Distorted microfiche

Many are the causes of warping (check my brain. They don't come much
more warped.)  But seriously, Eric Kesse probably said all that there is
to say about it, but I'll add a few more things.

Temporary curl is caused by the difference in dimensional behavior
between the emulsion and the base.  Normally this is measured in units
of 100/R where R is the radius of curvature in inches in accordance with
ANSI Standard IT9.10-1991 Method A.  There is another method (Method C)
that can also be used for fiche and sheet film, but there is no way of
converting units of method A into units of method C.  A curl of 150 in
100/R units means that the film looks like it is wrapped around a 1 1/3
inch diameter core.   A curl of 25 means that the film is curled as if
around an 8 inch diameter core.

Oh heck.  Let me just quote a pile of stuff rather than trying to
re-write the books so to speak.  Note that by convention positive curl
means that the emulsion is concave and negative when the emulsion is
convex.

    "The inherent tendency of photographic film to furl is caused
    primarily by the difference in humidity expansion between the
    photographic emulsion and the support.  Manufacture of a
    photographic film includes the coating of photosensitive,
    water-emulsified layer on a supporting surface.  Upon evaporation of
    the water, there remains a hydrophilic gelatin layer attached to a
    relatively hydrophobic plastic base.  A mechanical equilibrium
    exists between these two layers at a relative humidity which
    generally varies between 50 and 70 percent, depending on the film.
    However, when the relative humidity falls below the equilibrium
    point, the film loses moisture.  The dimensional contraction of the
    emulsion is greater than that of the support, and this differential
    causes the emulsion to pull the base into an arc and film curl
    results.

    Curl becomes greater as the relative humidity is lowered because of
    increasing differences between the equilibrium lengths of emulsion
    and support.  In addition, as the relative humidity falls, the
    modulus [the ease of stretchiness. A rubber band has low modulus
    while steel has a high modulus -- Doug] of the emulsion layer
    increases and eventually becomes greater than that of the support.
    This is a second factor contributing to film curl at low relative
    humidities.  In other words, at low relative humidities, the
    emulsion has both a higher contraction and a higher stiffness to
    pull the base into curled state.

    Negative curl appears when the relative humidity rises above that at
    which the emulsion and support are in equilibrium.  The emulsion,
    expanding from absorption of water, pushes the support into a
    negative curl configuration. Ultimately, a high enough humidity
    level is reached that the decreasing modulus of the emulsion assumes
    a value such that the emulsion no longer exerts any force on the
    support.  Film curl then becomes identical to that of the uncoated
    support.

    The influence of emulsion thickness on curl at low humidities is
    quite pronounced.  This is a direct reflection of the greater pull
    of thicker emulsion layers.  It is for this reason that non-gel
    backed color films generally have higher curl levels than non-gel
    backed black-and-white films.

    As the humidity rises, curl decreases.  However, the humidity at
    which curl becomes zero is a function of the emulsion composition.
    Emulsion composition also controls the existence and magnitude of an
    emulsion push above the equilibrium point.  At 85 percent RH, none
    of the emulsions have sufficient modulus to exert any curling force
    on the support. Consequently, the film curl at this condition is
    essentially the support curl.

    The primary effects of emulsion thickness on low relative humidity
    curl is illustrated by the variation of curl with the gel/base
    ratio. Emulsion composition, which is a secondary factor, accounts
    for the curl range at a given gel/base thickness ratio.

    Characteristics of the support also influence film curl at all
    relative humidities.  For some products (e.g., motion-picture
    applications), film support is purposely given a negative curl
    during manufacture.  This reduces the degree of positive curl at low
    relative humidities and gives a greater negative value when the
    relative humidity is high.  However, the curl amplitude (i.e., the
    difference between high humidity curl and low humidity curl) remains
    unaffected by the support curl.

    Some Kodak Estar-base [polyester - Doug] aerial films are backed
    with a gelatin coating that exerts a curling force similar to that
    of the emulsion layer.  The curl for such films is much less than
    for films which are gelatin coated on only one side; it is dependent
    upon the relative pulls of the gelatin emulsion and the gelatin
    backing.  The non-gel backing layers used for static resistance on
    some products have no curl-control characteristics.  The curl of
    these films depends only upon the forces of the emulsion layer and
    the curl behavior of the base."

Skipping ahead it is interesting to note:

    "...Photographic processing will change curl to some extent because
    it alters the mechanical properties of the gelatin layers and,
    consequently, their pull on the film support.  The films process to
    maximum density have less curl than the unprocessed films because
    processing removes silver salts from an emulsion and causes a
    reduction in its modulus.  Similarly, an emulsion processed to a low
    density will posses a still smaller modulus, resulting in still
    lower film curl."

