Quote Originally Posted by BobNewYork View Post
... Also, over-fixing in sodium thio will start to bleach the image - ammonium doesn't have these problems. ...
A simple experiment is to cut a piece of WT paper in half and fix one in a simple sodium thiosulfate fix, the other in a simple ammonium thiosulfate fix. I know that if you use a lith print, or even better POP, you will find that the ammonium thiosulfate bleaches the image much faster than the plain hypo.

I use very very dilute ammonium thiosulfate as a bleach for clearing highlights in POP and VanDyke prints...