CLAY MANUFACTURES. 683 
body is sold for several thousand dollars. The clays chosen are selected 
with reference to plasticity, shrinkage, liability to crack, color, ete.; in 
a mixture, at least one light clay is employed, and the aim is to keep 
the mixture of clays as light in tint as may be and still secure the other 
qualities necessary. The flint is used in the finest state of division, and 
is perfectly white, as is the spar also. The body mixture of kaolins 
alone would, if heated, be liable to crack without apparent cause, and 
would be infusible at the heats applied. 
By adding silica, which sometimes forms nearly one-half the mix- 
ture, the body is very much whitened, and the clay is much more like 
a stone-ware clay in composition, and is prepared to “fret” or vitrify on 
heating, but because of the purity of the reagents there is nothing 
present to cause vitrification with the free silica. Should this body be 
burned, all tendency to shrink or erack would be gone, but the bond 
would be very slight which would hold the mass together; a blow ona 
thin edge would give a dead, wooden sound, which well illustrates the 
lack of close union in the particles. By adding spar (which is powdered 
orthoclase, containing 14 per cent. potash) the mixture is complete; 
the color is corrected by the flint, as well as the tendency to shrink and 
crack, and with the presence of the spar the burning immediately causes 
a thorough vitrification of the whole mass to a homogeneous solid with 
a slightly glassy fracture. A blow ona piece of this mixture would 
give a clear, ringing sound, which also illustrates its state of combina- 
tion. As there is necessarily some small amount of iron in the clays used, 
its yellow color is usually counteracted by the use of a very little cobalt. 
The blue and yellow colors really unite to produce a green, but this 
color has not nearly so strongly marked a character as either of its 
constituents, and escapes observation. 
The glazes used in white-ware are much more complicated than for 
yellow-ware, and require perhaps the most skillful work of all to get 
just right ; there is more value placed on the composition of a good glaze 
than any secret about a pottery, even including the composition of the 
body. The constituents and way of mixing an ordinary glaze are as 
follows: Proportions of borax or boracic acid, or both, with flint, spar, 
clay, and Paris-white are mixed while dry and put into a sagger, which 
has been previously coated with a wash of flint. This precaution is 
taken because the liquifying of the glaze would allow some of the iron 
from the sagger clay to color it if it were not protected by the pure 
