the first characteristic is only to be gained at the expense of the
latter.
D wishes to print half-tones on an antique paper. In this case modern
printing inventions have bridged over some of the obstacles of the
past, and the offset press and extra-deep engravings have brought this
last requirement within the realms of possibility, but unless resort is
had to these new methods, the requirements again are irreconcilable to
each other.
It is evident, however, that only through technical paper information
can one solve such problems as necessitate a compromise capable of
giving the maximum possible satisfaction.
The structure of paper, machine made, results in the greater proportion
of the fibers in the formed sheet lying in the direction of the flow
of the stuff. This determines what is called the “grain” of the paper.
When paper is in the roll the grain of course is lengthwise of the web,
but in the sheet the cutting and slitting may be arranged so as to
leave the grain either lengthwise or crosswise of the sheet. This is an
important consideration for a number of reasons.
In the first place, it is easier to tear the paper with the grain than
across, as the fibers are parted rather than fractured in this way.
This is a point which might be utilized by printers when printing
detachable coupons.
Perhaps the most important consideration is the great difference in
folding qualities. Many a paper will fold very nicely with the grain
and crack badly if folded the other way.
Again, a great difference is noticeable in the flexibility of books,
dependent largely on whether the grain runs parallel or at right
angles to the binding. If flexibility is desired, the grain should
run parallel to the back of the binding. Occasionally a wide-paged
pamphlet, especially of light-weight paper, is improved by the rigidity
to be gained from having the fibers run at right angles to the binding.
It is also true that this increases the strength of the binding, as the
sewing or wire stitching passes around more fibers than if the grain
ran up and down the page.
Not infrequently does the middle signature of a pamphlet pull loose
from the binding. Usually in such cases the paper is not strong anyway,
but it could have had more resistance had the grain run at right angles
to the binding.
The tensile strength of a strip of paper is greater with the grain, but
its elasticity is greater across the grain.
A convenient way to ascertain the direction of the grain in papers
that do not show it clearly by folding is to cut two narrow strips a
few inches long, hold them by one end so that they coincide. When held
horizontally, if the loose ends do not part, it indicates that the
lower paper has its grain in the long dimension. If the lower paper has
its grain crosswise, the loose end will sag away from the top strip,
because, as above remarked, a paper is more flexible across the grain.
This test may be applied either to sized or unsized papers.
Another test is to cut a small square and moisten one side; the paper
will curl into a little cylinder and the grain runs parallel to the
length of the cylinder. This test cannot be applied to an unsized paper.
This leads us to a consideration of the effects of moisture and
humidity on paper.
It will be recalled from the chapter on Paper-Making (No. VI) how
plastic paper is in its moist stage, and how tenacious of water are the
cellulose fibers. It will also be recalled that there is considerable
shrinkage across the web of the paper from the time it leaves the
wire to the moment it is reeled. In fact, the very thing which makes
paper-making a possibility is the shrinking of each individual fiber,
occasioned by the expulsion and evaporation of the water, which has
served as a carrier from the machine chest to the wet end of the
machine.
This propensity of each individual fiber does not cease when the paper
is made, but persists forever. A cellulose fiber will absorb moisture
from the air in proportion to the relative humidity, just as the hair
in a barometer is continually shrinking or expanding as the weather
changes.
A definite percentage of moisture is normal to a cellulose fiber in
proportion to the moisture in the air. The fiber swells as it absorbs,
and shrinks as it gives off water.
Herzberg gives as the results of investigation with a good
writing-paper made of rags, sized with rosin, the following report of
the percentage of moisture retained under various degrees of relative
humidity:
Relative humidity Moisture contained
of the air, in the paper,
percentage percentage
100 21.5
90 13.5
80 8.9
70 8.4
60 6.5
50 5.6
40 3.4
30 2.3
In a sheet of paper, where thousands of fibers lie side by side,
the combined expansion is distinctly noticeable in the changing
dimensions of the sheet. This gives rise to difficulties in securing
accurate register in color-printing, owing to atmospheric changes. The
manufacturer may minimize this difficulty by a careful formation of the
paper and the regulation of the drying, so as to turn out the paper as
nearly as possible containing an average normal percentage of moisture.
