342 REPOET OF NATIONAL MUSEUM, 1899. 



present in the usual small amounts it produces an incipient vitrifica- 

 tion which makes the ware ring like a bell when struck, and is often a 

 help in selling. 



The extent to which iron may be present without detriment is a 

 point on which authorities do not agree. The Stourbridge clay of 

 England, acknowledged to be the most refractory clay known, has 2.25 

 per cent of iron on an average of 100 analyses, with extremes of 1.43 

 and 3.03. Gros Almerode clay, has 2.12, Coblentz, 2.03, New Castle, 

 2.32, and yet all these clays are famous. Test mixtures of iron and 

 pure kaolin have been run higher than this and have stood well, but as 

 a general rule it is unsafe to rely for fine qualities on a clay with over 2 

 per cent of iron, particularly if the other impurities are developed in 

 any amount. It is a well-known principle in chemistry that mixtures 

 of bases are much more active fluxes than an equal amount of any one 

 base; so with iron, its eflect shows worse when in presence of other 

 fluxing agents. 



The state in which the iron is present makes some difi'erence; if as 

 the sesquioxide, it takes more heat than when in the protoxide state 

 to combine in the clay, for iron will only combine with silica in the 

 protoxide state, and if that state is already developed, it is easier to 

 combine the sand and iron than if in the other oxide. 



Sulphide of iron has a bad effect on the clay since its decomposition 

 gives rise to the lower oxide of iron, besides the effect which the sul- 

 phur may have. 



Silicate of iron is also detrimental, since it melts at a comparatively 

 low temperature. On a piece of ware, iron in the uncombined state 

 imparts a buff' or red color; when combination begins and progresses 

 the ware is of a bluish-gray cast, deepening as the fusion of the iron 

 proceeds, and running to glassy black if much iron is present. 



Lime and magnesia act as fluxes on clays, but in any but the glacial 

 clays the comparatively small amounts present makes them but little 

 thought of as detrimental. They are probably present as silicates, 

 and as these are readily fusible, their action is evidently unfavorable. 

 When these bases are present as carbonates they combine at a higher 

 temperature than iron or potash. The Milwaukee bricks, as already 

 noted, are full of carbonates of lime and magnesia, and require a very 

 hot burn, but when once the lime and silica combine they destroy the 

 effect of 5 per cent of iron, enough to make the clay perfectly black. 

 A brick of this kind presents an even, fine-grained, vitrified appear- 

 ance on its fracture.^ 



^They (lime and magnesia) have also the remarkable property of uniting with the 

 iron ingredient to form a light-colored alumina-lime-magnesia-iron silicate, and thus 

 the product is cream-colored instead of red. Mr. Sweet has shown by analysis that 

 the Milwaukee light-colored brick contain even more iron than the Madison red 

 brick. At numerous points in the Lake region and in the Fox River valley cream- 



