124	MODERN PAPER-MAKING
The acidity of sizing agents is now controlled by pH value, as are many
other reactions in the paper-mill.
A solution of alum is always dirty and full of jute, hairs, etc., from the
bags in which it comes, so that it must be carefully strained through flannel
over a fine-mesh wire cloth when it is being put into the beater, or it may be
strained when being run from the dissolving tanks to the store tank.
The furnishing of dry alum direct into the beaters is not to be recom-
mended, even in news mills, as the hairs and other impurities contained in it
foul the sheet, and also are frequently the cause of breaks at the machine. The
provision of proper dissolving tanks is a simple and inexpensive way of avoiding
this trouble, and it is also more economical in the long run.
Loading.—There are two reasons for the loading of paper with mineral
substances. The first, and no doubt original, reason is for the purpose of
producing a cheaper paper by substituting loading for fibre. As there is no
record of the introduction of this process, we may conjecture that some astute
paper-makers kept the secret to themselves and reaped a rich harvest until it
leaked out.
The second is that the use of loading, or fillers, imparts special properties
to papers, and it is therefore a necessity for some classes of paper.
The most important of these fillers is china clay or kaolin. This is the
remains of very ancient deposits of felspar or granite. Its chemical composition
is about half silica (SiO2). The other half is made up of about 35 to 40 per
cent of alumina (A12O3), 12 to 15 per cent of water and traces of various sub-
stances such as calcium, magnesium, iron, mica, etc.
It is found in Cornwall and Devon, but huge deposits exist in many other
parts of the world. It is usually found close under the surface, and is dug out
so that great pits are made. It is not usable without treatment, as it is mixed
up with sand and grit, mica particles, etc., and must be refined and cleaned to
be of any use to the paper-maker.
The clay is broken from the face of the pit banks by directing against it a
jet of water at high pressure, or by men with picks, who break down the
cky and direct it into a flowing stream of water. It is allowed to settle
at the bottom of the pit long enough to let the heavier substances fall to the
bottom and the clay suspended in the water is pumped to the surface. It then
passes through 'mica drags' or shutes; the finer particles pass through
screens, leaving mica behind. The stream of water and clay is then run to
settling ponds or pits. From thence it is run to storage tanks and the clay
falls slowly to the bottom, leaving the clear water at the top. The water
being run off, the clay is dried in drying pans or kilns and broken up for use.
As may be expected, there are many grades of clay.   The best qualities