alteration in structure, and may in this state be flattened by
calendering to a smooth surface, and the presence of clay helps to
fill in the microscopic valleys between the fibers so that the surface
becomes level to human vision.
THE FOURDRINIER
Now to return to a sketch of the wet ends of paper-machines. The
Fourdrinier part consists of a head box, which resembles the case of
an upright piano. Where the keyboard might be, is a broad portal for
the passage of a stream of pulp, the width of the machine, onto a
horizontal, endless wire belt. This wire belt is suspended in a frame
some thirty feet long and held taut by being stretched over a number of
rolls. The large roll near the head box is known as the breast roll.
The still larger roll at the other extreme of the frame is called the
lower couch roll, on top of which is a felt-jacketed couch roll. The
wire is kept level by a transverse series of “table rolls” closely
set, and the under part of the wire is held down by stretch rolls.
Directly under the top part, and continuing from the breast roll for
about two-thirds the length of the frame, is a shallow tray called a
“save-all,” as it catches all the drippings which contain filler, and
some fine fibers which are returned to the screens by stuff-pumps,
maintaining a continuous circulation so that nothing goes to waste.
Into this save-all water may be admitted to regulate the consistency of
the stuff.
Near the couch roll the wire passes over two or three suction boxes,
and on top of the wire, between the suction boxes, turns a wire-covered
roll called a “dandy.”
On either side of the machine is a frame which may be contracted or
expanded. It carries a series of pulleys over which run rubber deckle
straps, the under parts of which rest on the wire and keep the wet
pulp within bounds. By this means the width of the web of paper is
regulated. As a little pulp leaks under these straps machine-made paper
has deckle edges on both sides of the web. Artificial deckle edges may
also be produced by squirting a fine stream of water upon the web near
the couch roll, but it is not possible to produce this effect across
the web. Except on special papers the deckle edges are trimmed off by
slitters at the end of the machine.
Near the flow box, running at right angles across the machine, are two
so-called “slices” about eight inches apart. These may be adjusted
at various heights from the wire, in order to regulate the thickness
of the paper. Their most important function is to make the thickness
uniform from one side to the other of the sheet, and to create a pond
which assists in forming the paper.
The frame of the Fourdrinier has a joint near the first suction box,
and a mechanical arrangement called a “shake” is located near the head
box to impart a lateral shaking motion to the frame while the wire runs
straight ahead, thus imitating the shaking of the hand mold.
Beyond the couch roll is a series of press rolls, between which run
endless felts to carry the soft, moist paper.
Then follows a large series of steam-heated cylinders. Next a stack of
iron calender rolls, and a set of reels. As soon as one reel is full
a new reel is started and the paper from the first reel is slit by
rotary slitters and made up into rolls of the desired widths on the
winder.
PAPER IN PROCESS
It is an almost dramatic moment when the machine is ready to start. The
machine tender opens the valves which admit the stuff from the flow
box and a stream spreads out onto the wire. At a given signal the back
tender starts the wire, and the endless white stream moves smartly
forward. Then ensues the mechanical imitation of making paper by hand,
only instead of forming sheet by sheet, the formation is a continuous
process in the web. The shake of the machine mixes the position of the
fibers in the “pond” behind the slices; the water runs like a downpour
of rain through the moving wire into the save-all, leaving behind its
burden of fiber, or “stuff,” as the mixture is at that stage called, in
a white film.
The suction boxes accelerate the expulsion of water, and the dandy roll
closes the fibers together as the film passes beneath it. Then the web
is carried between the couch rolls, when the water fairly pours out
in the squeeze. As the top roll is felt-jacketed, the film sometimes
sticks to it, as a slight suction is created in the pores of the felt.
The back tender stands by with a hose to wash down the paper if it
starts to adhere to the jacket. The paper is prevented from completely
going around this top roll by a guard board which is fixed across the
top. Many machines are now equipped with a “suction couch roll,” which
does away with the need for a top roll, as the water is sucked, instead
of pressed, out of the paper.
