Re: "Boric acid" and gum

[Ryuji Suzuki <rs@silvergrain.org>]

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: "Boric acid" and gum
Date: Fri, 31 Aug 2007 19:38:19 -0700

> 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.

To see if borax would do, what you needed to do is to make a
boric acid solution, titrate with NaOH to the end point pH of
about 9.2 at 25C, and use this as if it were borax
solution. (because it is identical to borax solution.) In
reality, the active agent in the crosslinking mechanism above
is borate, not boric acid, and the pH of 9 or higher would be
most effective. (At pH of 9.18 at 25C, 50% is in borate form
and 50% is in boric acid form.)

Crosslinking of PVA or other polyhydroxy polymers with borate
is via condensation reaction, which is quite different from
how common gelatin hardeners work. As it is condensation
reaction, the crosslinking is easily reversible by changing
the pH or adding more water.

> 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.

In photography, hardener is defined as an agent that
suppresses swelling of the polymer binder when immersed in
water or other processing solution. Even if you can make a
thin film layer of gum-borate coating, it will swell (and
dissolve away, as you noted) once immersed in water (as I
described above) and it won't qualify as a hardener, even
though borate crosslinks the polymer.

--
Ryuji Suzuki
http://silvergrain.org