BROWN PAPERS AND BOARDS
_Common Browns._--The raw material used in the manufacture of common
brown papers is chiefly jute and waste fibres of every description,
such as waste cuttings from boxboard factories, old papers, wood pulp
refuse, and other substances of a like nature. The jute, in the form
of sacking or old gunny bags, and the hemp refuse, in the shape of old
rope and string, are subjected to a slight chemical treatment just
sufficient to isolate the fibres to a condition in which it is possible
to work them up into paper. The bagging and string are cut up in a rag
chopper and boiled in revolving boilers with lime or caustic soda for
several hours at a pressure of 20-30 lbs., the lime being used when it
is desired to manufacture a harsh paper, and the caustic soda being
employed for the production of paper having a softer feel. The pulp is
not always washed very completely after the process of digestion, as is
the case with white papers, and it is often possible to extract from
brown papers of this class a considerable proportion of the alkaline
matter which has not been thoroughly removed from the boiled pulp.
The presence of this alkaline residue does not affect the quality of
ordinary brown paper, but is frequently a serious defect in the case
of middles or straw boards, which are afterwards utilised for boxes
and covered with coloured papers. The colour of the paper pasted on to
such incompletely washed boards is frequently spoilt by the action of
the alkali when moistened with the paste used, many aniline dyes being
susceptible to the small proportion of alkali present.
The stronger materials, such as jute or old rope and string, are either
used by themselves or blended with inferior raw material according
to the quality of the paper being made. The jute and hemp fibres are
generally beaten by themselves in the engine before the other materials
are added. The pulp is mixed with the required amount of loading, while
the sizing and colouring operations are carried out in the usual way.
The common brown papers are known by a variety of trade names which
at one time indicated the nature of the fibrous constituent, but at
the present day the name is no guide or indication of the material
used for the manufacture of the paper. The common heavy brown used for
wrapping sugar and sundry groceries made in heavy grey and blue shades
is a coarse paper made from cheap materials and containing a large
proportion of mineral matter. It is usually supplied under the trade
name of _royal_.
A somewhat lighter and stronger wrapping paper of a white or buff
colour, used for wrapping groceries, tea, and cotton goods, is that
known as _casings_, a name probably derived from the application of
this paper originally to the lining of cases.
_Manila papers_ so called were originally made from rope, but the term
is now applied to papers which may be made entirely of wood pulp.
_Rope browns_ are common papers made of fairly strong material of a
miscellaneous character, this name having been derived from the fact
that rope and similar fibre were at one time used exclusively.
_Wood Pulp Wrappers._--Most of the papers of the present day are made
from wood pulp, this material giving a thin, light, tough paper, which
is pleasant to handle and forms a great contrast to the dense, opaque,
heavily loaded, and inartistic specimens produced some years ago. Paper
of this kind, though apparently more expensive than common browns, is
really more economical in use. The paper is not only stronger, but it
is possible to obtain a larger number of sheets for a given weight.
The great advantage in the improvement of brown papers dates from
the introduction of the now well-known kraft papers, which are of
comparatively recent origin.
_Kraft Paper._--The term Kraft, meaning "strength," is applied to
a remarkably strong cellulose paper prepared from spruce and other
coniferous woods by the soda treatment, the special feature of the
process being an incomplete digestion of the wood.
The wood previously chipped into pieces 1 inch to 1½ inches in length,
is boiled with caustic soda, the digestion being stopped before the
wood pulp has been quite softened, and while the pulp is still too
hard to be broken up into isolated fibres by simple agitation in
water. The pulp after thorough washing is disintegrated by means of
an edge-runner, or some form of breaking engine, the first mentioned
probably giving the most satisfactory results, and converted into paper
by the usual methods.
The wood can also be reduced by the sulphate process, in which case the
chipped wood is boiled in a liquor to which about 25 per cent. of spent
lye from a previous cooking is added.
The best results are obtained by attention to the cooking process to
ensure an under-cooked pulp, by careful isolation of the fibres in a
kollergang, or edge-runner, which machine is capable of separating the
fibres without shortening them, and by proper manipulation on the paper
machine.