All of this is from Kodak publication M-62.

To those of you who haven't scrolled on ahead out of boredom, I will add
that storage under very dry conditions may result in permanent curl
which leads me to the next topic.

Permanent curl in good film is usually caused by cold (or plastic) flow
effects.  This property is related to core set.  When plastic film is
stored in such a way that it is distorted for long periods of time (such
as excessively dry conditions) it will curl.  Imagine a round track for
racing.  The person in the inside lane has a shorter distance to travel
than the person in the outside lane.  Since the film support is not
infinitely thin, when curled, there is a difference in length between
the inside surface of base and the outside.  Through the thickness of
the base, some of the plastic is being stretched while other parts are
compressed. Since film (and people) don't like to be under stress, the
polymer molecules flow to relax.  Triacetate film suffers from this flow
more readily than polyester support.  As core set, you may remember
trying to get a roll of curly 35mm film onto a reel in the dark.  The
interesting thing is that when triacetate film is processed (put into
water), the base absorbs enough water that it flattens out.  (Horribly
curly 35 mm rolls behave nicely and lie flat after processing.)
Polyester, since it has such a lower water absorption will not do this.

With polyester film you will be limited to putting pressure on the film
to make the base flow the other way.  Acetate base **could** be
flattened by the application of water **BUT** make darn sure that the
emulsion will stand up to this treatment.  With age, the gelatin slowly
becomes more soluble and adhesion to the base may be tenuous.  If you
simply try to flatten the fiche you will find that acetate tends to
recover more easily than polyester.  In fact you may never get complete
recovery with polyester unless the direction of stress is reversed.  The
curliness in either support will be influenced by the tightness of curl
that it was stored at (thus the lower the humidity is, the tighter the
permanent curl will be.) In addition, flow rate increases with
temperature so warm storage conditions will increase the curl problem.

For those of you who are still with me, here's a gem.  How do we
distinguish acetate and polyester film without tearing it?  If you have
a large amount of film on the same film (say from a large microfilming
project), if you grab (gently now) a stack of film and look at the
lights through the edge of the stack then you will have a good clue.
Polyester has such an high index of refraction that the critical angle
is low (remember high school physics?).  Total internal reflection
allows most of the light that enters from one side to bounce back and
forth between the two surfaces of plastic to eventually emerge out the
other side.  This is the same reason that fiber optics works and in fact
polyester can be used for fiber optics.  Triacetate film will be dark.
The stack must be fairly tight otherwise light slipping in between
sheets will show up although light between sheets and light passing
through sheets does look different. If you have some acetate mixed in
with a stack of polyester, you will find light and dark bands
corresponding to the two types of film.

What if you don't have lots of one type of film to mess with?  The
second method is called the cross-polarization test that depends on the
property of birefringence.  Take two polarizing filters.  Holding them
together, rotate one until minimal light passes through (in theory there
should be no light, but we aren't in a perfect world.)  Slide a piece of
fiche with a Dmin area (or close to Dmin) between the two polarizers and
hold the stack up to the light.  If the fiche is acetate, the polarizers
will still look black.  If you have polyester, you will see a bright
spectrum of colors much like looking at oil in a puddle on a sunny day.

These two tests will hopefully save you from destroying your fiche
collection.

There are also less probable causes for distortion in non-deteriorating
film.  Tightly packed stacks of acetate film with a high moisture
content (either left a very long time in a humid environment or not well
dried before storing) may have a preferential loss of solvents from the
edge versus the center of the film.  This solvent loss will result in a
dimensional difference between the center of the film and the edge
causing distortion.  I have to admit that this is a long shot and I've
only heard of it happening with motion picture film.

Finally there is acetate deterioration.  The physical deformation caused
by vinegar syndrome (as the motion picture people call it) is not really
a curl, but more a distortion.  The film will ultimately shrink 10 to
15%. In sheet film this starts out appearing as a ripple around the
edges and ends in the gelatin layer finally lifting from the film base.
If you smell vinegar then this is a likely contributor to your problem
although since it doesn't have a preferential direction like humidity
induced curl and plastic flow, it is unlikely to be your primary cause.
Please note that although the nose is the most sensitive detector that
we have (better than any simple chemical indicator) I don't recommend
sniffing boxes of film. It does cause eye, nose, throat and lung
irritation.  As Eric said, if you have this problem then store the fiche
under cool or cold conditions or deaccession them (toss them).  In
addition, buy the IPI Storage Guide for Acetate Film.  (Okay that was a
shameless plug, I admit it.)

Good luck Anita.  Yet again I got carried away here.

-Doug

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                  Conservation DistList Instance 7:25
                 Distributed: Friday, September 3, 1993
                        Message Id: cdl-7-25-002
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