there often appears to be some confusion between the two, leading to an
unreasonable prejudice against papers made from either class. The fact
is so generally known that news-print, one of the cheapest grades of
paper, is made from wood, that the partially informed person is prone
to think that all wood papers are of low quality, whereas paper of
permanence and excellent quality may be made from the high grades of
wood cellulose chemically prepared.
GROUND WOOD.--The mechanical, or ground wood, as its name implies, is
made by grinding logs from which the bark has been removed. The logs
are shipped, or floated from the lumber camps to the mills, where
they are cut to convenient length and the bark is removed. Next they
are taken to the grinders. One type of grinder consists of a vertical
grindstone encased in an iron jacket. There are three pockets over
its circumference into which the logs are placed. They are held by
hydraulic pressure against the revolving stone, over which flows a
stream of water, and are rapidly reduced to fibers. These fibers are
carried by the flowing water into a chamber below the grinders, passing
through a screen which catches the coarser bits, the fibers of suitable
size thus being separated from the rest. This pulp is still not
sufficiently fine or uniform, so it is pumped into screens and forced
through the finely perforated plates. The fibers are carried through
with a large quantity of water, and are formed into thick sheets by
means of a so-called “wet machine.”
WET MACHINE.--The wet machine consists of a vat, in which a partially
submerged hollow drum rotates. The surface is covered by a wire cloth,
and the hollow axle of the drum acts as a drain for the fiber-laden
water, which, in passing through the drum, deposits a film of fibers
upon the revolving surface. This soft pulp film, continuously forming,
is removed from the top of the drum by an endless felt running tangent
to it, and held in close contact with it by a couch roll, the pressure
of which causes the web of pulp to adhere to the felt.
The felt passes between two squeeze rolls, and the pulp adhering to
the upper roll is wound up until a certain number of layers have
accumulated, when it is cut across by a knife and removed as a thick
sheet.
[Illustration: WOOD GRINDER]
The sheets, folded to a convenient size, separated by alternate pieces
of sacking, are put in a hydraulic press and squeezed to remove the
water. The pulp is taken from the press about fifty per cent moist;
the sheets are separated from the sacking and are now ready for use
or for shipment. It is also quite customary to ship the pulp without
having pressed it. In this case it contains about 70% water, due
allowance for which is made in billing.
This pulp contains practically all the constituents of the original
wood, has little strength, inferior felting properties, and is not of
permanent character. Its utility results largely from its cheapness.
When made into paper with a suitable admixture of sulphite pulp, for
strength’s sake, it proves to be admirably adapted for the fast-running
newspaper presses, as ink dries upon it almost instantly.
It is also used in the making of boxboards, cheap cardboards, pie
plates, wall papers, etc. It should, however, be strictly excluded
from all papers of more than ephemeral purposes, because of its lack
of permanence. The appearance of a paper containing much ground wood
is inferior, as the color is poor and small shives of wood may be
discerned on the surface. An easy and reliable way to ascertain the
presence of ground wood is to moisten the paper with a drop of strong
nitric acid, which develops a dark-brown stain if ground wood is
present. Another good test is phloroglucine, which turns ground wood
to a bright carmine shade. The quantity of ground wood is roughly
indicated by the intensity of the stain.
BLEACHED GROUND WOOD
A quality of pulp intermediate between chemically produced wood
cellulose and ground wood is obtained by bleaching an especially finely
ground quality of pulp wood. This product is excellent as a filler
for medium grades of paper, as it is opaque--fine, and of fair color.
Nevertheless, it is open to the same criticism as other ground wood as
to permanence, though in a less degree.
CHAPTER FIVE
THE CONSTITUENTS OF PAPER--_Continued_
CHEMICAL WOOD-PULPS.--Chemical wood-pulps are obtained by a variety of
processes, all of which have as their object the isolation of the pure
cellulose fiber by the dissolution of non-cellulose components. The
same principles are applied to the treatment of esparto straw or other
plants. The character of the pulp depends not only upon the nature of
the wood, but also upon the solvents used and the duration and severity
of the cooking.
