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About paper grain



I thought it might be helpful to make a few comments regarding paper grai=
n.

If a paper does not have "grain", it does not mean that it doesn't expand=

or contract with changes in moisture content. It only indicates that that=

the properties in the direction of manufacture on a paper machine (called=

machine direction) are approximately equal to those perpendicular to the
machine direction (called cross direction). In a handsheet, there obvious=
ly
is no direction of manufacture but the directions are usually related to
the wires on the mold.

Paper grain arises from two basic mechanisms: fiber orientation and
dried-in stresses. In any machine-made paper I have been associated with,=

mostly fine papers, the grain comes almost exclusively from dried-in
stresses. One only need look at paper with a high-powered magnifying glas=
s
or low-power microscope to see that the fibers are laid randomly. If a
paper machine is brought to a crash stop and samples are taken from the w=
et
end through to the calender stack at the dry end, and those samples are
allowed to dry freely on mesh, the paper grain from samples taken from th=
e
wet end will have no grain while those taken progressively through the
machine will have increasing amounts of grain to a maximum at the wound
roll of paper. =


Where does this grain come from? It can't be fiber orientation since it
doesn't change significantly from one end to the other. Other more
sophisticated tests confirm that fact. It must be the way the paper is
dried. In fact, if the wet end samples are dried under constraint in a
laboratory, they will develop "grain" if constrained in one direction onl=
y.
Machine made paper is constrained in the direction of manufacture because=

it is pulled through the machine. It is not constrained in the cross
direction and is free to shrink as it is dried. Once it shrinks, it is mo=
re
prone to expand upon remoistening. If you have a sample of Mohawk Superfi=
ne
or similar less-calendered paper, look at the surface with a low angle
light first in the machine direction, then in the cross direction. When t=
he
light shines across the grain, you can easily see the large scale grain
which looks like grains of rice with the axes in the machine direction. I=
n
other words, you can tell the grain direction just by looking at it if yo=
u
aren't fooled by the felt marks. Smaller scale grain can't be seen withou=
t
an electron microscope.

Handsheets can have some oriented fibers depending on how the mold is
manipulated. As the water drains through the screen, the screen can act a=
s
a comb and drag the fibers in one direction. Usually, though, handsheets
have less grain because they are not constrained along one axis while
dried. The fact that many handsheets have less grain although the
possibility exists that the fibers could be more oriented again points to=

the importance of the drying method in contributing to the grain mechanis=
m.

What harm does it do to explain grain as arising from fiber orientation? =
I
think in a primitive sense as long as people keep their grain directions
straight it doesn't matter. However, an understanding of the mechanism of=

dried in strains is important for understanding dimensional stability,
curl, and distortion in single and multiply papers and boards but that is=

another subject. =


I hesitate to make this any longer lest the eyes glaze over. =


Dick Grant

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