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About paper grain - better copy



I'm sorry the previous post was poorly formatted. This
is another attempt.

I thought it might be helpful to make a few comments =

regarding paper grain.

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 =

obviously 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 glass 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 =

wet 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 the 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 only. 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 more prone to expand upon remoistening. If =

you have a sample of Mohawk Superfine 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 the 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. In other =

words, you can tell the grain direction just by looking at =

it if you aren't fooled by the felt marks. Smaller scale =

grain can't be seen without 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 as 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 mechanism.

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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