still can be used), and reaching to 20 units. A combination of three glasses in one colour, each of the value of one unit, will give the same effect as the one glass marked 3 units. A combination of red, yellow and blue in equal units of each will produce a neutral tint light or dark, according to the units employed.

In the analysis of an orange coloured paper three glasses were used, red having a value of 4.4 units, yellow, a value 2-8 units, and blue 12 units.

The colour of the paper is not permanently recorded in terms of the standard glasses used, although these are of course registered for reference.

The visual sensation produced by a colour may differ from that recorded by the standard glasses used. Three equal units of the standard colours absorb light and produce a greater or less degree of black. Hence when all three

are used, the smallest value is recorded as black. Thus, as the standard glasses used were:

a record which, to a colour analyst, is an exact measurement of the colour in figures, and which in words may be termed a dark orange-red.

Table XXII. gives a number of colour readings for some well-known papers.

Surface and Finish. These qualities of a paper cannot be recorded by any numerical expression. The amount of finish in a sheet of paper is largely a matter of taste, and the paper-maker can modify it to suit the requirements of his customer. Under these circumstances it is only possible to describe the paper in general terms.

Feel and "Handle" of the Sheet. These are definite qualities by which the merits of a sheet of paper are largely judged. Stationers and printers frequently decide between two papers by the difference of "handle" when the papers are rattled and shaken by hand. As a matter of daily experience it is found that good papers rattle considerably when shaken between the fingers, and this is usually regarded as a sign of good quality.

CHAPTER X

THE CHEMICAL CONSTITUENTS OF PAPER

Normal-China-clay-Sulphate of lime-Barytes-Agalite-Determination of ash--Nature of loading-Moisture-Starch-Sizing materials-Gelatine-Casein-Rosin size-Abnormal-Acid-Chlorine-Sulphur-Metallic impurities.

The Normal Chemical Constituents.

THE term "normal chemical constituents" may be used as referring to all the ordinary substances found in paper which are not actually vegetable fibres, and as distinct from adventitious matter in the shape of impurities.

Under such a classification would be considered:

1. Mineral substances added as loading.

2. Mineral matter used for improving surface.
3. Materials necessary for sizing the paper.
4. Pigments and colouring-matters.

However free from added mineral substances, all paper contains a proportion of constituents which are not actually cellulose. The most highly purified form of paper is the specially prepared "filter paper" of Messrs. Schleicher and Schüll, from which the natural mineral constituents have been removed by means of hydrochloric and hydrofluoric acids. This paper contains only 0.02 per cent. of ash.

In strong contrast may be noted many of the highly glazed, surface-coated papers used for half-tone printing, containing 33 to 38 per cent. of their weight of mineral matter, and 8 to 10 per cent. of common glue, leaving only about 50 per cent. of actual fibre.

Mineral Substances.-Although many substances have been employed from time to time as loading for papers, the list of those in common use is not large.

China-clay. This mineral is used in large quantity as a filler for news, cheap printings, and common writings. It is a natural product found principally in Cornwall, England, formed by the gradual disintegration of felspar, through exposure to air and moisture. It is a hydrated silicate of alumina, having the approximate composition represented by the formula:

3

Its specific gravity is 2.5. The better qualities are free from iron, the absence of which is desirable in clays used for the finer papers.

When the percentage of ash in a common paper exceeds 3.00, it may be assumed that the ash is due to china-clay, or kaolin, used as a loading.

The examination of a residue for china-clay depends on certain reactions of clay when treated with suitable test chemicals.

China-clay is insoluble in water, dilute acids, and alkalies. Concentrated hydrochloric acid in the cold decomposes it partially. Prolonged boiling with strong sulphuric acid decomposes it, giving an insoluble silica and a soluble sulphate of alumina.

Under the microscope the particles of clay appear irregular in shape

Silicates of Magnesia, under the more familiar names of agalite, talc, French chalk, asbestine, are occasionally found in paper, and may be recognised under the microscope by reason of their crystalline structure.

The exact analysis of the mineral matter or ash which is left behind in a crucible as the result of burning a known quantity of paper demands considerable chemical knowledge and skill. If it is known that the ash, for example, is either china-clay or pearl hardening, a simple test with hydrochloric acid, as already described, would be sufficient, but the examination of complex mixtures can only be made by a competent chemist.

Sulphate of Lime.-A white crystalline substance, somewhat heavier than china-clay, used chiefly for the better qualities of writings and printings. It occurs under many trade names, such as gypsum, powdered plaster, annaline, which are prepared by grinding the natural mineral; or as pearl hardening, satinite, terra alba, mineral white, prepared artificially by adding sulphuric acid to calcium chloride.

Sulphate of lime is partially soluble in water, and dissolves more readily in dilute hydrochloric acid. If the ash of a paper dissolves when warmed with dilute hydrochloric acid, the presence of this substance may be suspected and subsequently confirmed by chemical analysis.

A microscopic examination of the sulphate of lime will usually determine whether the natural ground product or the precipitated form of mineral has been used.

Barium Sulphate.-A heavy white substance having a specific gravity of 44. The natural mineral when powdered is used under the name of Heavy

I

Spar. The precipitated form prepared by adding sulphuric acid to a soluble barium salt, such as barium chloride, goes under the trade names, Blanc Fixe, Permanent White.

It is used in considerable quantities as a suitable mineral for coated papers on account of its purity of colour and brilliancy. When mixed with precipitated aluminium hydrate it is used as a paste for art papers under the name Satin White.

Barium sulphate is insoluble in water and acids. It can only be rendered soluble by fusion with a mixture of carbonates of sodium and potassium in a platinum crucible. The fused mass is boiled with water, filtered, and the residue dissolved in hydrochloric acid. The examination of a mineral residue in the ash of paper for barium sulphate is based upon this procedure.

Examination of the Ash of Paper.-A proper knowledge of chemistry is indispensable for an adequate analysis of the ash of paper. It is impossible

a

88

8

08

8

B

20

P

Fig. 117.-Sulphate of Lime.

a. Ground Product.
b. Precipitated Product.

to deal with the subject fully in an elementary treatise, and the following tests only indicate approximately the nature of the mineral residue.

(A) The ash is quite white.

If the ash is entirely soluble in dilute hydrochloric acid when heated in a testtube, china-clay, barium sulphate, silicates of magnesia are absent. If no effervescence takes place when the acid is added, carbonate of lime is absent.

The ash is probably Sulphate of lime. The presence of this compound may be confirmed by dividing the solution into two parts, adding some barium chloride solution to one, when a heavy white precipitate will be produced; and adding sufficient ammonia to the second to completely neutralise the acid, followed by a solution of ammonium oxalate, when a white precipitate will form.

If the ash is treated with dilute acid as described, any sulphate of lime present is dissolved out, and may be detected by the special test given. If the precipitates mentioned are only produced as mere traces, then the ash merely contains traces of sulphate of lime derived most probably from the hard water used in the manufacture of the paper.

Any residue insoluble in hydrochloric acid may contain china-clay, barium sulphate, or silicates of magnesia.

It is dried in a platinum crucible and then fused with a mixture of sodium and potassium carbonates. When cold, the mass is treated with hot water, filtered, and the residue dissolved in hydrochloric acid, which solution is then added to the hot water extract.

The silicates in this way yield free silica and soluble salts, and the barium sulphate gives soluble barium chloride.

If the solution is quite clear, china-clay and silicates of magnesia are absent.

The ash in this case is probably sulphate of barium. It is confirmed by adding some dilute sulphuric acid to the solution when a heavy white precipitate is obtained.

If the solution is not clear, but contains a flocculent gelatinous bulky