I use 5% strength (v/v) of 35% HCl (makes something like ~0.57M concentration if I'm not mistaking) for 4 minutes (agitating constantly and changing side facing the solution halfway), this is perfectly enough for printing good cyanotypes. (Which is a process that is very vulnerable to presence of alkali.) The bubbling and fizzing stops completely at about 3:30 or so. If you double the strenght (10% v/v) then 2 minutes processing is enough (tested), I just use a milder concentration to be more gentle to the paper, even if I haven't noticed any (short-term) adverse effect of using the stronger variant. 2 litres of the above solutions are enough to process an area equivalent to 5 full sheets of 22x30" sized paper (maybe more, but I haven't processed larger batches), it doesn't seem to slow down and loose strenght when used up to the previously stated capacity.
You can try to mix an acetic acid solution with a concentration something like ~0.6M to 1M and try several processing times (1, 2, 4 minutes and such). A good indication of enough processing would be the extinction of bubbling and fizzing...
5% HCl (v/v, 35% stock) doesn't smell offensive and is pretty safe to handle. (Half the strenght of the acid in our stomachs!)
Originally Posted by gattu marrudu
Last edited by Loris Medici; 05-27-2009 at 09:39 AM. Click to view previous post history.