As a photo educator, I want to make sure I have the best information to pass along to students when it comes to troubleshooting processes. A friend of mine and one of the best platinum printers out there today wrote this excellent pair of articles on a common problem – the dreaded “black spots”.

https://ianleake.com/blog/black-spots/

And a viable solution to the problem, at least part of the time:

https://ianleake.com/blog/why-oxalic-acid-combats-black-spots/

Ian does an excellent job of explaining the chemistry behind the issue in a simple, straightforward manner that even a non-chemist like me can understand.