The preparatory steps to any process by which chemical wood-pulp is
made are identical with the preparation of trees for ground wood,
only after the logs have been “barked,” they are reduced to chips by
a mechanical “chipper.” The ordinary practice in America is to sort
out any knotty or imperfect logs as they pass on a conveyor from the
“barker,” and if the log it too faulty it is discarded. As it is
desirable to have a uniform size of chips, the chips are passed through
a screen for this purpose.
The chips are stored in bins convenient to the digesters. The digesters
are of two types, rotary and stationary. The rotary type is horizontal
and the stationary is vertical.
After the digester has been loaded with chips, the chemicals are
introduced and the “cook” is carried on by means of high steam
pressure. The strength of the chemicals, pressure of steam, and
duration of cooking, are the principal factors in determining the
result from any particular wood. Slow cooking at low temperatures
yields the best results.
[Illustration: WET MACHINES WHERE THE PULP IS CUT OFF IN SHEETS, THE
BROWN CO.
To the right are the hydraulic presses for removing moisture from the
pulp. The pulp is shipped about seventy per cent moist.]
SODA PULP.--Soda pulp takes its name from the caustic soda which is
used as a solvent. Rotary digesters are employed in its manufacture.
The principal wood used for making soda pulp is poplar, though
chestnut and aspen are also used. Soda pulp is soft in texture and
of no great strength, but in combination with harder stocks it lends
mellowness to the sheet. It is almost one-third cheaper than bleached
sulphite pulp, quotations for February, 1915, being $2.20 to $2.35 per
hundredweight, whereas bleached sulphite was quoted at $2.80 to $2.95
per hundredweight. The prices since the war have risen over 100% and
were quoted in September 1919 at $4.75 to $5.00 and $5.75 to $6.25,
respectively. One reason for the difference in price between soda and
sulphite pulps, is that the soda is recovered from the spent liquor,
whereas in the sulphite process the liquors go to waste.
SULPHATE PULP.--The solvent used in making sulphate pulp is a mixture
of caustic soda, sulphide of soda and sulphate of soda. Sprucewood,
largely, is used and the pulp produced is exceedingly strong.
Unbleached sulphate pulp is used, notably, in the making of Kraft
wrapping-paper. The soda is recovered from the spent liquors.
SULPHITE PULP.--Sulphite pulp is produced by the use of bisulphite of
lime; this, being acid, necessitates a special brick lining in the
digesters, which are of the vertical type. Sprucewood is the best raw
material and yields a strong, fairly long fiber, capable of being
bleached to a good white color.
MITSCHERLICH PULP.--A special method for making sulphite pulp was
invented in Germany by Professor Mitscherlich. It varies from the
ordinary process in that the cook is continued over four times as long
under lower steam pressure, and yields a fiber of greater strength.
The steps subsequent to cooking chemical pulps of all kinds are
similar. After emptying the digesters, the soft, discolored mass of
fibers is washed and bleached. The yield of cellulose fiber is close
to fifty per cent of the air-dry weight of the wood. The shives and
undigested particles are removed by screening, and the pulp is either
run out like ground wood on wet machines, or made up into rolls,
or sheets, on a paper-machine. The soda pulp is shipped in rolls
and the sulphite in sheets, as this is the most favorable form in
which to handle them at the paper-mill. If the pulp is to be used on
the premises, it is made up into laps on the wet machine and is not
artificially dried. The so-called “air dry” pulp contains about 10%
moisture, and pulp containing not over this amount of moisture is
billed at its actual weight.
[Illustration: CYLINDER MACHINE FOR DRYING PULP, THE BROWN CO.
The web of pulp is shown as it passes from the cylinder mold over the
couch roll toward the driers.]
ESPARTO AND STRAW.--Esparto pulp is made by the soda process from a
grass obtained in the circum-Mediterranean countries, and is used most
extensively in England and somewhat on the Continent, but freights have
been prohibitive for American manufacturers.
Straw pulp is similarly made, and while occasionally used on medium
grades of writing-papers, its principal use in this country is for
strawboard and cheap wrappings. It is expensive to reduce to a clean,
bleached pulp on account of its knots, and the large quantities of
silicious matter it contains.
