Paul Taylor on wed 29 mar 00
Dear Merrilee
This letter is long and contains advice that is ninety percent inferior
to that given by someone with a full physical and chemical Knowledge of your
clay and glaze material choices. I am posting it so you can get an Idea of
the problems that glaze chemistry can overcome. Some of what I advise here
for that reason is speculative. That is the nature of working on glazes at
this level a lot more testing and heart acke is involved. However the glazes
we use do not have to have the same specifications as an industrial glaze
and some materials are not readily formulated . BUT for a clear glaze you
want qualities that industry also wants .
Your problem is not a simple one because you are using earthenware
technology at stone ware temperatures or the other way round depending on
your point of view.
Barium and lithium used to make cone six exciting but their seems to be
a lot of negative felling about their stability and the possible harmful
effects if they leach out. I am sceptical and wonder if a sense of
proportion has not been lost but since I do not use them it is not my
business- it is unfortunately yours. Not using these materials makes cone
six difficult since these materials can be substituted for some of the other
fluxes adding stability . Zinc is also a material used for cone six and used
to be used a lot before higher textures were popular. I have no experience
of using zinc.
It seems logical that a slight rise in temperature could be compensated
by leaving out some of the fluxes. However the difficulties come because the
clay does not seem to cooperate and the fluxes and lead starts to boil off
badly so lead silicates become un stable . Fluxes are also subject to
eutectics but I would not worry too much about that .
Lead is a flux that fits pots very well and Boron very much part of
some low temperature glass structures has very little shrinkage. Since lead
is definatly out at cone six, boron is the only reliable flux left. The
fluxes that suit stoneware are sodium/ potash and calcium . Calcium is
unpredictable at cone 6 it can not make up its mind to be a crystallizing
matting agent or a flux.
Unfortunately putting a lot of sodium in the glaze tends to increase its
crazing potential and although potash is better even the best feldspars have
a lot of sodium in them and because our frits are also sodium rich more
trouble.
So cone 6 is between two stools it too high a temperature for earthenware
fluxes to be stable and too low for stoneware fluxes not to cause crazing.
The glazes that are used contain a compromise between the two but we are
now using boron and sodium/potash. The glaze recipes that work at this temp
do not allow a lot of leeway in their make up . So using someone elses
glaze recipe may not be copying but the only choice given the strict limits
at cone six. This explains all the panic on this list concerning borax
containing materials. A small change in the borates analysis seems to make
a big difference.
Glazes that are crystalline seem to hold together well I suspect a
crystal structure is more elastic than a compleat frozen liquid ,and maybe
you can not see the crazing in these glazes.
The clay you are using probably fires well at cone nine. At cone six it
is not very vitrifyed this is great for oven ware no nasty silica structures
in a tight matrixes but getting a glaze on is difficult because it does not
shrink. The difficulty with putting flux in a body is that fluxes like iron
do not work in the steady way they aught, they enter the equation suddenly.
A discovey made by many a potter firing his earthenware too high . There
must be clays that fit cone six well. But I would suspect that the choices
are fewer and the clear glazes that fit them are also fewer.
One of the great helpers of the glaze fit is the glaze body layer in
earthenware. All the pot is completely covered to help prevent the crazing
that occurs if the body absorbs water. You have noticed I expect that old
earthenware crocks are inevitably crazed . I would theorize that unless the
pot has a good glaze body layer it will sooner or later crazes with age.
Most have to get their clay off the shelf. If you can buy one that suits
cone six all the better
but that is a matter for your supplier. He boasts that the clay fires to
cone whatever and usually does not supply expansion figgers for firing it at
cone six.
The firing will make a difference a slow crawl to temperature is more
forgiving than the blast to cone X. I do not know why and that as many
things may be my imagination.
The glaze thickness it important Ting ware is glazed very thin to
prevent crazing.
So you can judge from what I am saying that it is not impossible to get
harmony at cone six but the perameters are small because earthenware fluxes
are very reactive at that temp and stoneware fluxes you need such a high
proportion which introduces crazing.
Thank God for borax this seems to be the corner stone of most ^6 glazes
it fluxes well and shrinks little if you can find a form without loads of
soda in as well .
For your glaze
First make sure that the materials you are using are dry if not
compensate for this dry some out-mesure the loss and compensate for the
difference.
Substitute high potash spars for all or some high soda spars and if they
have a higher silica content all's the better because silica in this form is
already bonded with the fluxes which gives you more melt.
Put in as much silica as you can you will be surprised how little you
will need if you attend to the other things first. Also this is where
Eutectics comes in adding silica may not necessarily raise the temperature
of the melt.
