THE MAKING OF PAPER
THE MAKING of paper is closely related to that of textiles. In each
case the same basic materials are used. A mass of interlacing fibers
is formed into a continuous sheet, but the method of interlacing the
fibers and holding them together is entirely different. Paper resembles
felt more closely than any other form of fabric. The fibers are not
woven but are matted together and compressed. In the case of felt the
fibers of wool or hair are held together mainly by the microscopic
barbs that they possess while paper fibers are held together by means
of sizing.
Although paper gets its name from papyrus the latter was not paper, for
it was a felt of pith rather than fiber. The ancients produced a very
fair writing material from the rushlike plants that grew in the swamps
along the Nile. The stem of the papyrus was stripped of its bark and
the pith was cut into thin ribbons which were laid side by side to form
a sheet. Over these and at right angles to them was laid another layer
of pith ribbons. The material was then soaked in water, pressed into a
continuous sheet and dried. These sheets were then pasted together and
rolled up into scrolls. Unquestionably they played a most important
part in the civilization of ancient Egypt, providing, as they did,
a ready means of recording knowledge and disseminating it among the
people.
CHINESE INVENTION OF PAPER
Like so many inventions that are of vital importance to modern
civilization, paper is a Chinese invention. The Chinese made paper from
the fibers of trees, a material to which we have reverted in recent
years. The invention dates from about 150 A. D., when some one devised
a process of reducing the bark of the mulberry tree to a pulp, beating
it and boiling it in lye and then matting the fibers on a screen and
pressing them into a finished sheet. The process was introduced into
Europe by the crusaders in the 12th Century. In 1150 a paper-making
plant was established in Italy and soon the new writing material
was being made in increasing quantities and became a common article
throughout Europe. It was not until the end of the 18th Century that
machinery for making paper came into use.
In the old hand process which is still used to a very limited extent
for special papers, the pulp, after being reduced to a thin milky
fluid, was dipped out of a vat with a rectangular sieve which had a
wire screen bottom of fine mesh. The water drained off, leaving the
fibers upon the screen. As the pulp was draining the sieve was shaken
about to distribute the fibers evenly and interlace them as much as
possible. The depth of the sieve determined the amount of pulp that
was dipped out and hence the thickness of the paper. The soft, matted
mass of fibers was turned out upon a felt mat and left to dry. Soon
other fibers came to be used and the finest grades of paper were made
from linen and cotton rags. The rags were carefully sorted out and
washed after which they were stored in a damp place to rot for a number
of weeks. Then they were cut and shredded and boiled until they were
converted into a pulp. The complete process of making fine paper by
hand took from two to three months. Paper was made in small sheets
limited by the size of sieve that could readily be operated by hand.
The first man to make a continuous web of paper was Nicholas Louis
Robert, a Frenchman who received a reward of 8,000 francs from his
government for his invention. Robert’s patent rights were acquired by
the Fourdrinier Brothers (Henry and Sealy), of England. They developed
and perfected the machine and to them is ascribed the honor of founding
the modern paper industry.
USES OF PAPER
Paper is more commonly and extensively used to-day than any other
product of manufacture and is employed for an astonishing variety of
purposes, ranging from milk bottles to suits of clothing. One very
novel use we might mention, in passing, is to be found on the sugar
plantations of Hawaii. After the cane is planted the fields are covered
with a thick paper made, by the way, from sugar-cane fiber. This
covering prevents the weeds from growing, but the sharp spikes of the
sugar sprouts pierce the paper and grow through it. By the time the
paper has disintegrated and the weeds have a chance to grow, the cane
has such a start on them that they are kept shaded and cannot compete
with or choke out the cane.
TURNING FORESTS INTO PULP
Before we proceed to describe the modern Fourdrinier machine, we must
look into present day methods of preparing pulp. The finest papers are
still made from linen and cotton rags. Esparto grass is largely used in
Europe. This comes from Spain and North Africa. But by far the greater
part of the paper produced to-day is made from the fiber of trees.
Particularly is this true of our own country, which consumes half of
the world’s output of paper.
It used to be that only the poorest grades of paper were made from
wood pulp; now however not only newspapers but nearly all books and
magazines are printed on a forest product. Formerly only spruce wood
was considered suitable for paper manufacture, but the rapid depletion
of this raw material led to the employment of many other species of
trees and our Government scientists are constantly searching for and
experimenting with new woods for this purpose.
