A PRELIMINARY REPORT ON CLAYS OE ELQRIDA 
83 
It is believed by many that a highly calcareous clay is entirely un¬ 
desirable. This, however, is not always the case. 'Wisconsin clays 2 , 
highly calcareous, produce a good building brick. When a vitrified 
ware is not attempted and the lime is in a finely divided and evenly dis¬ 
tributed state, a good clay may contain as much as 20 to 25 per cent. It 
is interesting to note that the calcareous Wisconsin clays used in the 
manufacture of brick, are burned, with a few exceptions, at a much 
higher temperature than the non-calcareous ones * 1 . 
The lime contained in the silicates have no noticeable detrimental or 
beneficial effects. While they may act as fluxes, they do not cause the 
ware to soften rapidly. 
When lime is present in the form of the sulphate it will, when 
heated sufficiently high, be broken up into calcium oxide, (CaO), and 
sulphur trioxide, (SOs). The sulphur trioxide may cause blisters or 
cracks in the ware as it escapes. 
Magnesia —Magnesia may occur in the same form as lime, but the 
silicates in this case are the most important sources. In this form it 
acts in much the same manner as calcium silicate, but is not quite so 
active. While magnesia acts as a flux in firing it does not cause the clay 
to soften as rapidly as calcium and the points of incipient fusion and 
viscosity are more widely separated. Magnesium-bearing minerals 
which might occur in clays are biotite (black mica), hornblende, chlorite, 
dolomite, etc. The chlorite may be abundant in some partially decom¬ 
posed residual clays and the dolomite in some sedimentary ones. 
Alkalies —This term is used to include potash (K 2 O), soda (Na20), 
and ammonia (NHs). Ammonia is present in some raw clay, but as it 
is easily volatile it readily escapes when the ware is heated. Soda and 
potash,, called the fixed alkalies, are present in almost every clay. The 
complex silicates, as mica, greensand, orthoclase, etc., are the principal 
sources of the alkalies in clays. Some alkali carbonate may be held in 
clay by adsorption. They are regarded as the most active fluxes present 
in clays. Except in the case of refractory clays, they are desirable consti¬ 
tuents when in the form of silicates. By their fluxing action they bind 
the clay particles together in a dense hard body at a lower temperature 
than would otherwise be possible. It is for this reason that feldspar 
is used as a flux in many high-grade wares. There is practically no 
coloring effect exerted by the alkalies. 
2 Clays of Wisconsin, Wisconsin Geological Survey, Bull. XV, p. 18, 1906. 
1 Cl ays of Wisconsin, Wis. Geol. Survey, Bull. 15, p. 36, 1906. 
