64.

dependent upon its refractoriness, that is, at how high a temperature it fuses
the higher grades of clay having, as a rule, high fusion points. A long tem-
perature interval between the points of vitrification and fusion is also a
very desirable quality in a clay that is to be used in the manufacture of
vitrified ware. 7 :

The more important varieties of clay that are recognized in the clay
industry are mentioned below. Their suitability for use in the manufacture
of certain wares is determined by the degree in which they possess the
properties mentioned above, that is, their plasticity, resistance to fusion,
colour upon burning, and so forth. It must be understood that there are
many clays with properties intermediate between these groups.

KAOLIN.

Kaolins are white burning, residual clays that contain mostly silica, ©
alumina, and combined water with few fluxing impurities. When tested |
they usually show low plasticity, low air shrinkage, low tensile strength,
burn white, and are highly refractory, that is, they fuse at a high tempera-
ture above cone 80 (1730 degrees C.). Kaolin mixed with quartz, feldspar,
and bone ash, is used in the manufacture of high grade porcelains. Kaolin ~
is used also for making semi-porcelain whiteware, for floor and wall tiles,

as a filler in paper manufacture, etc. Kaolins are washed to free them. —
from the silica and unaltered rock that is present in all residual clay deposits.
The kaolin deposit at St. Remi, north of Grenville, Que., is the only one of
commercial size that has been opened up in Canada. It occurs in Pre-
cambrian quartzites. ; 2 a

cs BALL CLAY.

Ball clays are a variety of white burning clays that are very plastic.
They are used as a binding medium in the manufacture of whiteware or
semi-porcelain. There are very few known occurrences in the United
States. A clay occurring at Willows in southern Saskatchewan may
be considered a bail clay. se

REFRACTORY OR FIRE-CLAY.

Fire-clays are those clays that can withstand high temperatures with-
out fusing. This means that they are low in fluxing impurities such as
iron, lime, magnesia, and alkalis, and that the amount of free silica is also
low.2. The better grades of fire-clay. fuse at temperatures above cone 26
(1,650 degrees C.). Those that fuse at temperatures between cones 26 and
30, inclusive (1,650 degrees and 1,730 degrees C.) are classed as No. 3, a
between cones 30 and 33 (1,730 degrees and 1,790 degrees C.) as No.2,
and above cone 33 as No. 1 grade fire-clays. They vary in plasticity
tensile strength, and other properties. Most fire-clays are white or yellow-
ish white and burn to a light colour. There are instances of fire-clays
however, that are black or even yellowish red in the raw state. Fire-clays
are used in the manufacture of firebricks, retorts, furnace linings, and for
other purposes where resistance to heat aftec manufacture is of importance.
They are used also in the manufacture of floor tiles, terra-cotta, paving a
bricks, and so forth. >

Teas N. B., ‘‘Report on the clay resources of southern Saskatchewan.’ Mines Branch, Pub. No. 468, 1918 =

pp. 14, 15. : y ,
2Ries, H., Op. cit., p. 174.