a) Ammonium nitrate is a restricted substance at least in the EU see Annex I in https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32019R1148&;from=EN and even if you are a professional user (in German "Gewerblich") it can be difficulty to obtain a small amount.

b) Other suggestions:

1. Adding gelatin and glutaraldehyde to your cyanotype chemistry see https://illumina-chemie.de/viewtopic.php?p=72786#p72786 (in English) 6% are to much (emulsion is to viscous) so I cut it to 4% see https://illumina-chemie.de/viewtopic.php?p=72786#p72786 (in German only). Mixtures are stable for more than 1.5 years @ RT (room temperature) see https://illumina-chemie.de/viewtopic.php?p=78036#p78036
Modification would be to take only the gelatin and reduce the amount to 1 to 2% @ the potassium ferricyanide solution. Should gave you a additional layer with will stain your spots.....

2. Not tested: To replace the gelatin from recipe above with polyvinyl alcohol or polyvinyl acetate, which will make the same...adding a layer where the formed pigment will be withhold, covering the areas of the spots after development.

Udo

-----altphotolist@groups.io schrieb: -----
An: altphotolist@groups.io
Von: "Chris Seiberling,"
Gesendet von: altphotolist@groups.io
Datum: 17.02.2021 18:25
Betreff: Re: [altphotolist] Persistent cyanotype spots

Mike W: I’d be happy to run the test you suggest. Lacking institutional affiliation, I’m struggling to find a supplier for ammonium nitrate— there appears to be a reluctance based on other infamous uses (If anyone has a suggestion about sourcing ammonium nitrate, please let me know).