WASTE PAPERS.--The next largest source of paper-making fibers to wood
is the waste paper, such as old books, magazines, newspapers, binders’
waste, paper shavings and miscellaneous waste. This stock is collected
by regular packers, sorted, and sold by grade to the mills.
The poorest grade consists of a mixture of miscellaneous papers of all
colors and description. It is only used in the production of boxboards,
sheathing paper, and other coarse varieties, and without undergoing any
preliminary treatment it is shoveled right into the beaters.
A higher grade consists only of mixed papers, printed or unprinted.
Next is a grade containing no ground wood or colored papers, and above
this are graded old ledger and writing papers.
[Illustration: THE BEATER-ROOM, CRANE & CO.
The beater at the far end of the room is equipped with a washing drum.
This drum is lowered into the tub during the process of washing.]
Paper trimmings are divided into four classes, white and mixed, soft
and hard “shavings,” and are especially available, as they may be used
after sorting and dusting without undergoing further treatment, but it
is customary to macerate them in some sort of a pulper before placing
with other stock in the beaters. The printed waste must be boiled in
a solution of soda ash. This makes the ink removable. After about six
hours’ boiling, the stock is transferred to washers and treated like
rags. The ink and dirt having first been removed, bleaching solution is
introduced, and finally the stock is let down into drainers. In some
mills the draining is omitted, the excess bleach is washed out and an
antichlor added; then the stock is pumped over to a beating engine to
be mixed with the other ingredients preparatory to manufacture. This
process is less thorough, and there is more danger of getting residues
of bleach into the paper, as it is rather a nice matter to exactly
neutralize the bleach in the washer, and the maintenance of a uniform
color is endangered.
Printers, or others, who accumulate large quantities of waste papers,
will find that it pays to keep the various grades in separate
receptacles, as a better price may be obtained for it in this way.
Furthermore, by means of a baling press, the papers may be set aside in
compact bales, which occupy less room and are not so great a fire risk
as loose accumulations. The fact that 21.4 per cent of the paper-making
fibers, according to United States Census Report, 1909, are derived
from waste papers, indicates their importance as raw materials, while
their use lessens the drain upon our forests.
THE NON-FIBROUS CONSTITUENTS OF PAPER.
The non-fibrous constituents of paper are the mineral fillers, the
ingredients for sizing, and the coloring pigments and dyes. Mineral
fillers should not be regarded as adulterants. They are used, not as
a means for adding weight, but for the sake of certain effects which
are requisite in many papers. No filler is used on good writings or
ledgers, as the printing requirements do not call for a closely filled
surface or a mellow texture.
In book papers a varying percentage of clay is used, as it improves the
printing quality by filling up the interstices between the fibers and
increases opacity. Papers for half-tone printing require more filling,
in order to have smooth, level surfaces.
There are several kinds of filler in common use. The most common
is China clay, of which the cleanest and finest grades are
obtained principally in England. No equally good deposit has yet
been successfully developed in this country. Clay is a product of
the natural disintegration of feldspar. It is soft, plastic, and
non-crystalline.
Agalite and talc, which are silicates of magnesia, are also used. They
are cheaper and less desirable, both on account of color and their
crystalline nature, which is more or less damaging to cutter knives
and printing-plates. These fillers are used widely in the cheaper
book-papers, and can often be detected by holding a sheet against the
light, as the little, translucent crystalline particles then appear
like pinholes.
Sulphate of lime, commercially known under such names as gypsum, pearl
hardening, satinite, etc., is a white, crystalline substance. This is
used to some extent in paper-making, but principally as a coating.
Barium sulphate, prepared chemically, and known as blanc fixe, is used
largely for coating papers because of its brilliancy and purity of
color.
SIZING MATERIALS.--Starch was one of the earliest materials used for
sizing paper, and is used considerably in addition to other materials,
as it adds a hard, tinny character desired by the trade on certain
grades. Silicate of soda is also used to impart similar characteristics.
Gelatine, or animal size, is obtained by boiling down suitable animal
tissues. As a sizing agent, it is applied after the paper is made by
passing the web of paper through a vat containing the hot liquid size.
Casein, which is sometimes used as sizing, is more important in its
functions as an adhesive for the making of coated paper. It is prepared
by treating skim milk with weak acid.
