I had problems achieving smooth mid tones and highlights with a lot of papers I tried recently (Buxton excluded). Gold(III) chloride (HAuCl4) is used to change the color and contrast in Ziatype printing. Dick Arentz mentions in his book that gold chloride can be added to the sensitizer (development process) and "this may also reduce granularity".
I added 5 to 10% of 5% HAuCl4 of the Pd Volume to the sensitizer. This resulted in very smooth highlights and mid tones. There was a small color shift to cooler colors. Using NH4Pd I got very neutral tones and with LiPd a slightly less warm tone. I have to do more tests with different developers, contrast control and papers (e.g. still problems with Revere) but I would prefer to add some gold to the sensitizer than extensive pretreatment of the paper to get good full scale.
Irving Penn was also using Iridium together with Pt/Pd and it may have a similar effect as gold as toner. I found no references for what compound he used. Ir(III) chloride? I found no solubility data through my web search. Has anyone used any Iridium salts? What color shift one should expect.