The same conditions are responsible for wavy edges, which occur
principally along the cross-grain dimension of the sheets. The ends of
the fibers, being exposed, easily absorb moisture as paper lies in a
pile, but the moisture seldom permeates more than a few inches into the
pile. Therefore, the larger part of each sheet is unaffected, but the
fibers exposed to the air expand when absorbing moisture increasing the
area of the exposed end and, consequently, causing it to assume a wavy
formation which is suggestive of a ruffle.
When feeding such sheets to a cylinder press, much trouble may arise
if the waves occur along the “gripper edge,” which is usually on the
longer dimension of the sheet. In some instances the difficulty may
be avoided by ordering paper with the grain running the long way of
the sheet, which also offers another advantage in relation to securing
close register, namely this: the area of the sheet in square inches
will increase least through atmospheric expansion which occurs across
the grain if the cross-grain dimension is the lesser.
[Illustration: SUPERCALENDER STACKS, APPLETON COATED PAPER CO.
For a description of the Supercalendering process, see pages 55 and
56.]
CHAPTER NINE
APPRAISING AND TESTING PAPER
The appraisal of a specimen of paper differs from testing in that
an appraisal comprehends the value of an object in relation to its
usefulness and marketability, whereas testing is merely an arbitrary
method of expressing the chemical or physical properties of the
object. The knack of appraising can be acquired only through practical
experience; and the ability to make tests is gained only by careful
technical training.
In the majority of cases a satisfactory appraisal may be given without
chemical or physical tests, but these are cases when the superficial
characteristics, such as color, finish, feel, etc., are the prime
qualifications, and such considerations as fiber contents, freedom from
impurities, exact tensile strength etc., are of negligible importance.
Although experience, only, leads to the knack of appraising paper,
certain points might be suggested with benefit to the beginner which
would assist him to an earlier acquirement of the art.
COLOR.--Color being a purely relative term as applied to the variations
in so-called “white” papers, it is necessary to make comparisons
with accepted standards of the various grades in order to arrive at
conclusions.
In common parlance, white papers may be described as natural, light
natural, white, blue-white, pink-white. Natural papers are those
in which a minimum of artificial coloring has been added, and the
brilliancy of shade depends entirely upon the quality of the stock.
Almost all paper is colored to some degree while the stock is in the
beater, and the minimum quantity of order of paper, which any mill
will make on a special run is usually limited by the contents of one
beater, and, on account of the time required to wash up, the cost
of special colors is increased. Rose-pink and blue are the colors
used in modifying the natural color of any beater of pulp to produce
a white paper. The so-called “white color” of the cheaper grades of
papers is ordinarily gained by a comparatively heavy use of blue,
and by comparison with a white paper of good quality the blueness is
decidedly noticeable. In judging color, it is well not only to look at
the surface, but also to examine the paper when held up against the
light, making comparison with some acceptable standard, also noting the
clearness of the stock, as indicated by the sharpness of definition of
the shadows of the fingers which hold the sheet. This comparison is
affected, of course, by the bulk of the paper, but two papers of about
equal bulk may be fairly compared in this way. Any judgment as to shade
is, in part, only a question of taste. Permanency of color may easily
be determined by exposing a portion of a sheet to sunlight for a few
hours and noting any alteration in color.
FORMATION.--While examining a paper for color and clearness, the
formation of the sheet should also be observed. In general, a close,
even formation is to be desired. Fibers of the same approximate
length may be loosely or evenly formed, according to the skill of
the machine-tender. The longer the fiber, the harder it is to get a
close, even formation, and it should be remembered that these two
qualifications are to a greater or less extent contradictory.
FINISH.--Whatever the finish of paper, the two sides of an ideal sheet
would look exactly the same. In most papers made on a Fourdrinier
machine the impress of the wire is discernible, and there is a
perceptible difference in texture between the “wire,” or bottom, side
and the “felt,” or top, side, the one tending to reproduce the texture
of the wire cloth, and the other the weave of the felts.
