Nitrate film storage

Richard:

You may not have any problem at all with changing temperature in the
freezer. Extent of problems will depend on several factors:

    1) How is the film packed?  Single sheets hanging or on a rack will
    reach thermal equilibrium faster than film on rolls.  High packing
    density resists thermal changes more than low packing density.

    2)  Change in temperature.  Does the temperature change 20 degrees
    or 200 degrees?  (I don't know how high they go.)

    3)  Length of time at higher temperature.  The longer it sits at a
    high temperature the closer the film temperature gets to the high
    temperature.

    4) Thermal conductivity and thermal diffusivity of the film.  You
    have no real control over this.

The mechanics are that film changes from the outside in (just like
meat). We therefore have to use approximations.

The behavior is somewhat first order and therefore we can roughly
manipulate it to figure out how close to equilibration we are.  The math
gets a little messy so I recommend just using Kodak's numbers.

Kodak uses the formula:
              V
        t = C*-
              A

where t = time in hours to reach a given equilibrium level
      C = coefficient, in hours per meter or hours per foot, applicable to
          film
      V = volume of the film roll in cubic meters or cubic feet
      A = surface area of the film.

Percent of Temperature           Coefficient "C"
   Equilibrium                hours/meter        hours/ft

     50 %                       100                30
     60 %                       130                40
     70 %                       180                55
     80 %                       230                70
     90 %                       330               100

V/A gives us an idea of the packing density.

Suppose that we had film with a V/A ratio of 1, using the formula, it
would take about 100 hours to reach 50% equilibration.  We started at
say 0 F and the freezer is now at 40 F.  The change is 40 degrees.  Thus
in 100 hours under these conditions, the film will reach 20 degrees F.
Note that if we go another 100 hours, (C = 200), the % equilibration is
about 75%.  Add another 100 hours (C=300) and % equilibration is almost
90%.  This is why this change is first order.  It a unit of time to get
half way there.  In another equal unit of time we get another half the
distance there (1/2 + 1/2 of 1/2 = 3/4).  In another equal unit of time
we get another half of the distance (3/4 + 1/2 of 1/4 = 7/8).  And so
on. Thus with our example film, in 200 hours the film will be at 30
degrees. In 300 hours, the film will be at 35 degrees.  In 400 hours,
the film will be at 37.5 degrees and so on.

To give you some real life examples (from Kodak tests):

    a)a 6.6 inch X 14,700 ft roll on a 6 inch diameter metal core will
    reach full equilibration (in this test going from 70F to 120F) in
    about 17 hours.

    b)a 9 1/2 inch X 375 foot roll in a film magazine on a camera
    (simulated) reached equilibration (70F to 199F) in about 25 hours.

Note that % equilibration doesn't care how wide the temperature change
is. The wider the temperature difference between the film and outside
is, the faster the film is driven to change.  Therefore, if I wanted
example a) to go from 0F to 70F, it would still take 17 hours.  However,
if I put the film into a 140F room, it would reach 70F in about 4.5
hours.

I estimated that in 2 hours, a 100 foot roll of 35mm film with a 1inch
core would reach 77.55% equilibration.

Notice that if you stack rolls of film or pack sheet film into boxes,
you are effectively increasing the volume to surface ratio and therefore
are increasing the thermal inertia (the resistance to change in
temperature.)

The bottom line is that your freezer *may* defrost without your film
noticing the temperature change very much.

One note of caution though: in our lab fridge (frost-free) we had a
problem (actually we stacked too many boxes on it when we were confined
to three ex-darkrooms for the whole lab) and the compressor died.  The
heating cycle however, still kicked in.  We came in one morning and
found that the freezer was at 120 F (ack!!).  Warm alarms are no good if
no one is around to hear them. The best thing would be for the system to
shut-down if it got too high.

BTW we have all of our film samples (acetate and nitrate) in frost-free
freezers.  We did a rough profile with an electronic sensor of T & RH
(just spot checking every so often) and the temperature actually seemed
to hold pretty closely to -17 or -18 C (around 0 F) while RH changed a
fair bit.

We have in a project for funding which includes some better T an RH
equilibration studies.  Kodak's work was directed towards aerial film
for use in space (and therefore huge and weird sizes and formats.) Also
the influence of enclosures (particularly for RH equilibration)
including boxes and cabinets were not included in the Kodak study.
Typical cabinets and should have quite and influence on T equilibration
since they increase the thermal inertia.

PS the data and information above is taken (more or less) from Kodak
Publication M-62  "Properties of Kodak Materials for Aerial Photographic
Systems: Physical and Chemical Behavior of Kodak Aerial Films"--one of
three publications written for NASA with respect to shooting film into
space.

-Doug

                                  ***
                  Conservation DistList Instance 7:18
                Distributed: Wednesday, August 11, 1993
                        Message Id: cdl-7-18-003
                                  ***