Pfeiffer, Dan R (Dan) on mon 16 may 05
We mixed up my latest attempt to come up with a base glaze that would fix
the clay we are using. Using the limits in glazechem it looks to be just
right for the fix but I am not sure it will not run off the pot. :( the
viscosity index went from 30 to 40 and the surface tension when down a
little but without firing I can't tell it this is a big change or a small
one.
One test we did was to look at the specific gravity after we mixed it and
subtracted the weight of the water from the mix and did not get close to
what we just mixed in. ????? The thought being for a equal volume of glaze
minis the water we could get back the the dry weight. I could see why it
would be off a bit as the powders all have some water in them but I do not
understand why it is a lot off. The reverse causes a problem in that you
can't just add the total dry weight and say add 1.5 x the total and get a
specific gravity of 1.5. There must be some thing about specific gravity
that I don't know, maybe have it all wrong. Any one care to explain this for
me. My guess is there is some part of the solid that up to the weight of
water for it's volume does not count. The specific gravity of Quarts and
feldspar is about 2.75 so maybe this need to be the starting point and think
of the water as lowering it to 1.5???
Rather muddled thinking but maybe someone can sort it out. It would be a
great help to be able to backup when you have a big batch of gaze and latter
need to "fix" it.
Dan & Laurel in Elkmont Al
Potters Council Members
Dave Finkelnburg on mon 16 may 05
Dan,
You're on the right track thinking about how the density of the quartz
and other materials influences the density of the glaze slurry. Since water
has a density of 1 gram per cubic centimeter, and all the other ingredients
average about 2.65 g/cc (unless you use a LOT of iron or lead or some other
especially heavy ingredient), the mix is always somewhere in between. If
you weigh a known volume of glaze slurry you can always calculate the grams
of solids/cc.
There's more in the Archives about this, under Brongniart (or
Brogniart, the way I misspelled it at one time...drat!)
BUT, glaze slurry density only affects how the glaze runs when it melts
by how thick it causes the applied glaze to be. Glazes are more runny with
fluxes that melt at lower temperatures (sodium, boron, lithium, potassium),
and with less alumina and silica relative to the amount of flux.
The safest approach is to make a shallow dish of unglazed clay and fire
a vertical tile of your test glaze in the dish. If the glaze runs you won't
have any on a shelf, and you can check the glaze fit on the tile at the same
time.
Good glaze testing!
Dave Finkelnburg
----- Original Message -----
From: "Pfeiffer, Dan R (Dan)"
Sent: Monday, May 16, 2005 1:16 PM
> We mixed up my latest attempt to come up with a base glaze that would fix
> the clay we are using. Using the limits in glazechem it looks to be just
> right for the fix but I am not sure it will not run off the pot. :( the
> viscosity index went from 30 to 40 and the surface tension when down a
> little but without firing I can't tell it this is a big change or a small
> one.
>
> One test we did was to look at the specific gravity after we mixed it and
> subtracted the weight of the water from the mix and did not get close to
> what we just mixed in. ????? The thought being for a equal volume of glaze
> minis the water we could get back the the dry weight. I could see why it
> would be off a bit as the powders all have some water in them but I do not
> understand why it is a lot off. The reverse causes a problem in that you
> can't just add the total dry weight and say add 1.5 x the total and get a
> specific gravity of 1.5. There must be some thing about specific gravity
> that I don't know, maybe have it all wrong. Any one care to explain this
for
> me. My guess is there is some part of the solid that up to the weight of
> water for it's volume does not count. The specific gravity of Quarts and
> feldspar is about 2.75 so maybe this need to be the starting point and
think
> of the water as lowering it to 1.5???
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