Janet H Walker on tue 6 apr 99
It is interesting that a number of people have been posting about
having problems with their purchased dry-mixed glazes. It reminds
me of a possibly relevant interchange during the NCECA panel with
Dick Eppler and Jonathan Kaplan, on glaze technology. (It was a real
winner session for us technophiles by the way.)
Anyhow, during the question period, some one from the audience got
up and wondered aloud about how consistent dry-mixed glazes actually
were, both from bag to bag AND from the top of the bag to the bottom
of the bag.
He knew from his own experience that the pharmaceutical industry
does not dry mix powders unless all of the component powders in the
mix have the same particle size distribution. (You know the simple
real life side of this: "contents may have settled during shipping".
Just vibrating a box of cereal will give you a gradient of piece
sizes with the biggest ones on top and the teeny crumbs at the bottom.)
Now, we know already that particle size is a very big contributor to
things like the melt temperature of a batch of glaze. (Dick Eppler
stated in his presentation that the optimal particle size for
ingredients was "325 mesh, largely free of submicron size
particles." I think Hamer has something on this also; changing the
mesh size of either silica or neph sy can change the maturing cone
of the glaze.)
Glazes are mixed from a variety of ingredients that have quite
different particle sizes and distributions of same. We don't know
exactly what the glaze mfgr does. Probably they ball mill the glaze
batch and then put it into 50 pound bags. What are the chances that
the contents of the bags are actually the same?
If the person who was talking about Stokes law applying to the
particles as they tumble around in the ball mill was correct, some
of those bags will have more small particles and other bags will
have more large particles, just because of the physics of mixing.
But since we start off with various materials which have quite
different particle size distributions, logic says that the bags will
actually contain somewhat different glazes. (With me so far? For
example, the bags with lots of small particles will have less of the
ingredient that is mostly large particles, and so on.)
Then ship the bag out to the customer by UPS or common carrier
freight. When they get it, the powder will have further sorted
itself out, with more of the bigger particles on top and the smaller
on the botton. This implies that if one mixes quite small amounts
from the bag rather than the whole bag at once, that each batch will
actually be minorly different from the others as one proceeds down
through the gradient.
These are a lot of things to think about. The folks in the session
were inclined to discount the guy's worry that dry-mixed commercial
glazes were a bad bet if you need something consistent. But I don't
know. The mixing argument sounds pretty interesting as a kind of
"mystery factor" in why a number of people could be reporting
trouble with a commercial glaze. What if the particle size of one
raw material had changed but the manufacturing process had stayed
the same? It seems quite plausible that the resulting glaze would
not be quite the same as six months ago or whatever.
Anyone know more about any of these issues. Especially, can anyone
address the assumptions? You physicists and minerologists? Gavin?
Michael? Anyone else?
Waiting eagerly for the technical dirt to fly...
Jan Walker
Cambridge MA USA
Gavin Stairs on wed 7 apr 99
At 07:51 AM 4/6/99 EDT, Janet Walker wrote:
...
>Anyhow, during the question period, some one from the audience got
>up and wondered aloud about how consistent dry-mixed glazes actually
>were, both from bag to bag AND from the top of the bag to the bottom
>of the bag.
...
>Anyone know more about any of these issues. Especially, can anyone
>address the assumptions? You physicists and minerologists? Gavin?
>Michael? Anyone else?
Hi Janet,
The basic sorting physics is sound, but I can think of at least one way to
make the commercial glaze uniform: frit it. I don't know how many
commercial glazes are fritted.
Variegated glazes are a different matter. There you need to control the
particle size, and you don't want complete mixing. You could possibly frit
two or more batches, and then mix them. With large particle sizes, you
would probably need a binder/carrier, like Veegum or bentonite. That would
introduce another particle size/density disparity.
The variables in self sorting are size and density, so a mix of more or
less uniform size of different materials will also sort itself to some
degree. Size is the most powerful variable, though.
For a user of commercial glazes, I would guess that it might be prudent to
mix them up a bit before using, by tumbling the container, or inverting and
shaking for a while. You can also dip a sample from several layers in the
container using a long scoop. Another strategy is to use a whole container
at once. This is often specified for dry mix products. If the
manufacturer is doing proper quality assurance and control, the individual
containers will contain proper mixtures, but, as you note, this may no hold
for a sample taken from the top or bottom of the container.
I'm sorry, that's about all I can contribute on this subject.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
Michael Banks on thu 8 apr 99
I can't speak for the standard of products sold in North America, but in
Australasia two types of dry powdered commercial glazes are marketed. The
biggest volume marketed here by far, are fritted powdered glazes and are
sloshed over pots en masse by the majolica brigade. The products are very
consistent and seem not to be prone to settling or separation, being
basically frits with 1-3% added suspension agents.
The second type of product used here by the pottery fraternity are usually
stoneware glazes, including special effect glazes. I have some experience in
formulating and overviewing the manufacture of some of these. They consist
of heterogeneous mixes of minerals and could be expected to be prone to the
types of problem raised in Jans post.
My experience here is that careful quality control over the raw materials
and mixing is essential and problems encountered by customers are apparently
rare. The glaze components are not ball-milled, but simply mixed in a
purpose-designed dry powder mixer from -75 micron ingredients. In practice,
the different particle size and density minerals are well homogenised by the
powder mixer. To avoid the problems of settling and gravity separation in
transit (Stokes Law at work), customers are urged to mix up the entire bag
in one batch. The dry glaze is sold in a variety of bag sizes to
accommodate this.
It is likely that mixing of dissimilar dry ingredients in the pharmaceutical
industry would be much more problematical for a variety of reasons. But if
numbers of potters are having problems in your neck of the woods, either
their prozac, or their glaze is being badly mixed.
Michael
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
Jan Walker wrote:
(Snip)...Anyhow, during the question period, some one from the audience got
up and wondered aloud about how consistent dry-mixed glazes actually
were, both from bag to bag AND from the top of the bag to the bottom
of the bag.
He knew from his own experience that the pharmaceutical industry
does not dry mix powders unless all of the component powders in the
mix have the same particle size distribution.....
Waiting eagerly for the technical dirt to fly...
Jan Walker
Cambridge MA USA
Lee Marshall on mon 19 apr 99
Jeff,
i was the guy talking about Stokes law and the tendency of particles to
seperate according to particle size when mixing . Any university with a
Pharmacuetical manufacturing dept. or a powder technology guru will be very
familiar with this phenomenon.
Bill Marshall
lmarsh1220@aol.com
I disavow all knowledge of this person he has taken over my computer
Lee Marshall
| |
|