The paper produced under favourable conditions in this direction is
wonderfully tough and strong and may be quoted as the most recent
example of the fact that the latent possibilities of wood pulp have by
no means been exhausted or even thoroughly investigated.
_Imitation Kraft Paper._--If wood is boiled in water at high
temperatures the fibre is softened and much of the resinous matter is
removed. Such wood, if ground in the same way and by the same methods
as ordinary mechanical wood pulp, is readily disintegrated, and a
long-fibred pulp may be obtained. The process of boiling short 2 feet
logs of wood in a digester under a pressure of 20-50 lbs. has long
been known. The wood after boiling is partly washed and then worked
up into pulp by the usual mechanical process. The wood is easily
ground and yields pulp containing long fibres which in their physical
properties closely resemble those of pure wood cellulose, but the
original constituents of the wood are present almost unchanged, just
as in mechanical pulp. The product obtained by grinding is a very
tough flexible material of a brownish yellow colour, and the paper is
known as _Nature brown_. It is chiefly used for the preparation of
tough packing papers, for the covers of cheap pocket-books, and other
miscellaneous purposes. When this brown mechanical wood pulp paper is
glazed on both sides it is then known as _ochre glazed_, the word ochre
referring to the colour. When made up into light weight papers it is
sold as _imitation kraft paper_.
A great variety of wrapping papers are now made from wood pulp, such
as _sealings_, _sulphite browns_, _manilas_, _sulphite caps_, but the
distinctions between these papers relate chiefly to the amount of
finish, the colour and size of the sheet. The methods of manufacture
only differ in small details as indicated by these distinctions.
_Fine Wrappings._--The papers used for packing small goods such as
silver ware and other delicate articles are generally tissues, the
better qualities of which are made from rag, and the cheaper qualities
from wood pulp. These papers are known as tissue, crêpe, crinkled
tissue, manila tissue, and by a variety of trade terms.
[Illustration: FIG. 40.--Single Cylinder or Yankee Machine.]
Many of the fine wrappings of the tissue class and the somewhat heavier
papers known as M. G. Caps are manufactured on the single cylinder
machine, which produces a paper having a highly polished surface on one
side and a rough unglazed surface on the other side.
In the single cylinder machine the beaten pulp passes from the
stuff-chest on to the wire of the ordinary Fourdrinier machine and
through the press rolls, but instead of being dried over a number of
cylinders the paper is led over one single cylinder of very large
diameter which is heated internally with steam. The paper is usually
pressed against the surface of the cylinder by means of a heavy felt,
which is, however, sometimes omitted. The side of the paper coming
into contact with the cylinder becomes highly polished, the surface in
contact with the felt remaining in an unfinished rough condition. This
paper is said to be machine glazed and is known as an M. G. paper.
[Illustration: FIG. 41.--Section of Wet Press, or Board Machine.]
_Boards._--Cards, millboards, middles, boxboards, carriage panels,
and similar paper products are manufactured either on a _single board
machine_, by means of which single sheets of any required thickness can
be obtained, or on a _continuous board machine_, which is capable of
producing cards and plain or duplex boards of moderate thickness.
The raw material used consists, as in the case of browns and wrappers,
of every conceivable fibrous substance mixed with mineral matter and
then suitably coloured. The preliminary processes for the treatment of
the pulp are exactly the same as those employed in the case of brown
papers up to the point at which the beating has been effected.
SINGLE BOARD MACHINE.
The beaten pulp, diluted with large quantities of water, is pumped
continuously into a large wooden vat of rectangular shape. Inside
this vat revolves slowly a hollow cylindrical drum, the circumference
of which is covered with wire gauze of fine mesh. The drum is not
completely immersed in the mixture of pulp and water, so that as it
revolves the water passes through the wire, while the pulp adheres to
the surface. The water flows regularly into the interior of the drum
and runs away through pipes fitted at each side of the vat near the
axis of the drum, and the pulp is brought up out of the water until it
comes into contact with a travelling felt. The thin moist sheet of pulp
adheres to this felt, passes through squeezing rolls which remove part
of the water, and is finally carried between two wooden or iron rollers
of large diameter. The pulp adheres to, and is wound up on the upper
roller, the felt being carried back by the lower roller to the vat.
