(2) The elasticity of paper is partly permanent and partly temporary, the paper behaving to some extent like a piece of metal, since the total elongation produced does not remain permanent when the paper is released from tension. The total elongation, therefore, can be divided into temporary and permanent stretch, and these qualities can be easily measured in the following manner:

Two pencil lines are drawn on the strip of paper which is to be tested, at any convenient distance apart, the line being drawn at right angles to the length of the strip. After the paper has been fractured, the two pieces are removed from the machine and joined together again at the point of fracture, being laid flat upon a table for this purpose. The distance between the two pencil marks is accurately measured. The increase of distance between the two marks is a measure of the permanent stretch, while the total elongation indicated by the machine shows the total stretch. The difference between the total stretch and the permanent stretch is a measure of the temporary stretch. Some interesting details in connection with this matter will be found in chap. xiv.

Resistance to Wear and Tear. The methods for measuring the resistance of a paper to friction and folding are for the most part of a purely empirical nature, though in Germany machines are used in the Imperial testing institutes.

The crumpling test. The paper cut to some convenient dimension of about four or five inches square is crumpled up between the fingers, opened out and then turned over, crumpled a second time, and this operation repeated as often as desired. After each five or six operations, the number of holes appearing in the paper are counted, in order to see the rate at which the paper wears out. For very strong paper at least 120 operations are necessary to produce even a minute hole, but the average run of ledger papers, for example, will show holes much

sooner.

Example. A bank note paper gave the following results:

A record of this character is more satisfactory than a general statement that the paper is weak or strong.

The holes produced in the above case were exceedingly small and could only be observed when the paper was held up in front of a strong light. Although this method appears to be rather rough and ready, yet it is a valuable indication of the wearing qualities of the paper. This test, in conjunction with the test which measures the loss of strength due to folding, enables the observer to form quickly and readily a reliable judgment on the merits of two or more papers.

By plotting out the results of the above test in the form of a curve, it may be noted that the number of holes produced is almost in direct proportion to the number of operations, that is to say, up to the limit tried, the increase up to 110 operations being fairly uniform. By continuing the crumpling test it would be easy to find out whether the paper might be expected to wear out gradually or

Fig. 99. Curve to show the effect of testing Paper by crumpling.

to give way suddenly when the friction had reached a certain point. In the case of two papers which give similar results up to about 120 operations, it would be necessary to do this in order to determine finally and definitely the differences between the two.

In the Government testing institutes in Germany the papers are classified for resistance to crumpling according to the descriptions given in Table XV.

TABLE XV.

0. Extremely small.

1. Very small.

2. Small.

3. Medium.

4. Fairly large.

5. Large.

6. Very large.

7. Extraordinarily large.

The only machine for measuring the resistance to folding and crumpling is Schopper's machine, the general appearance of which can be seen from Fig. 100.

Resistance to Folding. The simplest method of measuring the effect of folding is to ascertain the loss of strength due to repeated folding of strips of the paper. Strips of paper of the same width and length as those taken for the determination of strength, are folded over and over in the middle a certain number of times according to the quality of the paper, and then tested for strength. The loss of strength measures the effect of the folding.

It is advisable to take several strips in each direction, and to note the mean results.

Example.-A paper having a mean strength of 30 lbs. before folding, shows after 150 foldings a strength of 18 lbs. The loss is 12 lbs., giving a percentage loss of 40 per cent.

A simple apparatus has been devised by Kirchner for producing the fold

Fig. 100.-Apparatus for testing the Resistance of Paper to Crumpling.

under definite conditions. It consists of a heavy roller working in a grooved block of wood which ensures that the amount of pressure acting on the strip is always the same.

One modification of the test is useful for some papers, especially those which have to be repeatedly folded in actual practice.

For this special folding test, a square piece of paper, six inches by six inches, is cut, the weaker and stronger directions being suitably indicated on the sheet. This paper is then folded in four, the finger of one hand being rubbed with even pressure along the folds away from the centre, one finger of the other hand being kept at the centre in order to prevent the paper tearing away at that point.

