I tried acidification with sulfamic acid on Zerkall book paper and did some Pd prints. This paper is only slightly buffered (close to neutral pH). Treatment with citric acid improves especially the mid tone but with sulfamic acid (0.5 M) pretreatment I got grainy highlights and mids (slightly better than untreated paper) but good dMax. Also the paper surface got softer so I was wondering if sulfamic acid is not only removing calcium carbonate but also interacting with the paper sizing.