Re: "Boric acid" and gum
"I had the joy of being in the bromoil class with a chemist physician who practiced whatever that field of medicine is where they determine risk and safety stuff at large corps. Anyway, he was a wealth of photo lore. One thing he told me was that the slime goop that kids play with is none other than gum arabic (or even PVA in some cases) mixed with boric acid. Boric acid makes gum go instantly hard (gum viagra?). Jim suggested using this as a sizing. Coat the paper with gum and then harden it in a 1-2% boric acid bath. As a person offlist reminded me, though, when I added acids to my gum mix I got more and more staining/non-exposure hardening in my gum layer as per the example on my website in the learning section on gum, so it may be an iffy proposition. I wonder if the residual boric acid would then affect subsequent gum layers, making it unsuitable for gum, but certainly this was a less toxic form of sizing to dink around with.
I understand it's called Gack if you make it with Polyvinyl acetate, SLIME if you make it with polyvinyl alcohol. (I read that today when I was googling the instructions for making it.)
Since this is a housecleaning day and I hate housecleaning, I mixed up some boric acid and tried to make SLIME by adding it to gum; I used up about a pint of gum with various trials and I couldn't make it work at all. I just happened to have some boric acid on hand, but no borax, so I couldn't see for myself whether borax works any better for turning gum into slime in the same way it turns PVA into slime.
Since the directions for SLIME call for a saturated solution of borax, I used a saturated solution of boric acid to try to make it a comparable test. The directions say to pour the borax into the material and start stirring like crazy, because it thickens up immediately. I did that; nothing particular happened (the gum stayed liquid.) The directions said that the more borax solution you use, the thicker and more viscous the gel you'll get, and the less of the borax solution you use, the thinner, more slimey and icky, the gel. I tried adding a lot, a medium amount, a little bit, and a tiny little bit. None of them had any significant effect except the tiny little bit seemed to make the gum slightly more viscous than usual, just a little thicker gum, but hardly a gel. I thought well, maybe it's still crosslinked enough to serve as a size, even though it didn't turn into a gel. So I spread a thin layer of that on a piece of glass and let it dry. After it was thoroughly dry, I dropped a couple drops of water on it, and the gum dissolved instantly.
In other words, I can't find any support for the idea that boric acid will serve as a hardener for a gum size.
It's called SLIME; it's a viscoelastic gel that boys love to gross girls out with. It's made by mixing PVA with borax (sodium borate) which is alkaline, not with boric acid. It's an experiment that's used to teach an easy lesson about polymerization, and results in a product that kids love, so it's a great science lesson for kids. Generally gum isn't used for this experiment because the results aren't as predictable as with PVA.