, limonite and lieniatite, are present in nearly all 

 clays. They may be introduced by percolating waters, or result from 

 the decomposition of any of the iron-bearing silicates, such as horn- 

 blende, mica or garnet. They are not infrequently distributed 

 through the clay in a very finely divided condition, or may form a 

 thin film around the other mineral grains. Limonite tends to color 

 the unbumed clay brown or yellow, while hematite imparts a red 

 color. Ferric carbonate may give gray tints. Mica is found in 

 most clays. Hornblende and garnet are probably wanting in a few. 

 Pyrite is present in many clays, specially stoneware and fire clays, 

 its yellow, glittering metallic particles being easily reco'gnizable. 

 These particles may be either fine grains, or large lumps, the former 

 of which have to be separated by washing, the latter by hand- 

 picking. Pyrite alters under the influence of weathering or burn- 

 ing to sulfate of iron, which is soluble in water and may indirectly 

 or directly act as a discoloring agent on clay wares, provided the 

 clay is not burned to vitrification. If burned to this point however 

 the pyrite acts as a flux (and according to Wipplinger^ a very strong 

 one) forming little specks, or larger ones, according to size of pyrite 

 grains, of fused ferrous alumina silicate. In all iron-bearing 

 minerals found in clays, the iron exists in one of two conditions, 

 viz,, as ferrous or ferric, and the fusibility of any given clay de- 

 pends somewhat on this fact, for the reason that ferrous coanpounds 

 lower the fusing point of a clay. In burning any clay the ferrous 

 salt will be changed to the ferric condition, provided the fire is 

 oxidizing in its action, but if the action is reducing, the iron 

 will remain in the ferrous form. The action of weathering 

 agents in nature is often sufiicient to o^xidize the iron in the clay, 

 so that in most clays more ferric than ferrous iron will be found. 

 Evidence of this change in the condition of the iron can often be 

 detected by the red or yellow color of the upper or more porous 

 layers of the clay, the lower layers being colored gray. A gray 

 color may at times be produced also by the presence of organic 



1 Keramik, p. 26. 