At a given signal the back tender starts one edge of the film forward,
by a skilful slap of the hand, which picks up the edge of the film and
transfers it to the felt carrier between the press rolls. The remainder
of the web is made to follow the lead of the first section, till
finally the full width is transferred to the first felt, which carries
it through the first series of press rolls.
An arrangement similar to the guard board, called a “doctor,” runs
across the top press roll, so that the paper may be allowed to roll
up if desired, while the machine tender regulates the flow of water
until the consistency of the stuff is right. The doctor also keeps the
press roll clean. Quite often the long end of paper first started at
the couch roll is passed right along from the first felt to the second,
carried through the second set of press rolls, and the third, if three
there be, to the steam driers, and thence over the entire battery of
driers, through the calenders onto the reel.
From the press rolls it is led by the back tender, assisted by a third
hand, and if all goes well the paper may be winding up on the reel
inside of ten minutes.
But there is many a chance for mishaps before the wet end of the
machine is adjusted and the heat in the driers is regulated to a nicety.
The weight of the paper depends upon the quantity of stuff let onto the
machine, the dilution of the stock, and the speed at which the machine
is run. Given a certain volume, the faster the wire runs the thinner
the stuff is spread, and vice versa. Before things are settled down,
considerable worthless paper may be turned off.
The width of the web is controlled by the distance between the deckle
straps. These are adjustable, but an allowance of ten inches or so must
be made for the shrinkage of the web in drying.
The preliminaries to a run of paper may be likened to the make-ready on
a printing-press, though they do not, as a rule, last nearly so long.
Yet this is the reason why small odd sizes and odd shades of paper are
not popular with the manufacturer, unless he can get a sufficient extra
price to compensate for the “make-ready” costs.
WATER-MARKS.[C]--The water-mark in paper is effected by raised
lines on the dandy roll. The design, being impressed in the moist
web, displaces the fibers and leaves thin areas in the paper, which
consequently show when the sheet is held against the light, as they are
more translucent than the adjoining areas.
[C] The study of ancient water-marks is quite fascinating in
connection with early Printers’ marks. See “A New Light on the
Renaissance,” by Harold Bailey.
[Illustration: FOURDRINIER MACHINE, S. D. WARREN & CO.
View showing the “Fourdrinier” part of a modern book paper-machine.]
CHAPTER SEVEN
PAPER-MAKING--_Continued_
TECHNIQUE.--The importance of the formation of the sheet on the
machine wire is the same as on the hand mold, as subsequent pressing
and calendering can only modify faulty formation. The stuff should
be uniform and even in texture. The press rolls must be ground with
absolute accuracy, and slightly crowned to allow for their sagging.
Otherwise water would be unevenly expelled from the web, possibly
causing a damp streak throughout the entire run of paper, which would
show in the finished product.
If a portion were pressed too hard it would contain less moisture as
it reached the driers and become dry before adjacent sections. If the
paper were calendered, the moister parts would take on a smoother
surface than the drier parts.
Another feature to be closely watched on particular papers is to
eliminate, as far as possible, the impress of the weave of the wire
cloth, which is left in the under side of the web. This can be
accomplished to so fine a degree, by a skilful man, that the difference
between the two sides of the paper is scarcely discernible. The
fineness of weave of the one cloth also is an important bearing in
securing an even sided sheet.
Thus we see that it is well-nigh impossible to reduce the making of
paper to an exact science, and a reasonable variation must be expected,
both in weight and finish. The successful management of a paper-machine
depends, from start to finish, on careful, experienced judgment and
alert attention. If the beater-man dilutes one batch of stuff more than
another, the variation will show the minute the altered stuff appears
on the machine, and only an immediate readjustment at the wet end can
avoid considerable variation in the product. Then, from end to end,
the long machine must be watched carefully, so that the pressing,
drying and calendering may all be kept uniform. A bungler should find
no place in the machine-room, but it is desirable that consumers
have sufficient appreciation of human limitations, as applied to
paper-making, to admit proper allowances for normal variations.
