Lili Krakowski on tue 9 may 06
This is a sequel to my Basic Internet Glaze Course, whxih can be found in
the Archives for Weel Three of April 2005. I want to thank the
ClayArt Glaze Guru and the two ClayArters who were generous enough to be =
my
First Readers, and whose suggestions I gratefully used.
ABOUT RECIPES
In conjunction with the Basic Internet Glaze Course we need to look at
recipes: what they are/aren=91t, can/can=92t do.
Like most potters, I have notebooks full of recipes, and many more in boo=
ks
and magazines. Some recipes are for temperatures, or techniques I don't u=
se.
But even when I eliminate all glazes with lead, and limit my list to cone=
s
4-6,electrically fired, I have infinitely more than I ever could need.
Why do I keep them? Well, I do a lot of glaze testing just for my own
enlightenment. And some recipes are desirable references. But I really ne=
ed
far fewer than I have=85As do most potters.
What is all this about?
1: A GLAZE is a mixture that melts silica onto a clay surface to form a
glass. As the molten glass would run off and form a nasty puddle, alumina=
is
added to hold it on the pot, and fluxes--"melters" or melting agents-- ar=
e
added to melt the silica at a pre-selected temperature.
The types and proportions of fluxes to alumina and silica vary with the
chosen temperature --the one at which the clay body (on which the glaze i=
s)
must be fired for best results (vitrification being the principal
consideration).
From this very simple basis, great complications grow!
Long ago, "primitive" pottery was glazed by painting ash, or soda, or lea=
d
(probably mixed with some liquid ) onto a pot, and the fire pulled enough
alumina and silica out of the clay itself to achieve some sort of glaze. =
Not
a good one, but something.
Today we have many materials--some mined, some commercially blended from
mined products-- available to us, anywhere in the world , and from
everywhere in the world. They allow us to create remarkably sophisticated
glazes designed to achieve a certain look, color, or texture.
(Beyond that, we have tremendous control over the way we fire,
allowing us to modify the final appearance of the glaze in the melt. This
relates to recipes only indirectly, and will not be discussed here)
Alumina and silica are reliable, steadfast, unimaginative. With a few
exceptions, they affect glaze only in their constant, consistent
roles. We should be more grateful to them. They are the behind-the-scenes
worker ants. They are the wonderful butler and cook who make the grand ho=
use
party possible. Without them, nothing.
It is the fluxes, of which there are many, which give a glaze its
distinctive look. It is the interplay of the fluxes that achieves
color, texture, "food-safety" and the like.
Fluxes come as one-active-ingredient-only materials ( such as Calcium
Carbonate, Zinc Oxide, Lithium Oxide) OR as blended ones.
Some "blends" are found in nature. Examples: Dolomite ( magnesium plus
calcium), talc (magnesium and silica), and several others. Note that talc=
is
a flux plus silica. A blended material may include not only the sought-af=
ter
flux, but other fluxes, as well as silica and alumina.
FELDSPARS are mined. They are complex materials, introduced into a
glaze to supply sodium, potash, lithium. Cornwall Stone, Nepheline Syenit=
e,
Plastic Vitrox , used just like feldspars by potters, are not considered =
so
by geologists.
FRITS--which are glazes fused in crucibles, then ground into fine
powder--are "synthetic feldspars", permitting the introduction of fluxes,=
as
well as boron. Some fluxes are found only in water-soluble form. Colemani=
te
now is gone from the US market, and Gerstley Borate is getting very scarc=
e.
Neither was a great treat, because their composition was so variable. The
other sources of boron are water soluble--so frits are our best, most
consistent, almost-insoluble source of a most useful ingredient. ( Many
frits, by the way, will work as rather inadequate glazes just by
themselves.)
Fluxes are so interesting because while calcium, for instance, has little
effect on glaze texture or color, soda, potash, lithium, magnesium, zinc,
barium, strontium can and do.
THE FLUXES DICTATE THE LOOK OF A GLAZE. If a glaze is to have a shiny,
satiny, or rough finish,--that depends heavily on the flux used. That is
best seen on untinted glazes. Colorants added to the glaze proper reveal =
the
influence of the fluxes, often very strong. Cobalt will be blue with
lithium, potash, soda, and zinc; purplish with magnesium. Copper gives
turquoises with barium, soda, potash, lithium and strontium, but grassy o=
r
bottle-greens with the other fluxes. Chrome, which gives a rather dull
green, turns brown in the presence of zinc, and gives pink/reds when
combined with tin oxide in high calcium glazes. In other words: the fluxe=
s
affect the finish of glazes and the color of tinted ones.
It should be added that titanium, an opacifier, and part of rutile, (whic=
h
is used as a colorant) affects both finish and color. Small
amounts, as found in clay --both body and slips-- also can have a marked
effect.
Having said all that--what about recipes?
For that we see PART TWO
Lili Krakowski
Kim Overall on wed 10 may 06
Lily,
I haven't gotten into glaze chemistry very deeply because it seems so
very complicated. Your descriptions however makes it easier to
understand. You're a great writer, too. Clayart's Grand Dame who can
teach us all a thing or two, or three or more.
Perhaps you should start us on a weekly,
or monthly (if we have to wait that long),
literary lesson? :>
Kim in Houston
Kim Overall on wed 10 may 06
Harley:
HOWDY! Yes, I was her mug exchange partner!
Please tell Janet CONGRATULATIONS AS NZP PRESIDENT for me.
I love my salt glazed mug and the beautiful pictures of her
studio/home. It's exactly what I would love to have here in Texas.
Quite a spread.
Off Topic: My husband's father was Chef on the Rangitiki on
all but her maiden voyage until it was decommissioned.
He's set up a website in tribute, http://www.rms-rangitiki.com/
Briefly, we are planning a trip to New Zealand probably next year.
If convenient geographically, I would love to come see the
NZPG potters there. It's a long way off though so as itineraries
become realized, I'll definitely be googling.
Thank you Harley and all the best to you both and
to all New Zealand potters!!!(Australians, too as my step-daughter
and her family live near Melbourne).
Kim in Houston
Gay Judson on wed 10 may 06
Thanks, Kim for the alert. I did not receive Part 1 or 2 or 3. Your
note sent me to the archives and there they were--I'm printing them
out now. =20
I am getting a lot of Clayart posts now--but can tell from other posts
that I am missing a lot...
Gay Judson San Antonio, TX
> -----Original Message-----
> From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Kim
> Overall
> Sent: Wednesday, May 10, 2006 2:05 PM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: About Recipes: Part I (of three)
Janet Smith on thu 11 may 06
Hi Kim .... Just noticed your name on this email and it just struck me that
you were the last person Janet did the Mug exchange with a couple of years
ago . Thought you might be interested to learn she has gone on to be the
current President of the New Zealand Potters Society this last easter ....
Harley Smith
On 5/11/06 7:05 AM, "Kim Overall" wrote:
> Lily,
>
> I haven't gotten into glaze chemistry very deeply because it seems so
> very complicated. Your descriptions however makes it easier to
> understand. You're a great writer, too. Clayart's Grand Dame who can
> teach us all a thing or two, or three or more.
>
> Perhaps you should start us on a weekly,
> or monthly (if we have to wait that long),
> literary lesson? :>
>
> Kim in Houston
>
> ______________________________________________________________________________
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