Check this out
http://www.cicada.com/pub/photo/zs/

 Since getting repeatable results with any Polaroid product is
difficult if not impossible...

There is very little contrast control available with Polaroid p/n films.
Processing the film for 1 minute yields results nearly identical to
processing it for 30 seconds.

There was an article in View Camera magazine a year or so ago about
adjusting contrast with Polaroid films.  As I remember pre-exposure was used
to reduce contrast and selenium toning was used to increase contrast.  The
results were supposedly very close to N-1 and N+1.

I use Polaroid Type 55 occasionally in 4x5 but try to confine it to scenes
that are nearly normal in contrast.  I find it to be an excellent film in
those situations but the base is rather thin, so it has a tendency to curl.
It can be scratched more easily than traditional film but careful handling
can eliminate that issue.

Sherman
http://www.dunnamphoto.com