The brown stain is caused by lack of neutralization of the paste in the trap of the 8x10 film. In our 600, SX70 and Spectra films there is a strip of woven trap material at the top of the film. This woven material contains some acid. The paste in the pod contains alkali. The acid in the trap material is there to neutralize the alkali from the paste that ends up at the top of the frame.
The 8x10 film does not contain any neutralizing agent in the trap. Originally in the Polaroid times the 8x10 film was a peel apart film and any paste that was caught in the trap area was peeled off with the negative and discarded so no neutralization was needed. We do not have the specific positive and paste to create a peel-apart system and have chosen to make a integral 8x10 film instead. Now the positive and negative stay together and the alkali stays in close proximity to the negative and sheet.
Due to the lack of neutralization and because of the close proximity some of the alkali from the trap area will slowly migrate to the picture area. There is can react with some of the chemistry and create a brown stain.
Currently we do not have a elegant solution for this problem. There are 2 options:
•Cut away the trap area of the frame and discard it
•Squeeze out the paste from the trap area. This is a messy and delicate task because the paste contains alkali.