Loading.
China clay or kaolin is sold in the form of large lumps of a white or yellowish-white colour. It is formed by the gradual disintegration of felspar by means of the action of air and water, and consists essentially of a silicate of aluminium. Its quality depends upon its whiteness and its freedom from coarse micaceous particles. It is usually prepared for admixture with the pulp by making it into a fine cream with water in a vessel provided with stirrers; it is then passed through a fine sieve in order to remove any impurities it may contain, and is then run into the beater. The clay or other filling material is usually run into the beater as soon as the latter is charged with pulp, so that by the time the beating operation is concluded, a perfect admixture of pulp and clay is effected.
Sulphate of lime, or “pearl-hardening,” is usually sufficiently pure to put direct into the engine. It is made by decomposing a solution of calcium chloride, with sulphate of soda, and is precipitated as a fine brilliantly white powder, consisting of CaSO4 + 2 H2O.
Two distinct forms of precipitated calcium sulphate are met with in commerce, differing from each other by their microscopical features, the one consisting of flat tabular crystals (Fig. 41), the other of fine needles (Fig. 42). Another form, erroneously called precipitated pearl-hardening, is also sold: it consists of the finely ground native mineral.
FIG. 40.
FIG. 41.
Some of the finer qualities of paper are made without addition of any loading material whatever, though such papers are of course the exception. The proportion of clay or {132} other material that can be put into a fibre depends to a certain extent upon the nature of the fibre, and upon the degree of fineness to which it is reduced in the beater. The amount added by different makers varies considerably, from {133} two or three per cent. to twenty, and even in rare cases to thirty per cent.
FIG. 42.
FIG. 43.
A new loading material called “agalite” has been lately introduced, possessing certain advantages over china clay, {134} or calcium sulphate. Agalite 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 it therefore, when added to a paper, 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 per cent. 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 per cent. is actually “carried” by the pulp. Figs. 40, 41, 42, and 43 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.
FIG. 44.
When papers contain such excessive quantities as 15 or 20 per cent. of clay, it cannot be to the advantage of the consumer, and should be looked upon as an adulteration. 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. The following is a very convenient plan in cases where a platinum crucible or dish is not obtainable:—Take a weighed piece of the paper to be examined, from 2 to 4 in. square, according to the thickness, roll it into a narrow hollow cylinder. Round this wind a weighed piece of platinum wire about 1⁄50 in. thick, as in Fig. 44. Hold this by means of a pair of crucible tongs in the flame of a Bunsen burner until it is completely burned. If the wire is carefully wound round, and especially if the roll of paper is made conical, the ash will be securely held in {135} position. Those who do not possess a chemical balance of the ordinary form will find a convenient substitute, which will answer the purpose of weighing the paper and ash with sufficient accuracy, in the spiral balance invented by Prof. Jolly[11] illustrated in Fig. 44. It consists of a spiral of hard wire A, which is suspended in front of a mirror B, upon which millimetre divisions are marked. A small float D, dipping under the surface of the water in the vessel E, is provided for the purpose of steadying the spiral and allowing it to come quickly to rest. The balance is provided with a light pan made of a thin plate of mica, and suspended by very thin platinum wires. For the present purpose, however, the pan is not necessary, and it can be replaced by the roll of paper and platinum coil as shown in the drawing.
[11] This balance can be obtained from Nalder Bros. & Co., Westminster.
The method of using is exceedingly simple, as the increase in the length of the spiral is in direct ratio to the increment of weight. The position of the spiral is ascertained by placing the eye in a direct line with the small glass bead C and its image in the mirror, and noting the corresponding division on the scale. The position of the bead can be altered so as to bring it to any desired point on the scale by raising or lowering the upright rod F, which is kept in position by the screw G. The balance stands on a foot provided with levelling screws.
It is evident that where proportional weights only are required it is not necessary to know the value of the spiral, but if the balance is to be used to ascertain actual weights, the coefficient of the spiral must first be determined. This is done by noting the increase in length after the addition of a one gram weight. The spirals are made of different thicknesses of wire, which of course give varying degrees {136} of sensibility: the most useful is one which gives with one gram an extension of 100 millimetres: one mm. being therefore equivalent to one mgrm.
The following experiment will illustrate the method of using the balance and of calculating the results of a determination of the amount of mineral matter in a paper:—
| Position of bead on scale | 100 | mm. |
|---|---|---|
| Position of bead after attaching platinum wire | 151 | „ |
| Position of bead with wire and roll of paper attached | 200 | „ |
| Weight of paper expressed as millimetres 200−151 = | 49 | „ |
| Position of bead with wire and ash attached | 156·5 | „ |
| Weight of ash expressed as millimetres 156·5−151 = | 5·5 | „ |
49
Weight of paper
expressed in
millimetres.
:
100
::
5·5
Weight of ash
expressed in
millimetres.
:
11·224
Percentage
of ash
in paper.
In order to ascertain the percentage of mineral matter actually added to a paper it is necessary to deduct from the amount of ash obtained a certain quantity due to the mineral matter in the fibre of which the paper is composed. This amount varies with each particular fibre, and with the method by which it has been prepared. The following table gives the percentage of ash yielded by the various pulps in a perfectly bleached state. In all cases the fibres were previously treated with a dilute solution of hydrochloric acid, in order to remove any carbonate of lime or other bodies which might have been introduced in the boiling and bleaching processes: the percentages are calculated on the dry substances:—
| ash per cent. | |
|---|---|
| Pulp from linen rags | ·25 |
| Pulp from cotton rags | ·10 |
| Pulp from esparto | ·50 |
| Pulp from straw (soda process) | 4·80 |
| Pulp from straw (Leunig process) | 1·36 |
| Pulp from wood (soda process) | ·50 |
| Pulp from wood (bisulphite process) | ·06 |
| Pulp from wood (mechanical) | ·40 |
If the paper contains calcium sulphate the ash obtained may consist partly of calcium sulphide, due to the reducing action of the carbon found on ignition, and the amount will {137} therefore not represent the true amount added. The ash should be moistened with a few drops of sulphuric acid, and again ignited, in order to reconvert it into calcium sulphate. It should also be borne in mind that the sulphate of lime as present in the paper is combined with two atoms of water CaSO4 + 2 H2O, and therefore that every part of calcium sulphate obtained represents 1·26 parts of “pearl-hardening” actually added.