Paper Manufacture

Loading.

Loading paper with mineral substances is an im portant auxiliary treatment, and it should not be considered as an adulteration, pure and simple. To writing and printing paper loading gives opacity: to process papers a highly glazed, compact surface suitable for fine-screen illustration work : to wrapping papers, ochre-brown and other colours. Colour tinting is also, of course, a loading. Other than hand made papers unloaded papers are altogether exceptional. Two to 3% of loading and up to io% is common and 20 to 30% in specially surfaced papers, anything more than which is certainly an adulteration, greatly weakening the strength of the paper. The larger proportions of loading, particularly for surfacing with supercalenders, are added to the surface with an adhesive, commonly casein or starch, and thereafter the special surface developed by glazing. The cheaper papers are glazed on the paper machine in the calenders as described : the better class of very highly glazed papers and those that are tub sized are also frequently given an additional glazing by supercalendering them or by plate glazing them between plates of zinc or copper. The supercalender, for this purpose, is a stack of rolls alternately of heated cast iron and of compressed cotton or paper so that the paper at each nip is between iron and cotton. Paper, though made in the roll on the machine, is usually sold in the form of sheets. A number of reels of paper, on their spindles, mounted in a stack, are fed, as a pile of webs, between two rollers : a series of revolving knives slits them longitudinally as they emerge from between the rollers, in effect into strips which are cut again transversely by the scissors action of a movable upper knife, working periodically against a lower fixed knife. The cut sheets fall on to an endless felt for stacking: they are thereafter sorted, usually, into three classes known as "perfect," "retree" (French retirer, to discard) and "broke." The retree are sold as second quality and the broke returned to the mill for repulping, as waste paper.

The introduction of esparto and wood pulp has greatly increased the number of grades of paper in use. Paper testing and valuation have therefore become a matter of great importance. The prin ciples of paper testing that have gradually proved themselves of value are partly physical and partly chemical. The physical tests involve determinations of strength, elasticity, stretch or expan sion on moistening, resistance to wear and tear by crumpling and folding. Ingenious machines, of various patterns, have been devised for making such tests under strictly comparable standard conditions. In hand made papers the values determined are found to be independent of direction but variable from place to place in the same sheet : in machine made papers the values determined are more independent of locality but they depend always on the direction in the sense that the paper is always weaker longitudi nally than it is transversely, and this is referable to the circum stance that at the moment of the formation of the web from the wet pulp, on the wire of the machine, the wire is continuously moving forward. Sizing quality is judged by capillary effects,

such as the rise of water up a strip of paper dipping in water, or the spreading of a blot of ink on its surface.

The chemical tests employed are mostly those of moisture content, i.e., loss of weight by drying at I oo° C; ash content, by incineration and weighing of the ash remaining: the nature and percentage of sizing, usually gelatine or rosin, the gelatine being identified by extraction of the paper with hot water and testing with tannic acid solution, which gives a gelatinous precipitate with gelatine (it can be estimated quantitatively by Kjeldahl's determination of nitrogen, if required) and rosin being extracted with alcohol, and the loss of weight, thus arising, is taken as equal to the weight of rosin present. Acidity and alkalinity are determined by ordinary indicator dye, colour reactions (litmus, congo red, methyl orange, etc.). The "fibre furnish," i.e., the kinds of fibre present and their approximate percentages are determined by microscopic examination of prepared slides, suitably stained (see FIBRES).

Standard Sizes.

Paper is sold in sheets of different sizes and is made up into reams containing from 48o to 516 sheets in Great Britain, Soo in the United States; these sizes correspond to dif ferent trade names, as foolscap, demy, royal, etc. ; the following are the ordinary British sizes: Classification.—The following list includes the chief papers.

Rag.

(I) Hand made, drawing, ledger, and bank-note and book papers.

(2) Machine made drawing and other papers, often mixed with high grade sulphite wood pulp.

(3) Filter papers.

(4) Cigarette papers, heavily beaten, frequently with about pp% chalk as loading: the so-called India paper, for printing Bibles, etc., is very similar.

(5) Tissue papers similarly, without loading (suitable for wrapping silver).

(6) Imitation parchment, similarly by heavy beating. This paper is grease-proof but not water-proof.

(7) Wax surfaced wrapping papers requiring some strength in one direction, e.g., toffee wrappers.

(8) Vegetable parchment. The cellulose is hydrated by passing through a bath of 66 per cent sulphuric acid and thereafter very thoroughly washed. It is water-proof as well as grease-proof.

(9) Vulcanized Fibre—Made by gelatinizing paper, generally of cotton, with concentrated zinc chloride solution, then building up required thicknesses by lamination ; after which the zinc chloride is leached out and the homogeneous mass is dried.

(io) Willesden paper made similarly with cuprammonium hydroxide solution instead of zinc chloride solution.

Esparto.

(I I) Writing and printing papers, with chemical wood, especially sulphite pulp.

( 12) Magazine papers frequently contain high percentages of esparto.

(13) Blotting papers, often with soda wood pulp. Chemical Wood (a) Bleached.

(14) Sulphite pulp writing and printing papers.