Rosin size, the most widely used size, is produced from rosin by
cooking with soda ash, which produces a soft soap. The soap when mixed
with water by agitation assumes a milky appearance. In this condition
it is poured into the beater after all other ingredients have entered,
and is precipitated by the addition of alum as a resinate of alumina.
IMPURITIES IN PAPER.--Impurities, either chemical or physical, are
sometimes found in paper, owing to lax methods or inferior materials.
Free acid occasionally occurs, and in some cases would be very
deleterious. In papers that are to be bronzed, for example, this
acid would tarnish the bronze. Needle papers, and paper for wrapping
steelware, must be acid-free, otherwise they will cause rusting. The
presence of free acid may only be determined by an analyst.
Sulphur, which may give rise to the formation of sulphuretted hydrogen,
exists sometimes as an impurity in paper. It causes a brownish halo to
appear around printed letters, because of its action on printing-ink.
It would also cause oxidization of jewelry, mounted upon cardboard
containing sulphur residues.
Free chlorine, or chlorine compounds, the result of inadequate draining
of the stock, may cause final disintegration in the paper. It is the
duty of manufacturers to guard against this and the other deficiencies
noted.
Mineral impurities in paper are not uncommon. Minute particles of iron
worn off the machinery, or getting into the stock in the shape of
wire stitching, can often be discovered by the use of a magnet test.
In photographic papers this must positively be excluded, but in most
papers, if the particles do not show as specks, and are not large
enough to make trouble for the printer, they are not a serious menace.
CHAPTER SIX
PAPER-MAKING
We have now reviewed the various steps preparatory to the process of
beating, and this process is perhaps the most important of all. The
output of a mill depends, first, upon the quality of stock which is
furnished to the beaters, and secondly, on the way the stock is handled
in the beaters. A formula, better known as a “furnish,” is prepared by
the superintendent and given to the beater engineer. This tells him
exactly how to blend his raw materials. Very few papers are made from
one kind of material alone, most papers being a mixture of different
fibers, with the addition of mineral filler, sizing and coloring. All
the ingredients are put together into the beating engine with a large
volume of water similar to a washer, minus the washing drums.
BEATING.
The process is called beating because it has displaced the original
method of maceration by mallets and later by the machine described in
Chapter I as a “stamper.”
The ultimate characteristics of the paper are dependent upon the
handling of the beater roll and the character of the knives. For
example, a blotting-paper is made by a quick beating with sharp
knives. This cuts the fibers clean and short and leaves them in a most
absorptive condition. The very same fibers, treated with dull knives
and slowly beaten, would have an entirely different character. Their
ends would be teased out and ragged, and in the process of manufacture
they would part very slowly from the water absorbed. The paper produced
would have the characteristics of a writing-paper, hard and strong.
This instance will afford some idea of the wide variation in results
which may be brought about by varying the treatment in the beaters. So
important is this step in manufacturing that it has been said with a
good deal of truth that “the paper is made in the beaters.”
After the process has been continued a sufficient length of time, the
stuff is emptied into a chest called the “Jordan chest,” because it
acts as a reservoir for another type of refining engine known as the
“Jordan.” This engine is conical in shape and the inside is lined with
knives. A cone-shaped plug, also shod with knives, fits into this
shell, and by the turn of a screw may either be moved in or out, thus
varying the space between the two sets of knives. By this adjustment
the refining of the pulp which flows through the engine is regulated.
The stock passes through one or more of these “Jordans” into the
machine chest. Thence it is pumped to a level higher than the machine,
and flows through “sand settlers” to a screen. The “sand settler” is a
long, open trough containing a series of baffle boards which collect
any sediment, preventing it from getting into the paper.
Screens are of various types, the main feature consisting of bronze
plates pierced with fine slots through which the fibers are forced. The
object is to give uniformity to the stock which reaches the machine,
and to exclude any knots of stock, strings or foreign substances.
The width of the slots is varied to suit different stocks--some slots
being as fine as 10/1000 of an inch.
We have now described the process of paper-making up to the point where
the stuff is formed into paper, and must pause for a description of the
paper-machine itself.
PAPER-MACHINE.