Substitute boron for soda/potash. by changing the fritt or substituting
it for some of the spars
A good grinding of the glaze will also lower the firing temp and the
finer the materials the less soda you will need to use.
Glaze thiner.
If You do a little of all these it is better than doing one of them a
lot.
Iron seems to be a good flux so temocuse and high iron glazes seem to do
well at all temperatures. Maybe it is because you can not see the crazing or
that iron glaze structures like lead has a lot of give in it.
You can see how an understanding of glaze chemistry is an advantage
especially if you are familiar with formulas . It is not that you will be
breaking down all your glazes to their Seeger or proportional analyses but
it will give you more confidence in your guesswork.
I do not Know about the glaze and body formulas that suit cone 6 I deal
mainly with stoneware glazes and I have not got a familiar feel for the
temperature
the reasons above put me off firing lower than cone eight and most of the
potters know seem to end up firing higher not lower and the suppliers here
are very much geared up for either earthenware or stone ware and not much in
between which makes you very much on your own at cone six. not a lot of
choices for materials designed to fit this temperature.
My Thoughts on learning glaze technology are . Forget about how learning was
done at school.
Nigel Woods books are great for understanding stoneware glazes- lots of
pictures. Seeger(unity) formulas are difficult to get your heard around .
But unlike school you have no tests and lots of time to take glaze
calculation slowly. so if you understand a little a day you will get there
and I doubt if you are as illiterate as me.
It gets confusing because of the different ways of approaching it by so
many people.
There are four basic ways of constructing glazes
Formulas and calculation. Some use proportional analyses and others
Unity formula.
Triaxial and biaxial blending.
Adjusting recipes.
And bunging stuff in a bucket.
learn one at a time but use all four
Regards Paul T
----------
>From: Merrilee F Pascaris
>To: CLAYART@LSV.UKY.EDU
>Subject: Re: cone six glaze quest
>Date: Mon, Mar 27, 2000, 12:25 am
>
>----------------------------Original message----------------------------
>Has anyone else done any testing on this?
>
>I mixed up the following glaze: RR3/00 Cone 6 Clear (using Custer
>Feldspar and Ferro Frit 3134) .
>
>I also tested it with 1.5% cobalt carbonate and another with 2% copper
>carbonate. All crazed on our particular clay body, which is from a local
>supplier (Rovin, Taylor, MI). It's a gray stoneware clay (RO72), cone
>6.
>
>I tried adding 10% more silica, and it still crazed, but less. The
>colors were bright and clear.
>
>Ron Roy's Black #3 works very well on this clay body, as well as other
>glazes that I've gotten from Clayart, so I have lots of other bases that
>I can use, but I'd really like to learn how to revise glazes. I guess
>this might be a good way to start. I have Insight, (bought it at NCECA
>last year) but to be honest, haven't done too much with it.
>
>I'm open and willing to do what it takes. Any guidance would be
>appreciated. Thanks to all for your sharing and support.
>
>Merrilee Pascaris
>Birmingham Bloomfield Art Center
>Birmingham, MI USA
>
>On Sat, 18 Mar 2000 15:21:00 EST Ron Roy writes:
>Original message
>Here is a revision of the clear that will work with the
>chrome-tin reds and pinks. It has not been tested -
>for melt, durability or colour response but I will bet it will pass all.
>If anyone tries it I would appreciate knowing what happened.
>
>RR3/00 Cone 6 clear (untested)
>
>G200 - 22.0 (Custer should work just fine)
>Ferro frit 3134 - 24.0 (fusion frit f12 will work)
>Wolastonite - 23.0
>EPK - 23.0
>Silica - 8.0
>Total - 100.0
>
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Jonathan Kaplan on thu 30 mar 00
While I certainly do not wish to clog the list with lengthy replies about
the nature of glazes at specific temperatures, there is one point that I
could add that would be of great help to others.
With some of the information currently circulating regarding fluxes at cone
6 and associated problems with lithium carbonate and strontium carbonate
used in this range, as well as galze stability, why not design and work
with glazes that have a broader range of temperature and are fritted?
We have been using Fusion F38 frit for many years in our glazes and have
designed glaze bases that are both gloss and satin matt, receptive to both
commercial stains as well as raw earth oxides.We have used this material to
make glazes that have varying degrees of opacity and quite rich surfaces.
F38 is a strontium based frit.
F38 is available in 50# bags from RABCO is Sykesville MD. Ask for John Ray.
Best,
Jonathan
Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139
jonathan@csn,net
http://www.sni.net/ceramicdesigin
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