Re: clearing dichromate stain
On Jan 5, 2007, at 8:47 PM, Christina Z. Anderson wrote: I wasn't assuming you were endorsing alum; I was simply asking "infinitely better than what?" in response to your statement "I use potassium metabisulfite, finding it infinitely better." Better than sodium bisulfite? Better than alum? That was all I was asking. All the information I've seen from chemists here, as well as my own observations, give me to believe that the bisulfites and metabisulfites are interchangeable. And as you say, alum isn't seriously considered as a clearing agent now, so it wouldn't be terribly useful to say that potassium metabisulfite is infinitely better than alum. So, infinitely better than what? That's all I was asking. It was to show the substances in use throughout gum's history. In fact, if you read the list carefully there were contradicting statements on there from the various sources.Yes, I'm quite aware that the historical compilation is full of contradictions,, factually inaccurate statements, and outdated recommendations, and as a result I'm not sure I see its utility for present day workers. This is also true of sodium bisulfite; they work the same in my experience. I also did Livick's method of spray bottle pot metabi and it was fun to watch. But did you compare it with sodium bisulfite? It also rinses out of the paper easily and well, being more soluble than others. But we hashed that topic out on this list a year or so ago I think and I don't feel it needs to be rehashed again. If you search under Alberto Novo he had some great input on the topic. If I remember correctly, you didn't agree with him or me at that time.I did search under Alberto Novo, and found nothing about the solubility of potassium metabisulfite ; all I found from Alberto relating to the subject of clearing agents was a discussion about whether sulfite can work for clearing, which is a different issue. It was a fascinating discussion, and well worth re-reading. Alberto and I had a lively conversation (you didn't appear in that thread for me to agree or disagree with) that makes me nostalgic for the good days of the list, when we had such open discussions, but when we did our tests and compared them, our results were very similar. What our experiments showed together was that if you treat a stain with sulfite, it will turn the stain from brown to blue-grey just as bisulfite does, but it takes longer. In my test, after an hour in sulfite the stain was the same pale shade of blue-grey as after five minutes in bisulfite, but in Alberto's test the sulfite-treated stain was a slight shade darker blue-grey than the bisulfite-treated stain after an hour. One can assume that the slight difference probably has something to do with the pH of our water. But as I said, that's a different issue. But then I searched on "potassium metabisulfite" without reference to Alberto Novo, and came up with a discussion in which you asserted that potassium metabisulfite is more soluble than sodium bisulfite and therefore requires a shorter wash time, the same assertion you repeat above. When I asked for evidence or a source for that assertion, it was traced to Scopick, with no experimental findings of his own or citation to other findings to back it up; on the basis of such lack of empirical support I did say I would need to be convinced that relative solubility has anything to do with wash times. No scientists or others on the list weighed in on the assertion that metabisulfite will wash out of paper faster than bisulfite, so I asked a physical chemist I was working with about this; he replied, as I reported back to the group at the time, "You are quite right that the solubility of the compounds in water cannot be a factor in washout times for the solutions." And since a metabisulfite becomes a bisulfite in solution, like I said before, I would need to be convinced with some persuasive evidence that there is a difference in washing ease or washing speed. So I find the statement that this topic was "hashed out," implying that the hashing decided the issue in favor of the idea that metabisulfite washes out faster, rather puzzling. As far as I can tell from following that discussion from beginning to end, Alberto Novo didn't participate in that discussion, so I also find the reference to Alberto Novo rather puzzling. I'm sorry; sweeping categorical statements ("potassium metabisulfite is infinitely better") that I can't see a factual basis for raise my antenna, and then when I'm sent for proof of the assertion to a discussion where no one offered support for the assertion, then there's this red flag. Maybe you're talking about a different discussion, or different information from Alberto Novo, that the search engine didn't bring up for me, in which case it would help if you could be more specific. Katharine
|