CYLINDER MACHINE.--The cylinder machine, invented by John Dickinson
about ten years after the Fourdrinier, is much the same as the wet
machine described in Chapter II, with the addition of press rolls,
driers and calenders. The single-cylinder machine is used for making
light-weight tissues and other thin papers. Cylinder vats can also be
arranged in series, as on board machines, so that the webs formed on
each cylinder can be combined. This is accomplished by an arrangement
of felts which run tangent to the cylinders, picking off the formed
paper automatically from each successive mold.
The felt runs between squeeze rolls, so that the various plies of paper
are pressed together, forming a single thickness. Machines of this type
can make very thick sheets, and are used for making bristol boards,
blanks, boxboard, strawboard, etc.
The number and arrangement of driers on any machine depends on the
product to be derived. Fast-running machines, such as the large news
mills are equipped with, have necessarily a large number of driers,
as they turn off fifty tons or so a day and require a great drying
capacity. Slow running machines, such as are used in fine writing-paper
mills, need a much smaller number, as the average fine writing-paper
machine produces little over three or four tons a day.
[Illustration: CYLINDER VATS, MADE BY THE PUSEY & JONES CO.
The felts which convey the paper are omitted so as to get a clearer
view of the molds.]
HARPER MACHINE.--There is a type of Fourdrinier called the Harper
which differs from it in that it is turned end for end. A long felt
carries the paper from the couch rolls back over the Fourdrinier part,
delivering it to the first press. This is considered advantageous in
making very light papers which otherwise are with difficulty led from
the couch to the press rolls and are apt to break down in the passage.
YANKEE MACHINE.--There is even one type of machine known as the
“Yankee” which has but one drier of very large diameter. This is used
in making machine-glazed wrapping papers, which are very smooth on the
side of the sheet which comes in contact with the drier and rough on
the other side. The “wet” end of this machine is a Fourdrinier type.
The arrangement and number of smoothing and calender rolls is also
dependent on the class of paper to be made. Most writing-paper machines
have no calenders at all as the surface is obtained on special
machinery such as platers and sheet calenders after the paper comes
from the drying loft. One can easily appreciate that, while the general
principles of all paper-making are identical, there is a call for a
wide variety of arrangements, such as those cited, to meet the varying
requirements of different classes of paper.
SURFACE SIZING.--Surface sizing, or animal sizing, necessitates a vat
with squeeze rolls. The paper is first run over enough driers to dry
it; then introduced into the vat of hot size. On the cheaper grades the
size is dried on the machine by a special skeleton drying apparatus,
but the better grades are cut off and piled up by the “lay-boy” at the
end of the machine, then transferred to drying lofts and hung up over
poles to dry. Hence the term “loft-dried.” Any special finish has then
to be applied sheet by sheet.
FINISHING PAPER.--Finishing paper is accomplished either on the
paper-machine itself, or after the paper is turned off on the machine
it may be treated by special apparatus.
WOVE AND LAID PAPERS.--A so-called wove paper is made with a plain
dandy, covered with fine wire cloth the same texture all over. Laid
paper is really a water-marked paper, in which the whole surface is
marked by a specially constructed dandy which imprints a mark in
imitation of the early hand molds. There are heavy lines running with
the grain of the paper and lighter lines running across.
ANTIQUE.--An “antique” surface is obtained by skipping the calender
rolls and leaving the paper rough as it comes off the felt to the
driers. A medium finish is obtained by a slight calendering, while the
highest machine finish, and the so-called English finish, is obtained
by a heavy calendering.
WATER FINISH.--A common method of obtaining a high finish on heavy
papers is by the use of “water doctors,” which keeps two or more of the
calender rolls moist, dampening the paper while it is being calendered.
The surface thus imparted is called a “water finish.”
SHEET CALENDERS
Fine writing-papers may be finished in a variety of ways. A plain,
smoothed surface is obtained by passing the sheets, which are
automatically fed, by a system of tapes, through calender stacks,
called sheet calenders.
PLATING.--Plating was first resorted to as a means of smoothing paper
in the sheet, but when a linen, or pebbled, or any other special finish
is desired, it is also accomplished in a plating machine. This consists
of two heavy rolls. The sheets of paper, with a metal plate top and
bottom, are passed through the rolls under heavy pressure. If a linen
finish is desired, pieces of linen are placed between the plates on
both sides of the sheets so that the linen texture is embossed into the
paper. Similarly any other substance may be used for other effects.