When the sheet on the upper roller has attained the desired thickness,
it is immediately cut off and transferred to a pile of similar sheets,
a piece of coarse sacking or canvas being interposed between every wet
board. The dimensions of the full-sized board are determined by the
diameter of the upper roller and its length. A roll 74 inches wide and
14 inches diameter will give a board 74 inches by 44 inches.
As soon as a sufficient number of wet boards has been obtained they are
submitted to pressure in order to remove the excess of water and at the
same time compress the material into dense heavy boards. The pieces of
sacking are then taken out and the boards dried by exposure to air at
the ordinary temperature or in a heated chamber.
[Illustration: FIG. 42.--Double Cylinder Board Machine.]
The dried boards are finished off by glazing rolls. These rolls
compress the boards still further and impart a polished surface. The
amount of "finish" may be varied by the pressure, number of rollings,
temperature of the rolls, and by damping the surface of the dry boards
just before they are glazed. The boards are cut to standard sizes
before or after glazing.
_Duplex Boards._--If the single board machine is fitted with two vats
instead of one, it is possible to manufacture a board with different
coloured surfaces. A board coloured red on one side and white on the
other is manufactured by having one vat full of pulp coloured red and
the second vat full of white pulp. The thin moist sheets from the two
vats are brought together and passed through the glazing rolls, which
cause the moist sheets to adhere closely to one another, the double
sheet of pulp so formed being wound up on the rollers at the end of the
machine. The board is then dried, glazed, and finished in the usual way.
The same principle is occasionally adopted on the Fourdrinier
machine for duplex wrappers. Thus a common brown pulp is worked up
in conjunction with a dyed pulp to produce a brown paper having one
surface of good paper suitably coloured. The brown pulp flows on to the
wire of the paper machine, and after it has been deprived of part of
the water at the suction boxes, a thin stream of coloured pulp, diluted
to a proper consistency, flows from a shallow trough, placed across and
above the wire, on to the wet brown web of paper in such a manner as to
completely cover it as a thin even sheet of coloured pulp. The adhesion
of the latter to the surface of the brown paper is practically perfect,
and the weight of the couch and press rolls ensures uniform felting of
the fibres.
_Middles._--This term is applied to a thin or thick cardboard made
of common material, the colour and appearance of which is of little
importance for inferior goods. Boards of this kind are covered
subsequently with papers of all colours and qualities, and the origin
of the word "middle" is easily seen. The manufacture of a board
consisting of two outside papers of good material and a middle produced
from common stuff is effected by the continuous boxboard machine,
unless the board is too thick to be passed over drying cylinders,
calendered, and reeled, in which case the boards are produced on an
ordinary wet machine and the paper pasted on the surface of the dry
board.
The term is, however, now also applied to a common paper made of
mechanical wood pulp with perhaps a little chemical pulp, used for
tram tickets, cheap advertising circulars, common calendar cards, and
similar purposes, to which no outer surface of a special character is
added.
CONTINUOUS BOARD MACHINE.
This machine differs from the single board machine in that the finished
board can be produced from the pulp at one operation. It is used
principally for cards and boards of moderate thickness which can be
wound up in the form of a reel at the end of the machine.
The mixture of pulp and water is pumped into two or more vats and
formed into a number of thin sheets, which are all brought together
between squeezing rolls and passed through heavy press rolls which
compress the several layers into a compact mass. The thick sheet
obtained is dried over steam-heated cylinders which are placed at the
end of the press rolls, and calendered. The whole process, indeed,
resembles that of ordinary paper-making, the main difference being the
method of producing the wet sheet or card.
Some machines are constructed with six or seven vats and forty to fifty
drying cylinders, and are capable of turning out a large quantity of
finished material.
The board can be made of uniform quality and texture throughout, or be
finished off with high-grade paper on one or both sides. In the latter
case the constituents of the "middle" part are waste papers and raw
material of inferior quality, the outer surface of wood pulp, white or
coloured according to circumstances. The variety of papers and boards
which can be produced is due to the fact that the several vats of pulp
are independent of one another and can be filled with any kind of paper
stock. The combined sheets forming the ultimate board are dried on
the ordinary cylinders, calendered, and reeled up at the end of the
machine.