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So I am sitting here with a couple bottles of ink, and was thinking to myself, "Old Government recipes added hydrochloric acid to their inks to prevent sediment, but I don't smell a trace of HCl on any of these inks." HCl is hydrogen chloride gas dissolved (and fully dissociated) in water, so when the ink is used, the gas will come out of solution when the ink evaporates and leave, not affecting the Ph of the paper. As most paper is sized with CaCo3 anyway, it might react and neutralize in any situation. In any case, the amount is low. The recipes call for 2 ml pre litre. As a concentration is not given, I am going to assume that it is concentrated HCl, to play on the worst case scenario. Two ml of conc HCl is .0275 moles of HCl, which in a litre of water will provide a ph of 1.6, which not bad. The worst case one is the indian recipe, which calls for 5 ml of conc HCl. This gave a Ph of 1.15 or so. Nasty, around the same as stomach acid. My guess is that this will be the final ph of the resulting ink as well, as the tannic and gallic acids should not be full dissociated, and the increase of H+ from the HCl will shift them to be more dissolved then dissociated. This Ph seems a bit low, and might be harsh on fountain pens. Adding less or weaker acid might help with this. The iron in solution should help protect stainless steel nibs, and the HCl should not attack stainless or iridium anyway. Plastics are hit an miss, but should be fine. Off gassing might cause corrosion in if they have unprotected steel in the cap or whatever. Now, the main point of all this is, do you think it is worth the risk adding acid to modern IG inks to reduce sediment formation? To get the effect needed, a single drop of conc HCl is all that is needed per 100 ml. (A drop on average is between .1 and .2 ml) Any thoughts? I have liters of conc HCl around, but I dont feel like frying a pen if someone knows something I dont. I am pretty sure the indian version of the ink will give small bubbles if you put it on a carbonate. Thats strong stuff. Might not even need a biocide, as not many bacteria can live in a Ph of 1, but they exist, and the PH would select for them so I guess you probably would. If sediment has formed, then adding a drop or two of HCl might save an otherwise dying ink.