Some manufacturers have perfected their processes to a degree that
renders these differences imperceptible. Papers made on cylinder
machines of more than one vat are apt to be more even-sided, as the
contact with the wire of the molds is less protracted and there is
considerable pressing of the web between two felts as it is carried
along.
The evenness of the finish, and the fineness of texture over all parts
of a sheet, may best be judged by holding it aslant to the light. This
also discloses whether the paper is “fuzzy” or free from lint.
Fuzz, or hairiness, usually occurs on the wire side of the sheet. This
is due partially to the stock, soda pulp being especially likely to
fuzz. It is also due to overdrying, and sometimes to the action of the
suction boxes, which if worked too hard cause the surface fibers to
stand on end.
“Hairiness,” or fuzz is more apt to occur on antique and other light
finishes, but calendering will not entirely overcome it, and such
papers as would be fuzzy uncalendered, become fuzzy with handling.
In fact, the durability of the surface may well be tested by rubbing
the paper between the fingers. In this way, too, one judges the “feel,”
which of all qualities of paper is perhaps the most difficult to
express, but usually described as hard, soft, mellow, harsh, rough,
smooth.
In highly calendered papers, well closed and evenly finished, the light
will be reflected uniformly, as from a well-polished table-top; but
if the formation is “wild,” there will be a blotchy look as the small
knots of unevenly distributed fibers cause thick and thin areas, and
the thick ones get harder squeezing through the calender rolls and,
consequently, a higher finish.
Another cause for unevenness in finish is a variation in the thickness
of the paper as it is made on the machine. This unevenness runs
lengthwise in streaks, and may originate on the wet end of the machine
if the pulp is not deposited uniformly.
Again, the pressing may be faulty at the press rolls, causing a thin
streak. Naturally, the thin part of the paper dries more readily
than the thick, and as even surfacing depends partly upon even
dissemination of moisture in the sheet, a poorly pressed sheet would
have a faulty finish. Dirty felts also cause uneven drying, as water
can not be evenly squeezed through a felt the pores of which are
partially choked. Lastly the unevenness may be caused by the calender
rolls themselves being in poor condition.
It is easy to detect thin areas by examining paper in a pile, as a pile
of papers of uniform thickness will be practically level on top.
Papers for half-tone printing, whether coated or uncoated, should be
even in formation, thickness and surface, otherwise the printer’s
“make-ready,” which is designed to offset inequalities in the plates,
will be discounted by inequalities in the paper.
There are some special papers in which unevenness in formation and
finish are intentional, on account of the unusual effects thus gained;
and other papers, such as wrappings, where such niceties of the
paper-makers’ art are of little importance.
OPACITY.--Opacity may easily be judged, although it is difficult to
express it in any accurate terms, by placing the papers to be compared
side by side over a printed page, the relative merits in this respect
may be immediately perceived.
SIZING.--Sizing may be approximately judged by moistening the stock
and noting the rapidity of the absorption, or tested by drawing lines
with ink and watching to see if they spread afterward. Absorbency in
blotting-papers may be measured by submerging two strips equally and
noting how high the ink is drawn up into the strips. Such papers as are
made without any sizing and are ordinarily called “water-leaf.”
The sizing of coated papers should be neutral, but is frequently
alkaline or acid, since alkali is used to neutralize the lactic acid of
the casein. This may be detected by taste. The retention of a piece of
coated paper in the mouth for a few minutes will reveal through the
taste any tendency of the coating to sour.
WEIGHT AND BULK.--Weight and bulk may be closely approximated by a
practiced hand, but they must also be considered in relation to finish,
as pointed out in the preceding chapter.
There are many convenient forms of micrometer gauges for measuring the
thickness of paper and any one who has much to do with paper should be
provided with one, as it is unsafe to depend entirely upon judgment
when a thousandth part of an inch may account for ten pounds difference
in the weight of a ream of paper or cause serious variations in the
bulk of a book.
