A PRELIMINARY REPORT ON CLAYS OE ELORIDA 
81 
clay. Indeed, sand is often added to some brick clays having a high 
shrinkage in order to reduce the shrinkage. 
Contrary to the belief of some, the addition of quartz does not raise 
the fusion point of a clay. Quartz alone is very refractory, but in the 
presence of highly aluminous low-flux clays its refractoriness is re¬ 
duced. A high percentage of silica for a very refractory clay is not 
desirable. 
Alumina —-Alumina is one of the common constituents of all clays 
and is derived not only from kaolinite but many of the other silicates as 
well. Alone it is highly refractory, but in the presence of fluxes its fusion 
point is lowered. 
Iron Oxide —This includes both the ferric oxide, Fe203, and the 
ferrous oxide, FeO. The iron oxides in clay are derived from such 
iron minerals as limonite, hematite, pyrite, siderite, and also from such 
silicates as mica, hornblende, garnet, glauconite, etc. 
Iron acts as a strong coloring agent in both the burned and un¬ 
burned clay. Its range of color influence is from a very faint cream, 
through yellow and buff, to all shades of red, brown, and blue to black. 
The resulting color, however, is not solely determined by the quantity 
of iron oxide present but also by the texture and distribution in the clay, 
the form of the iron, whether ferric or ferrous, condition of the kiln 
atmosphere, whether oxidizing or reducing, and the neutralization effect 
of other constituents as lime. 
The iron oxides likewise exert a strong fluxing action, thus lower¬ 
ing the fusion point of the clay. Ries 1 states that “this effect will be 
more pronounced if the iron is in a ferrous condition or if silica is 
present.” The iron oxide probably enters into combination with silica, 
forming an easily fusible silicate. 
Iron may therefore be either a desirable or an undesirable ingredient 
of clays. In the lower grades of ware its coloring and fluxing action is 
beneficial. The fluxing action reduces the temperature at which burning 
must be done. In other wares, such as white wares and refractory prod¬ 
ucts, it is detrimental. It is obvious that in white-burning clays the iron 
content must be very low. 
iRies, H., Clays, Their Occurrence, Properties and Uses, p. 85, 1908. 
