A PRELIMINARY REPORT ON CLAYS OE ELORIDA 
105 
They should be dense-burning, have a low shrinkage, and be free from 
warping. The absence of soluble salts is an essential property. Most terra¬ 
cotta is made from a blend of several clays and is commonly fired at 
about cone 6 or 8. Gray or buff-burning fire clays are commonly used, 
but in most cases an artificial color is applied to the surface of the ware. 
Sewer-pipe Clays —Clay for sewer-pipes should have a low shrink¬ 
age, high strength, and be free from warping and cracking. It should 
be vitrified, hence a clay high in fluxes is best, but at the same time it 
should have a long firing range. Vitrification should be reached at cone 
3 or 4, but some clays used vitrify at a lower temperature. This type 
of ware is commonly covered with a salt glaze. Clays suited for sewer- 
pipe manufacture are similar to those used for paving brick. A dark- 
colored sewer pipe is most commonly called for on the market. 
Brick Clays —Clay for common brick is usually impure, often sandy, 
high in fluxes, of medium plasticity, variable strength, color and porosi¬ 
ty. They are in most cases red-burning but sometimes gray, cream, 
buff, or yellow. Common brick-clays are widely distributed. 
Face brick, or sometimes called front or pressed brick, requires a 
better grade of clay than common brick. They must have a uniformity 
of color in burning, freedom from warping or splitting, absence of solu¬ 
ble salts, good hardness after firing and low porosity. The shrinkage and 
transverse strength are variable. Front brick may be red, white, cream or 
buff after firing. Such clays are commonly fired from cone 1 to 9. Clay 
for face brick should be more plastic than that for common brick so that 
the ware may retain its form better and present smooth faces and square 
corners. Plasticity, however, is cot an important factor in dry-press 
methods. - The clay should have a good firing range and should be 
burned so hard that it cannot be scratched with a knife. 
Clay for paving brick should have the same general properties re¬ 
quired of a sewer-pipe clay. Ordinarily such clay is burned to the point 
of vitrification but from, some clays the toughest product is obtained 
before vitrification is reached. Such clays should have fair plasticity and 
good transverse strength. The firing range should be at least 150° C. 
Hollow block, fire proofing, and drain tile all require clay of the 
same characteristics;, in fact, they may all be made from the same kind 
of clay. The properties of clay suitable for these products is quite 
variable. They should have a fair transverse strength, burn to a hard 
but not vitrified body at a comparatively low cone (03-1), and should 
have sufficient plasticity to flow smoothly through the peculiar shape 
of die necessary to form them. 