The paper having been folded once, is opened up, smoothed out, then turned over and folded again along the same creases. This operation is repeated a certain number of times, the appearance of the centre of the sheet being carefully recorded after each ten foldings,

H

Table XVI. shows the record for one paper tested in the manner described. Naturally, the remarks are only of comparative value, as they cannot be expressed in numerical terms. The appearance of a small hole at an earlier or later stage of the test is a fairly good indica ion of what may finally be expected.

120

Minute hole.

Slightly larger.

Slight crack near hole in one direction.
No change.

Slight crack near hole in another direction.
No change.

Hole cracked in both directions, 11th inch each way.

After 120 folds, more or less, the paper was opened out, and two strips each five inches long, cut each way of the sheet, as shown in Fig. 102 c.

The strips were

Fig. 102.-Diagram to illustrate Test for Loss of Strength due to Folding. (a) Paper six inches by six. Dotted lines indicate position of folds. (b) Paper Arrows indicate direction of rubbing. (c) Paper

folded in four.

after folding. Diagram showing method of cutting test slips.

then put into the machine, and tested for strength. The paper naturally broke

across the fold or crease, giving the results set out below:

The original strength of the paper in its normal condition was:

Thus, the paper having an original bre king weight of 30-6 lbs. still retained sufficient strength after being folded 120 times to require 18.8 lbs. to cause fracture.

=

30.6 less 18.8 =

Lo-s due to test 11.8 lbs., which amounts to 38.5 per cent. Nature of Tear. Just as paper behaves differently when it is tested for strength in the two directions of the sheet, so it gives varying results when the "tear" is examined in two directions.

The common method for making a rough comparison between two sheets of paper is to make a slight tear in the sheet of paper in both directions, a judgment being formed as to the general condition of the paper from the appearance of the torn edges and the amount of force required to tear the sheet. Such a comparison is apt to be misleading, as it does not always follow that a paper which tears better than another is necessarily stronger. In the case of handmade papers, for example, the tear is by no means uniform all over the sheet. A paper may show a strong tear at the deckle edge and a weaker tear if the deckle edge is first removed. Considerable practice is required in coming to a decision that one paper tears better on the average than another, for a very little consideration will serve to show that it is difficult to establish precisely the relation of the "tear" of a paper to its "absolute" strength. The whole of the subject requires a good deal of investigation, and the results would certainly be interesting and instructive.

Generally speaking a paper will tear most easily in the direction which shows the greatest breaking-strain. A greater proportion of the fibres which compose the sheet point in that direction and consequently the tear takes place in between the fibres, so to speak. On the other hand, when the paper is torn across the direction of greatest strength the torn edge will show a far larger number of fibres protruding from the sheet, the tear being across the greater number of fibres rather than parallel with them.

Sizing Qualities.-The extent to which a paper is sized is determined in a rough and ready manner by the application of the tongue to its surface. If the moisture sinks in rapidly the paper is regarded as being only slightly sized.

Of course an empirical standard of suitability is thus set up, which probably in many cases is sufficient for the purpose, but such a method is quite inadequate when the degrees of sizing have to be compared more closely.

The methods generally employed for testing the water resistant properties, or sizing qualities of paper, may be described under three heads. Attempts have thus been made to express the extent to which a paper has been sized in numerical terms, and although such terms are arbitrary, they afford some degree of comparison.

The test for the strength of sizing may be made: (1) by the writing of characters or lines on the paper with different strong inks.

Three kinds of ink are usually applied to the surface of the paper. The strongest and most penetrating is the ink prepared from logwood, a medium ink being the ordinary fluid obtained from iron and gallnuts, while for weak inks alizarine and aniline colours are used.

The inks can be applied to the surface of the paper by means of a soft quill pen, or better still, with a drawing-pen, the points of which are well rounded, so that the surface of the paper is not scratched in any way on the application