Because of the insatiable appetites of our large newspaper printing
presses and because a newspaper is not intended to be an imperishable
document but a fleeting record of passing events, quality is of little
importance in the preparation of pulp for newsprint and everything
must be sacrificed for speed. The pulp is made of “ground” wood
as distinguished from the chemically prepared wood pulp known as
“sulphite.” The wood is first stripped of its bark and then cut into
pieces running from two to four feet in length, depending upon the
size of the magazine of the grinder. The wood is pressed against a
revolving grindstone by hydraulic pressure and at such an angle that
the fibers torn off will be as long as possible. The fibers are borne
off by a stream of water which carries them through a set of screens.
These remove any unground slivers, and eventually the pulp is drained
of water and pressed into laps, or, if it is to be used immediately,
it goes directly to the beating engine. Here it is mixed with a
certain amount of “sulphite pulp,” usually from 20 to 25 per cent. To
correct the yellowish color of the pulp, a small amount of blue and
red coloring is added. To give body to the paper from 5 to 15 per cent
of clay is mixed into the pulp and, to bind the fibers together and
give the paper a finish that will be at least partly nonabsorbent, a
quantity of rosin and soda ash is added with enough alum to precipitate
the rosin upon the fibers. The beater thoroughly mixes the mass of
material and converts it into a fluid of the consistency of thin milk,
which then passes on to the Fourdrinier machine.
GETTING DOWN TO THE “BONES” OF THE WOOD
Paper made from ground wood pulp soon discolors and becomes brittle
owing to the presence of the lignin and resin of the tree. The
superiority of sulphite pulp is due to the fact that chemical processes
are used to remove these disturbing ingredients, leaving only the
“bones” of the tree, that is, the pure cellulose fiber. Spruce wood is
usually used and after being freed of its bark and thoroughly cleaned,
all knots are cut out and it is sawed into short lengths which are
then split and cut up into little chips from ¼ to ½ inch long. These
chips are boiled under pressure in a solution of sulphur dioxide
and slaked lime. The “digesters” in which the chips are boiled are
large steel cylinders lined with brick set in cement and coated with
litharge and glycerin, so as to prevent the sulphite liquor from coming
into contact with the metal. A steam pressure of seventy to eighty
pounds is maintained for eight to ten hours and then the digesters are
discharged into vats. The sulphite liquor in which the resinous matter
is dissolved is drained away and the pulp is washed.
Soft woods, such as poplar, basswood, etc., are boiled in caustic soda,
producing what is known as “soda pulp.” “Sulphate pulp” is made by
boiling in a solution of sodium sulphate, to which a certain amount of
sodium carbonate and caustic soda are added. This process is used for
producing pulp for so-called “Kraft” paper and brown wrapping paper.
Formerly pulp was always made in the paper mill and hence the mills had
to be located near the source of wood supply, but to-day the making
of pulp is carried on as an independent industry in remote forest
regions where water power is plentiful. The pulp is pressed into laps
and compressed into bales weighing about 400 pounds. These are sent
over long distances by rail or steamship to paper mills located in or
near the big cities, so that the paper may be made near the points of
consumption.
Baled paper pulp has to be reconverted into a smooth milklike fluid.
A shredding machine is used for this purpose, which tears the pulp
apart and mixes it with water. Then it goes to the beater where it
receives its quota of sulphite pulp, coloring matter, clay, and sizing,
as explained before. After an hour of treatment in the beater, it
passes on to a tank in which it is further diluted with water and is
then put through a Jordan engine. This consists of a conical cylinder
in which is placed a conical roll. Both cylinder and roll are formed
with knives that extend longitudinally, but the revolving knives are
set at a slight angle to the fixed knives, somewhat after the fashion
of a lawn mower. The conical form permits of careful adjustment of the
roller in the cylinder. The pulp mixture enters at the smaller end and
discharges at the larger end. This machine gives the pulp its final
beating and reduces it to a very smooth consistency.
THE FOURDRINIER PAPER-MAKING MACHINE
From the Jordan engine the pulp is passed on into the Fourdrinier
machine, which we are now ready to investigate. The principal element
of this machine is a traveling belt of copper wire gauze which takes
the place of the screen or sieve used in the hand process. To confine
the pulp to the belt there are two endless bands of rubber, known as
“deckles,” along each edge of the screen, which travel with the screen.
To insure proper interlacing of the fibers, as in the hand process,
the screen is shaken laterally at the rate of about 300 vibrations per
minute.
The fluid pulp enters the machine through a flow box, where a constant
hydraulic head is maintained, so as to preserve a uniform discharge to
the wire gauze. The milky stream issues through a narrow opening in
the flow box, known as a “slicer.” This may be adjusted to alter the
width and also the thickness of the stream. No one would suspect from
the appearance of the milky fluid that it is filled with fibers, but a
microscopic examination of a drop of the thin liquid shows that it is
filled with tiny particles floating in the water like logs in a river.
It is interesting to watch this stream, as it passes out upon the
screen, change magically before one’s eyes, from a milky stream into
solid mass of pulp.
