OK well I'm completely confused ( what else is new? ). That one I tried with 10% acetic & 12% AgNO3, after I took it into the sun and it didn't change, I just put it back in my closet. Yesterday I noticed the silver was all reduced -- must have happened when it finished drying. Even darker than the first one. I have no idea what's going on with it. Also, I know that some people have used oxalic acid, acetic acid or tartaric acid instead of citric to restrain spontaneous fogging on salt prints... so what I'm seeing doesn't make any sense from that POV either. Maybe the tartaric acid is reacting with some specific thing in the paper I'm using.