A clay having been selected in accordance with the tests described, it
becomes necessary to prepare it for use. A fairly large supply should be
obtained and stored in a dry place. Most natural clays need some kind of
cleansing for there are almost always foreign substances present. This
cleansing is accomplished by reducing the clay to the fluid known as
slip. The necessary appliances for making slip are as follows:
A large sieve of quarter-inch mesh.
A small wire sieve of about 14 meshes to the inch.
A large barrel.
Two galvanized pails.
The clay is, after drying, powdered and sifted through the large sieve.
One of the pails is half filled with clean water and the clay, handful
by handful, is sprinkled into it. The clay rapidly absorbs the water and
sinks to the bottom. The addition of clay is continued until a small
mound rises through the water, when the whole is left to soak for an
hour. The bared arm is then plunged into the pail and the mass stirred
vigorously. A stick or paddle will serve, of course, but the potter
learns a great deal by the feel of the clay and therefore the hand is
best. It is said that he is a poor sailor who will not dip his hands in
the tar bucket and in like manner, he is a poor potter who fears the
slip tub. This stirring will tell a good deal about the probable working
of the clay. It may be stony or sandy or greasy. The large stones and
roots will have been removed by the sieve but now, after thorough
mixing, the slip is poured through the small sieve into the barrel. Both
pails may be kept going at once, one being filled while the other is
soaking and so on until the barrel is full or, at least, a good quantity
of slip has been prepared.
If the clay prove very sandy it should be washed. The mixture in the
pail having been well stirred is allowed to stand for a definite time,
say one minute. The slip is then poured into the second pail and it will
be found that a quantity of sand has settled. This is thrown away and
the slip in the second pail is examined. If enough sand has been
removed, the slip may be poured into the barrel, using the fine sieve as
already described. If still sandy the process should be repeated, the
settling being for two minutes. Experience is the best guide in this
operation but all the sand should not be removed.
When the barrel is full of slip it is allowed to stand over night when
some inches of clear water will be found at the top. This is removed
with a siphon which may be made of a piece of lead or rubber pipe. The
removal of the water results in the thickening of the slip and the
contents of the barrel should be thoroughly stirred with a long wooden
paddle to insure a uniform consistency. If the slip is found to be still
thin another settling and removal of the water will thicken it.
The slip thus prepared will keep indefinitely, provided that it is not
allowed to become dry by evaporation. It improves greatly with age. This
is the material which is used for casting as will be described later but
for plastic work it must be still further thickened. A shallow box may
be procured and made water-tight and the slip, when poured into it, will
thicken much more rapidly than in the barrel, but it is better to have
some shallow plaster dishes as the plaster itself absorbs the water and
thickens the clay. Instructions for making these dishes appear in the
chapter on plaster.
These directions will suffice for the preparation of a natural clay but
it is sometimes desired to prepare a white body either of earthenware or
porcelain. These bodies do not exist in nature and therefore a mixture
must be made. The ingredients are kaolin or white porcelain clay, ball
clay or plastic potters' clay, ground quartz or flint, and ground
feldspar.[B]
[B] Georgia Kaolin and Tennessee Ball Clay may be procured from the
John H. Sant and Sons Company, East Liverpool, Ohio, and flint and
feldspar from the Golding Sons' Company, Trenton, N. J., or the
Eureka Flint and Spar Company, Trenton, N. J., in quantities of not
less than one barrel or sack.
A suitable mixture for earthenware is--
Georgia Clay[C] 20 parts by weight
Tennessee Ball Clay 30 " " "
Flint 35 " " "
Feldspar 15 " " "
---
100
and for porcelain--
Georgia Clay 45 parts by weight
Flint 35 " " "
Feldspar 20 " " "
---
100
[C] If English china clay can be procured it will make a whiter ware
than Georgia clay.
The earthenware will be creamy in color and porous at an ordinary fire.
The porcelain will need a hard fire and will be white and translucent.
It is, however, non-plastic and hard to work. The preparation of these
mixtures of course necessitates a pair of scales but otherwise the
treatment of the mix is the same as that of natural clay. Washing is not
necessary but the clay must be powdered, mixed with the flint and spar,
and sprinkled into water as already described. In place of the wire
sieve, however, a silk lawn of 120 meshes to the inch should be used.
The lawn is simply a fine sieve and is named because of the material
(also called bolting cloth), with which it is covered. Have a carpenter
make a box without a bottom. Cypress or oak should be used and this
should be a full half inch thick. Four strips of the same thickness are
also to be provided. The box may be of any convenient size; eight inches
square and four inches deep is about right. The sides should be fastened
together with brass screws to avoid rust and a piece of lawn is strained
tightly across the bottom and secured with copper or brass tacks. A
strip of coarse muslin folded and laid along the edges will help to
prevent the lawn from tearing, the tacks being, of course, driven
through both muslin and lawn. Then the four wooden strips are set upon
the muslin and secured with brass screws. The completed lawn is then a
tray of which the bottom is formed of lawn. The strips of wood beneath
serve to protect the lawn when placed on a table as well as to assist in
holding it firmly.[D]
[D] Silk lawn of any desired mesh may be purchased by the yard from A.
Sartorius & Company, 57 Murray Street, New York City; or brass
sieves ready for use from the W. S. Tyler Company, Cleveland, O.
For storing clay in the plastic state there is nothing better than
stoneware jars. These may be had of any size and a tinman should make
close-fitting covers. Earthenware covers do not fit tight and are always
getting broken. A little water is poured into each jar and a support
provided for the clay so that it does not rest in contact with the
water. Under any conditions clay will slowly harden so that not too
large a stock should be kept. Slip, on the other hand, keeps well so
long as some water is always on the top and it is not a long process to
stiffen it into clay.