The paper-machine may be considered in three parts: The wet end where
the paper is formed and pressed, the middle, where it is dried, and the
dry end, where it is calendered, slit and wound.
There are two distinct types of wet ends--the Fourdrinier and the
cylinder. Both are mechanical reproductions in continuous process of
the steps taken in the ancient hand methods, a brief consideration of
which impresses clearly on one’s mind the rationale of the machine.
HAND PROCESS.
The tools of the primitive paper-maker consisted of a pulp vat for the
fiber-laden water, a frame, or mold, across which was stretched a mesh
of closely woven wire, and a removable frame, known as the deckle,
which fitted around the edge of the mold to keep the moist pulp from
overflowing and to help regulate the thickness of the paper.
Grasping the mold by two opposite sides, the vatman submerged the mold
in the water; then raised it out, holding it level. By this means a
film of pulp was caught up, being deposited on the bottom of the mold
by the passage of the water in which the fibers had been suspended.
A lateral shaking motion served to knit the fibers together, and to
deposit them as evenly formed as possible all over the mold. As the
water drained through, the film of pulp solidified. Then the deckle
frame was removed, and there, on the top of the mold, was a sheet of
moist pulp. The edges of this sheet would be thin and feather-like as a
result of the pulp leaking under the deckle. Hence the term deckle edge.
It required a great deal of skill to remove this film, while preserving
it intact. This was accomplished by inverting the mold and pressing the
sheet upon a moist felt cloth. If the act was skilfully performed, the
mold could be lifted away from the sheet, leaving it unbroken upon the
felt. Then it was covered by a second piece of felt and the process was
repeated until a small pile had accumulated.
The pile was removed to a screw press, wherein as much water as
possible was squeezed out of the paper. Cellulose fibers have a strong
affinity for water, however, and it is said that under any pressure
which such a pile could withstand, without becoming crushed and
gruelly, the paper would retain water equal to one-half its weight.
Hence, the last vestiges of moisture, excepting of course that amount
normally retained by air-dried paper, had to be removed by evaporation.
In the old days, this was accomplished by hanging the sheets over poles
to dry.
After that, if the paper required sizing, the sheets were dipped one by
one into a pot of animal size, then dried once more. Lastly they were
finished to the desired surface by being placed between smooth plates
and pressed.
FIBER CHARACTERISTICS.
A few moments’ consideration of the changes which the fibers undergo
from their condition of isolation as they exist mixed in the vat, to
their status as components of a sheet of paper, will help to make clear
much that seems obscure about the behavior of a sheet of finished
paper, as well as to explain the reason for the different processes
executed on the paper-machine.
The fiber is a hollow, collapsed tube, the ends bruised and frayed by
the treatment in the beating and refining engines. Absorptive in nature
to a marked degree, it swells with the water it takes up and is limp
and flaccid. As the mold is raised horizontally out of the vat in the
process of forming sheets, all the fibers which had been suspended in
the water which passed through the meshes of the mold are caught like
so many fish in a net, and lie spread in a limp, impressionable mass
over the surface of the mold until they are transferred by the “coucher
man” to the felt. Little alteration can take place in the general
position of the fibers after they have been “couched,” consequently
the formation of the sheet is the most important stage of the process.
As the water is pressed out, each fiber contracts to some extent, and,
from a consistency like gruel, the formed sheet passes to a more stable
state, wherein it can be gently handled without disintegrating.
[Illustration: FOURDRINIER MACHINES, CRANE & CO.
A good view of the surface-sizing vat is obtained in the machine on the
right hand. The paper is being slit just before its introduction into
the vat.]
As the drying proceeds there is a marked shrinkage in the dimensions of
the sheet, caused by the shrinking of each individual fiber, until the
fibers are thoroughly set, enmeshed one with the other.
The addition of size glazes over each fiber and makes it less
susceptible to moisture. The addition of clay permeates the structure,
filling up the interstices. Up to a certain point the clay does not
materially weaken the structure, as a certain percentage of empty
air space would exist without it. Beyond that point the clay will
fill places that conceivably would be filled by fibers, and having no
adhesive strength, the structure of a sheet overloaded with clay is
weakened in proportion to its overload.