A Text-book of Paper-making by Charles Frederick Cross Edward John Bevan J. F. Briggs

Author:Charles Frederick Cross, Edward John Bevan, J. F. Briggs
Language: eng
Format: epub
Publisher: Spon & Chamberlain
Published: 1907-03-25T05:00:00+00:00

Fig. 43.

FiQ. 44.

in water—which occasions loss on the machine. This is, of coarse, minimised in long runs on the machine, when^the ' back water ' becomes saturated with the sulphate.

PAPER-MAKING.

Some of the finer qualities of paper are made without addition of any loading material whatever, though such papers

Pig. 46.

Pig. 46.

are of course the exception. The proportion of clay or other material that can be retained by a fibre depends to a certain extent upon the nature of the fibre, and upon the degree of

SIZING, LOADING, COLOURING, ETC.

223

fineness to which it is reduced in the beater. The amount added by different makers varies considerably, from 2 or 8 p.ct. to 20, and even in rare cases to 80, p.ct.

Other mineral filling agents having a certain fibrous character have been from time to time introduced, possessing certain advantages over china clay or calcium sulphate. Agalite (Fig. 46) is a mineral of the nature and chemical properties of asbestos : it consists of nearly pure magnesium silicate. Its structure is more or less fibrous, like that of asbestos, which, as is well known, can be spun and woven, and even made into paper, and therefore, when added to a paper, it forms a part of the fabric itself. It is even claimed that it assists in keeping back some of the finer fibres that invariably find their way through the meshes of the wire-cloth, and it is said that 90 p.ct. of the amount added to the engine is found in the paper. In the case of china clay it is well known that only from 40 to 60 p.ct. is often actually * carried * by the pulp. Figs. 43, 44, 45 and 46 show the appearance of china clay, pearl-hardening and agalite when viewed under the microscope, magnified 200 times. The nature of agalite is such that it assists the paper in taking a high finish. This is probably due to its ' soapy ^ nature, a feature which is characteristic of asbestos, French chalk, ' soap-stone,* and other magnesium silicates.

The f oDowing analysis of china clays may be cited :—

Alumina Iron oxide Calcium Magnesium Silica . Water .

Sp.gr

varying from 2*5 to 2*8.

The following is an analysis of * agalite':—

Magnesia Iron oxide Alumina Silica . Water .

Sp.gr

varying from 2

6 to 2*8.

When papers contain such excessive quantities as 15 or 20 p.ct. of clay, it cannot be to the advantage of the consumer, and should be looked upon as an adulteration.

Analytical Determination of Loading,

It is a matter of some importance to be able to determine rapidly and accurately the amount of mineral matter in a paper. The usual method is to ignite a weighed quantity of the paper in a platinum crucible until the ash so obtained is either white or a very pale grey. From the weight of the ash, the percentage of mineral matter is easily calculated.

In certain cases it may be necessary to take into account both the normal ash of the paper and that due to other added mineral matter.

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