QUALITY AND STRENGTH.--Quality and strength may be approximately judged
by tearing the paper in both directions of the grain and observing the
fractured fibers, but these matters are to be more accurately estimated
by mechanical and chemical tests.
It will be observed that cleanliness in paper, and most of the
foregoing characteristics of paper, do not lend themselves to
mechanical tests, but are properties which require the judgment of an
expert.
CARDBOARDS.--In judging thick papers, such as bristol boards, it is
customary to see if they are snappy. An idea of their fibrous strength
may be had by folding in various directions. Pasted cardboards may be
distinguished from unpasted by burning, for if paste has been used the
layers of paper will split apart as the paper burns. This burning will
also give a slight idea of the amount of filler in the stock, as the
ash will be greater as the filler is increased.
PAPER-TESTING.--Tests applicable to paper may be divided into three
classes--microscopical, physical and chemical.
The purpose of microscopical tests is to determine the kind and
character of the fibers, and the proportion of each kind, also to
assist in determining the nature of mineral filler and of impurities.
It is also used in estimating the percentages of the various kinds
of fiber. Chemists are able to estimate this within five per cent.
A minute sample of paper is prepared by boiling in a one per cent
solution of sodium hydroxid, in order to remove everything from the
fibers themselves. The resulting mite of pulp is placed on a slide with
a dissecting needle, the excess moisture is removed and a stain is
added. This stain gives different characteristic hues to the different
kinds of fibers. The color and form of the fibers as observed through
the microscope disclose their character to the trained eye.
By counting the different kinds of fibers under observation, the
analyst estimates the proportions in which they existed in the sample
of paper.
The physical tests are more familiar to most persons, and include (1)
weight per ream, (2) thickness, (3) bursting strength, (4) tensile
strength, (5) folding endurance, (6) absorption, (7) expansion.
1.--There are two kinds of paper-scales. The most common kind gives,
directly, the ream weight from weighing a single sheet, and is of such
convenience that almost all paper-users could well afford to have one.
Sensitive paper-scales for small samples, 4 by 4 inches in size, are of
great assistance also, and should form part of the equipment of every
paper-dealer.
2.--The thickness is determined by a micrometer gauge measuring to
one-thousandth of an inch. In gauging thin papers it will prove more
accurate to take four thicknesses, as the error in reading is thus
quartered. The following table of bulks, which shows the number of
pages per inch from a gauge of four sheets, will be found convenient:
======================+==========
Thickness of four | Number of
sheets in thousandths | pages to
| one inch.
----------------------+----------
8 | 1,000
8½ | 941
9 | 889
9½ | 842
10 | 800
10½ | 762
11 | 727
11½ | 696
12 | 667
12½ | 640
13 | 615
13½ | 593
14 | 571
14½ | 552
15 | 533
15½ | 516
16 | 500
16½ | 485
17 | 471
17½ | 457
18 | 444
18½ | 432
19 | 421
19½ | 410
20 | 400
20½ | 390
21 | 381
21½ | 372
22 | 364
22½ | 356
23 | 348
23½ | 340
24 | 333
24½ | 326
25 | 320
25½ | 314
26 | 308
26½ | 302
27 | 296
27½ | 291
28 | 286
28½ | 281
29 | 276
29½ | 271
30 | 267
30½ | 262
----------------------+----------
3.--Bursting strength is determined by a variety of testing-machines,
constructed so as to record the pressure per square inch which may be
exerted before rupturing the paper.
In a government bulletin, Report No. 89, United States Department of
Agriculture, the following criticisms of this test are made: “This
pressure is generally believed to represent the mean strength of the
paper--that is, an average of the strength across and with the sheet.
This is not true however, experience indicating that strength as thus
determined more nearly agrees with the strength of the paper in the
cross direction, with the minimum strength rather than with the average
strength of the paper.
“Among other objections to testers of this type, is that to a certain
extent the operator can influence the results at will, and even with
the greatest care there is quite a wide difference between different
tests of the same paper.”
