This thread got me to thinking...I have a bottle of oxalic acid. So, I went down to my darkroom, made up a solution of 10% oxalic acid and brush it onto the paper that I use for Kallitype printing (Rising Stonehenge, which is acid-free).
Let it dry, coated it with sensitizer and exposed it. Once it hit the developer (sodium citrate), it stained like hell, with weird orange colour lines running through it. Yuck.
I mixed up a 5% solution and soaked a sheet of paper in it and this time it worked not too badly but I could see very subtle uneven densities...maybe I didn't soak the sheet long enough in the 5% solution. So another sheet in the solution and I gave it an hour soak. This time it worked really well. Blacker blacks and nice cool tone.
I then tried putting drops of the 5% solution in with the sensitizer. Ten drops (mixed in with 22 drops each of the ferric oxalate and silver nitrate solutions for an 8x10). It worked, but seems slightly grainy compared to the print that received the oxalic acid bath for an hour.