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Re: About paper grain



>I have always heard that paper has a memory, meaning that the way it is
constrained when dried the very first time creates a grain and/or shape
(perfectly flat or cockled) that the paper is inclined to return to if it=

is
moistened and dried again without restraint. Maybe Richard can comment on=

how much actual change might occur with subsequent environmental
influences.<

I like to think of paper as being composed of a lot of tiny springs
that have been stretched to different degrees. When paper is
dried, they become "frozen" in position. When the paper is
remoistened, they can relax somewhat and try to return to
their unstretched state. =


Cellulose, like plastic (and taffy)  is viscoelastic, that is, it has a =

tendency to flow under tension. This will release tension over
time. Moisture is a great plasticizer and the higher the moisture,
and also temperature, the faster the flow and stress relief.
Humidity cycling as with the seasons or even the day-night
cycle work to release these stresses over time. =


When wet, paper can expand in the cross direction by 2 to 3%.
In the machine direction, it expands 10 times less and might
actually shrink a little depending on type of paper and how
it is stretched during manufacture. Dimensional changes due to
humidity cycling will be substantially less than that but it can =

be enough to give you fits when trying to repair a tear, for instance.

Cockle is due to nonuniform dimensional changes in the
plane of the paper. These are due to nonuniform drying that,
in turn, is caused by the way the fibers are distributed in the sheet. Th=
e
papermakers' term for it is formation. Hold a piece of
poorly formed paper like toilet paper up to the light and look
through it. You will see light and dark clumps of fibers that
are thick and thin spots in the paper. These dry at different
rates causing differential stressesbetween clumps and are =

directly responsible for causing cockle.

By the way, cockle should not be confused with dishing and
fluting which occurs at the edges of a stack of sheets, or
edges of a book. In this case, the edges of the stack
exchange mositure more rapidly that does the center of
the stack and this results in the edges being longer (or
shorter) than the center. This will cause wavy patterns along
the edges if they have absorbed moisture and expanded,
or tight edges which form dish shapes if they have given
up mosure and shrunk. If you make end sheets by pasting
papers together and then placing them between blotters,
you will have some beautifully distorted paper edges if you
don't change the blotters often so that the moisture goes
primarily into the blotters and not to the air around the
edges - one practical example.

I hope this has been helpful.

Best regards,
Dick Grant

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