4.--Tensile strength is determined by clamping a strip of paper of
standard dimensions in a machine which exerts a uniform tension until
the strip breaks. The breaking strength is shown on the recorder,
and the amount of stretch before breaking is also registered, thus
indicating the elasticity of the paper. The best known instrument of
this sort is the “Schöpper,” but the machine is very costly, hence is
rarely found except in well-equipped laboratories.
5.--Folding endurance is determined on a machine which folds a strip
of paper back and forth in a slot, the strip being clamped at either
end to a spring device which maintains a uniform tension. The number of
double folds which the strip withstands is automatically registered.
This test is favorably regarded as an indicator of durability, but the
apparatus is expensive and not easily available, hence this test fails
of frequent use.
6.--The absorption tests are applied principally to blotting-paper, and
consist in suspending equal widths of paper so their ends are submerged
in a beaker of colored water. The height the water rises in a given
time demonstrates the capillary attraction.
7.--Expansion is estimated by taking strips of uniform dimension,
dipping in water and measuring the expansion.
Chemical tests are for the determination of (1) the percentage of
mineral filler; (2) the percentage and nature of sizing materials; (3)
qualitative test for starch, acid, sulphur, chlorine, glue, filler
material, dyes, ground wood.
The amount of filler may be determined by incinerating a piece of paper
of known weight. As the filler is non-combustible, the weight of the ash
determines the percentage of filler, although allowance must be made
for the amount of water of crystallization driven off from the mineral.
Tests for acids are important in papers used for mounting tarnishable
substances, such as jewelry.
Tests for sulphur or chlorine are important in determining the
chemical purity of the paper, since such residues militate against the
permanency of color and strength of paper.
The presence of ground wood is easily determined by a drop of either
strong nitric acid, which turns the paper brown, or a drop of
phloroglucine, which gives a reddish-brown tint from contact with
ground wood. Aniline sulphate produces a yellow tinge.
The presence of starch may be ascertained by using a dilute solution of
Iodine which leaves a black stain in contact with starch.
Note.--For more technical information see “Paper Technology” by
R. W. Sindall.
CHAPTER TEN
PRESSROOM DIFFICULTIES
Technical difficulties with paper in the pressroom arise from many
sources. They may be conveniently classified into three groups:
Difficulties for which the manufacturer is responsible; difficulties
for which the printer is responsible, and difficulties due to
atmospheric and other natural conditions not entirely within human
control. Let us consider some of the first group.
UNIFORMITY.--Probably the most frequent source of trouble is lack of
uniformity, either in weight, thickness or finish. This is chargeable
to carelessness on the part of the paper-machine tender. A run of
paper which varies in weight will naturally vary in thickness, and,
obviously, this could account for uneven color in presswork. These
variations would not necessarily be accompanied by a variation in
finish. To make paper uniform in all three respects necessitates,
firstly, uniform consistency of the pulp--or “stuff,” as it is
technically called--at the point where it flows onto the machine. A
uniform volume of stuff and uniform speed of the machine are also
demanded. The speed of the machine and the volume of stuff are quite
readily controlled, but as the amount of water used by the beater-man
in preparing the stuff is usually judged by the appearance of the pulp
in the beater, there are always such variations as are peculiar to this
human factor.
The difficulties of the machine-tender may often be traced to the
beater-man, not only on account of the amount of water in the mixture,
but also because of the irregularity in the length of fiber from one
beaterful to another.
Assuming that the stuff is right and the formation on the machine is
good, the pressing of the paper next demands close attention. It is
obvious that any unevenness of pressure will result in the water being
expelled unevenly from the web of paper, with a consequent variation in
thickness. In this case there would also be a damp streak in that part
of the web where the pressing was too light.
The result is that such paper can not be dried evenly all the way
across the machine because this damp streak will still have an excess
of moisture after the adjacent areas of the web have become properly
dried.
FINISH.--In running through the calenders the damper portion will
take a higher finish. It may even be so damp as to cause a blackening
or crushing of the paper; whereas, if the moisture is sufficiently
evaporated from this streak, the rest of the paper may be so dry that
it will not finish smoothly enough.