Re: reticulation

[Wayde Allen <allen@xxxxxxxxxxxxxxxx>]

On Wed, 21 May 1997, John Fleshin wrote:

> >>>>>>>>>>>>>>It is my understanding that the cracking (reticulation) of the
> gelatin layer is due to mechanical stress caused by uneven swelling (this is the
> mechanism used to make collotypes) or thermal shock. The stress causes the
> gelatin film to crack, but there are NO cells to rupture.  Take a look at
> the bottom of a dried up mud puddle and you should get the idea.>>>
> 
> Sounds good, but what is the reason for the cracks following lines of
> density change?  If it occurs after fixing,  I guess the undeveloped silver
> is gone, but how does this affect the gelatin?  I imagine the reticulation
> is different depending upon when it occured?

The development often causes some degree of hardening or tanning of the
gelatin depending on exposure.  This changes the extent to which the
gelatin will swell or stretch.  The greatest degree of hardening should
occur in the regions where the most silver is reduced (highest density). 
Since some developers tan the gelatin more or less than others, I would
assume that what you use would make a difference here. 

There are by the way, several processes that use this variation in the
swelling of the gelatin to actually form the image.  Namely the bromoil
and collotype techniques. 

Yes, without knowing any better, I would assume that you might notice some
difference in the reticulation pattern depending on if it occured before
or after development, or even after using a hardening fixer.

>  Will it occur in unexposed
> film?  Does anyone know what the manufacturers do to prevent it?

Hmmmm...I don't know.  With sufficient thermal shock I would imagine, but
then the plastic might also crack or the gelatin even separate from the
base.  Gelatin films seem to be pretty tough when dry.  They soften when
wet though, which I would guess makes the gelatin film tear easier.  The
swelling caused by the water should also add considerably to the internal
stress.

As far as putting anything in the emulsion to prevent reticulation goes, I
can only speculate.  I've tried making gelatin solutions and coating them
on overhead projector slides with varying degrees of success.  Getting it
to stick is one problem, getting it to dry evenly without buckling is
another.  In my case, I added a bit of sugar to act as a plasticizer,
although there are many possibilities, glycerin, etc. .  I would imagine
that these additives, whatever they may be, would affect the propensity
for a gelatin layer to reticulate.  The subing layer used to get the
gelatin to stick to the plastic may also have an affect.

> I always heard that the thicker the emulsion the more likely to reticulate,
> and this also might be tied into the fact that Konica IR is a very old
> formula, from the fifties, when it was sold as a regular film.  

Sounds reasonable to me.

> Would be nice to know the thicknesses of various emulsions if this is a
> variable worth persuing.  Seems to be heading toward a reticulation web page?

Perhaps you could get the manufacturers to tell you, and then correlate
these data with reticulation experiments?  Then again someone may already
know the answer.  In any case, I suppose you could get the dry emulsion
thickness by measuring the thickness of a snippet of film, removing the
emulsion in something like an ammonia(?) wash and/or hot water, measuring
the plastic base, and taking the difference.

- - Wayde
  (allen@xxxxxxxxxxxxxxxx)


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End of Infrared-Digest V0 #25
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