EMBOSSING.--Embossed papers are usually finished from the roll by
running between iron rolls with embossing patterns engraved upon them.
An extra strength is required of paper for this purpose, otherwise the
pattern will cut through the sheet.
SUPERCALENDERING.--Supercalenders are machines, apart from the
paper-machine itself, for making high-finished paper. The rolls vary
in number. Each alternate roll is made of hard paper. In treating
uncoated stock there are also one or two steam boxes to moisten the
paper before it is calendered. This softens the surface fibers, and
they can then be rolled flatter and hence take a shinier surface. The
alternate rolls in a stack for calendering coated papers are made of
cotton, and no steam boxes can be used, because the moisture would
injure the coating.
The paper is run through the calenders in the web. All smooth, or
special, finishes are gained only at added cost. Where the process
takes place on the machine, more breakage is occasioned and more paper
has to be sorted out, as the hard-finishing accentuates spots in the
paper, and little lumps of fibers, which would pass unnoticed in an
uncalendered or antique paper, are squashed down and blackened by
calendering. Hence the higher cost of such papers.
Supercalendering and plating bring into play different workman, so that
the labor cost is increased, and any finishing, sheet by sheet, is
necessarily slow and more costly than that accomplished in a continuous
process from the roll.
COMBINING.--Many kinds of papers, as photo-mounts, double-thick covers
and cardboards, are made by pasting two or more thicknesses together.
This was formerly done in the sheet, but most of the pasting is now
effected in the web. The papers are run over a paste roll, combined,
and passed either through a drying chamber or over a battery of driers
like those of the paper machine. The pasted paper is lastly made into
rolls and taken to the finishing room to be sheeted.
COATED PAPERS.--Coated papers are made by covering the surface of
ordinary paper with a veneer of clay, mixed with some adhesive, as
casein or glue, and suitably colored.
The process is done from the roll; the paper first goes through the
machine where the liquid coating is brushed onto the surface, passing
directly in automatically formed festoons through a long, heated room
to dry, and finally is rewound. The rolls are then taken to the
supercalender room and the paper is given the desired finish.
Dull-finish coated papers require a special kind of coating and receive
very light calendering after being coated.
High-finished coated papers of the best grades are double-coated and
run several times through the calenders.
Another method of producing a high finish is known as “flinting.” In
this process the paper is mechanically polished by smooth flint stones
and gains a very high luster. Such papers are most widely used as box
covering. A similar effect is obtained by friction calenders, which
consist of two chilled iron rolls with an intermediate roll of hard
paper. The top roll rotates at a higher speed than the others.
[Illustration: COATING ROOM, APPLETON COATED PAPER CO.
This view of the “wet end” of the coating machines shows the rolls
going through the coating process, the web of paper traveling along the
drying racks appears in the background.]
The coating may be dyed to any color desired, so that coated and
glazed papers are obtainable in a wide variety of shades.
[Illustration: FINISHING-ROOM, CRANE & CO.
The machine on the right is a plater.]
GUMMED PAPERS.--Gummed papers are made by passing the web through a
machine, which coats it with glue, after which it passes over drying
apparatus and is wound into rolls ready for finishing.
Gummed paper for labels is usually finished in sheets, while for
sealing tape and box stays it is ordinarily made up into rolls.
WAXED PAPERS.--Waxed papers are made by applying a coating of parrafin.
This renders the stock water proof, and it is used largely as a wrapper
for food products.
GLASSINE PAPER.--By a special treatment in the beaters and jordans
cellulose fiber is so treated as to become hydrated. This hydration
makes the paper produced grease proof, and by heavy supercalendering
the character of the sheet is again greatly altered, it becoming almost
perfectly transparent. In this state it makes a most attractive and
hygiene wrapper.