[Illustration: FIG. 71.--DIAGRAMMATIC VIEW OF THE FOURDRINIER
PAPER-MAKING MACHINE
A. Flow Box
B. Copper Gauze Belt
C. Rollers
D. Deckles Confining Pulp
EE. Pulleys
FF. Drums
G. Suction Boxes
H. Dandy Roll
I. The Final Suction Box for Drying
J. Couch Roll
K. Canvas Belt
]
In Figure 71, which is a diagrammatical view of a portion of a
Fourdrinier machine, the flow box is shown at _A_ and the copper gauze
belt at _B_. This belt is supported on a series of rollers, _C_. The
deckles which confine the pulp along the margins of the screen are
shown at _D_. These belts travel with the wire gauze for a certain
distance and then pass back over the pulleys, _EE_, while the wire
cloth passes on and around the drums, _FF_. As soon as the pulp flows
out upon the wire gauze, the water it contains begins to drain away
through the gauze. But it does not do this fast enough under the mere
pull of gravity and it must be helped by means of suction. At _GG_
there are boxes or troughs that fit against the under side of the wire
gauze. In them a partial vacuum is maintained by means of suction
pumps and a large part of the water remaining in the pulp is sucked
out here. A dandy roll is shown at _H_, which presses lightly upon
the moist pulp, squeezing out the moisture in it, and at _I_ there is
a final suction box which further dries the web of fibers. It is the
dandy roll that puts a patterned surface on the paper. If the face of
the dandy roll is of finely woven wire cloth with diagonal mesh, a
“wove” paper is produced, while if it is of such surface as to impress
parallel lines in the half-formed paper, “laid” paper is produced.
Water marks are also produced by placing a suitable raised or depressed
pattern on the dandy roll, producing a corresponding thickening or
thinning of the pulp. Even after passing the last suction box, the
thin web of paper is still very wet and so flimsy that it cannot be
handled. Right here comes the most delicate part of the whole process.
The web must be picked off the wire cloth and carried across a narrow
gap to a felt belt which conducts it through another set of rollers,
A felt-covered roll, _J_, known as a “couch” roll, presses down upon
the web and the latter clings to the felt in place of the wire cloth
which passes down around the roller _F_. In starting a web of paper
very skillful manipulation is required to transfer the web from the
couch roll to the felt belt, but after it has once bridged the gap it
has enough tensional strength to pull itself across the gap. The felt
belt carries the web through a pair of heavy rollers which press out
more of the water. Because the paper passes through the rolls with the
felt belt it receives an impression from the felt on the under side.
This is now corrected by running the paper through a second pair of
rolls in the reverse direction, and on a second felt belt, so that both
sides of the paper receive the same impression. From here on the paper
passes through a long series of steam-heated drying rolls and emerges
eventually as a perfectly dry sheet.
Our largest Fourdrinier machines make a web 200 inches wide and they
can be run at a rate of 800 feet per minute, or more than nine miles
per hour. The paper is made in lengths eight miles long, which are
wound up into rolls to be fed into the huge printing presses of our
metropolitan newspaper plants.
The difference between the making of news print and finer papers lies
chiefly in the quality of pulp used and the quantity of clay and sizing
added. After the finer grades have issued from the drying rolls, they
are put through calenders to give them a glazed surface. This consists
virtually in ironing the paper between smooth rolls of iron on one
side and compressed paper or cotton on the other. Just as in ironing
clothes, the paper must be moistened before being ironed and steam is
used to supply the necessary moisture. In some cases a high glaze is
given to one side only of the paper by means of friction. The paper
passes between two rollers of different size. The smaller one travels
faster than the larger one and consequently it slips on the paper,
giving it a very smooth surface.
THE CYLINDER MACHINE FOR MAKING PAPER
There is another type of paper-making machine which is used for coarse
papers, pasteboard, etc. This is known as a cylinder machine. The wire
gauze is formed into a cylinder which revolves in the liquid pulp. A
slight suction is maintained in the cylinder which causes the water to
flow in through the gauze; leaving the fiber drained on the surface.
The cylinder revolves, carrying the drained pulp on its surface until
it encounters a felt roll that picks off the web and carries it to the
pressing and drying rolls. One disadvantage of this type of machine is
that the cylinder in dragging the drained pulp through the water causes
the fibers to be combed in a direction parallel to the run of the paper
web, so that the paper, while strong in the direction of its length, is
comparatively weak in the transverse direction. This is corrected to a
certain degree by the use of “riffles” or devices that produce eddies
which disturb the even flow of the pulp.
In the making of pasteboard, a number of cylinders contribute their
webs to form a single sheet of several plies.