I should have been more clear why I'm asking! I've made prints using sea water and sea salt too. I haven't proved this yet, but I think the sea salt I'm using gets ever so slightly dark ( not obvious fogging, but subtle and difficult see if you aren't looking carefully ) right when the silver nitrate is brushed on. It's possible this is due to the sulfates or bromines in it, so I'm wondering if there might be an easy away to remove the sulfates.
In fact, I've just finished reading Mark Kurlansky's book,
Salt- a World History, and I've read a lot about salt production. I'm definitely interested in making my own salt for printing. It's pretty neat: the calcium chloride in sea water makes salt "sour" and the magnesium makes it "bitter" ( the word "bitter" comes from "bittern" which is the dregs from a salt evaporation pond! ). If you start boiling or evaporating sea water, the calcium chloride precipitates first, so it can be removed by letting it settle and then decanting. Then the sodium chloride starts to crystallize, and the
fleur de sel forms a thin layer of crystals on the surface -- this is scraped off by hand and sold for a lot of money! If you keep going, finally combined magnesium-potassium salts start crystallizing and the trick for good tasting sea salt is to harvest it at the right time, before it is too bitter but when it has some sea salt "taste".
By the way, Mark Kurlansky also wrote a book called
Paper, Paging Through History, all about the history of paper making.. I just finished reading that one too! You can guess where this is heading