CHAPTER EIGHT
THE PHYSICAL AND CHEMICAL ASPECTS OF PAPER
The size and weight of a sheet of paper of any given quality and
finish are its most obvious features, and when we speak of the weight
of a sheet of paper we refer not to the one sheet, but to the weight
of one ream of similar sheets. Most papers are ordered on a basis of
ream weight for a specified size, as, for example, 25 by 38, 50-pound.
Blanks, cardboards and cover-papers, especially the first two, are more
frequently ordered on a basis of bulk, as two-ply, three-ply, etc., and
thick or double thick in the case of covers. The thinner covers are
usually designated by their ream weight, though frequently quoted, as
are the heavy-weight covers, the blanks and cardboards, in price by the
hundred sheets.
The reason for this difference is probably that such stocks are sold
in comparatively small lots, so that it is simpler to bill them in
accordance with the number of sheets than to figure the weight of a
small number of sheets and multiply by the pound price.
Another thing which facilitates the system is that these kinds of paper
are carried in standard stock sizes, as the majority of orders are too
small to be made in special sizes.
The relation between thickness and weight of a given paper is
approximately a direct ratio. For example, given a sheet of machine
finish 25 by 38, 50-pound, four sheets of which bulk .011 of an inch,
the bulk of the same finish and quality, in 25 by 38, 60-pound, can
be approximately ascertained by the equation 50 : .011 :: 60 : x, the
answer of which is .0132.
The difference in bulk between two papers of the same weight depends on:
1. The finish.
2. The percentage of mineral filler.
3. The nature and treatment of the fiber.
For example, on a bulk of .015 of an inch to four sheets a
supercalendered paper would weigh about 65 pounds, a high machine
finish about 60 pounds, a text or medium finish about 50 pounds, an
antique about 40 pounds. In other words, the density of any given piece
of paper is proportionate to the amount of calendering it receives.
Naturally, the antique paper, lightly pressed and uncalendered, is
loose for texture and full of minute air pockets, so that it is light
for bulk, while the supercalendered paper is squeezed to a hard, dense
sheet containing little air space.
If the proportion of mineral filler is great, the weight will be
still greater in proportion to the bulk, as the specific gravity of
the mineral is greater than that of the fiber, and the fine particles
tend to fill completely the small interstices between the fibers, so
that the air space is reduced to a minimum. If, in addition, a surface
coating is added, we get a paper with the highest possible percentage
of filler, and consequently a glazed coated paper has less bulk in
proportion to its weight than any other kind. Such paper contains from
30 to 40 percent of mineral.
The nature of the fiber brings about a difference, in that some fibers
have thicker walls and smaller canals than others. The treatment causes
a variation, in that a quick beating with sharp knives leaves the
fibers more nearly in their original shape than a prolonged beating
with dull knives, which breaks down the structure of the fibers and
draws them out into minute fabrillæ.
The strength of a paper of given quality will also to a certain extent
be proportionate to the duration of beating, as well as the amount of
pressing and calendering received. The amount of sizing and the drying
also affect its strength.
An antique paper, having large air spaces and loosely knit as it is,
has not the tensile strength it would possess if pressed and calendered
to a greater density.
The addition of loading adds to the weight without increasing the
strength, as it has no binding properties. Moreover, the bulk, in
proportion to the weight, is lessened by the introduction of filler.
Consequently it is axiomatic, that of two given papers of equal weight,
finish and quality of fiber, the one containing the less filler will
be the stronger, as well as bulkier. The addition of filler, however,
increases the opacity, gives mellowness, and improves the printing
quality by equalizing the texture of the surface.
The addition of sizing tends to increase the strength of paper, owing
to its adhesive properties, but if liberally used it detracts from the
mellowness and gives the sheet a tinny “character.”
The length of the fiber also affects the strength, as long fibers give
greater strength and better folding quality than short. It is not
possible to get as close formation with long as with short fibers.
Hence occasions frequently arise wherein customers ask for
characteristics which are somewhat contradictory.
A desires a light, bulky paper with a high finish, but a bulky paper
with high finish must, in the nature of things, be heavy.
B desires a very strong, thin, but opaque paper. It is obvious that the
strength of a thin, opaque paper can be but a relative factor, while
thinness and opacity are irreconcilable features.