I am a sculptor experimenting with cyanotypes. I am trying to work on drywall board (with the paper coating removed) and plaster (that I am pouring). I have had sporadic success, but am having a problem with yellowing after I wash. If I put the developed image in a bleach solution, the yellow goes away, but as it dries, it returns. I have been using gelatin hardened with glyoxal, and have also tried glutaraldehyde, formalin, and chrome alum, and while all produce acceptable images, the yellowing appears. I have also tried spray starch, shellac, and gel medium to seal the plaster, but have not gotten satisfactory results.
I am using the traditional formula with distilled water.
I am trying to determine the source of the yellowing and how to either get rid of it or avoid it in the first place.
Any suggestions would be greatly appreciated.