LOADING 125
are free from grit and sand, and the particles are very fine and white. These
are used for coating papers. Most paper-makers' clays contain more or less
mica, according to the quality, and this shows up in paper as glistening specks.
Clay is of a colloidal or plastic nature when mixed with water. The old cir-
culating tanks used in some mills, combined with strainers, to separate the
clay from grit, etc., have in many mills given place to centrifugal machines,
which are more positive and continuous in their action.
Another important filler is calcium sulphate (CaSCX), known also as gypsum,
terra alba in its natural state, and as pearl hardening, crown filler, satinite, etc.,
in the artificial production. Gypsum and terra alba are the ordinary mineral
ground fine and have a crystalline structure. Pearl hardening is prepared by
precipitation with acid and is of a fine white colour, clean and free from grit.
It is partly soluble (i Ib. to 45 gallons of water), and therefore a great deal is
lost if the back water of the machine is not well conserved. It can be used to
improve the colour of high-class papers. As satin white, in paste form, it is
used for paper coating.
Magnesium silicates in the form of asbestine, agalite and talc are sometimes
used as fillers, the particles being of a fibrous form, and well retained by the
paper; they are also 'soapy' and give a high finish. Barytes and bkac fixe
are forms of barium sulphate. The latter is the precipitated quality and is sold
in paste form for paper coating.
Almost every paper of any substance, except hand-made and some very
expensive all-rag papers, contain mineral loading. All printing papers require
china clay, which enables the paper to have a close regular finish, and is very
absorbent of printer's ink, besides allowing a cheaper paper to be produced.
It also takes colours well and brings up the brightness of the shades. Used in
moderate quantity in blottings, it gives a soft smooth feel, taking away a good
deal of the harshness of the fibres, and helps in the retention of the dyes, which
in this case have to be used without a mordant.
Calcium sulphate fillers are not used to such an extent as china clay, but
produce much the same results, except that they do not give the same limp
feeling and are less inclined to make the paper soft and flabby.
All these loadings, however, reduce the bulk of the paper, being twice the
weight of fibre, bulk for bulk.
A loading which has received much publicity during recent years is titanium
oxide. The particular merit claimed for this material is that it imparts a greater
degree of opacity than the commoner forms of loading. There is no doubt
that used in the correct proportion, which varies according to the degree of
opacity required, a superior result can be achieved in this respect, but it is a
very expensive material, and it is doubtful whether the weight saved by enabling