Kris Griffith on mon 24 feb 97
------------------
Roy, If you take the barium out of this glaze, would it then be suitable =
for
functional items? I am looking for a red at =5E6 Oxidation, one that has =
depth in
the surface, rather than a flat color, for use on functional ware. Any
suggestions?
Kris
----------
From: Ron Roy=5BSMTP:ronroy=40astral.magic.ca=5D
Sent: Sunday, February 23, 1997 9:26 AM
To: Multiple recipients of list CLAYART
Subject: Re: Cream breaking Red
----------------------------Original message----------------------------
=3E----------------------------Original message----------------------------
=3E------------------
=3EHello=3B
=3E
=3EEarlier this week the glaze =22Cream breaking Red=22 ---Cone 6,or 7 maybe=
even 5
=3Eor fired to 2195 in 4hours 9 minutes and soaked for 35 minutes.
=3E
=3E35 Gerstley Borate
=3E15 Soda Spar
=3E10 Whiting
=3E5 Barium Carb
=3E10 Flint
=3E13 Tin
=3E2 rio
=3E
=3EI'm sort of interested in using this glaze. Maybe changing the Barium to
=3EStrontium for the safety factor. The material that slows me down ablit is=
the
=3Ehigh level of tin oxide--------It costs soooooo much. At 13 parts the =
expense
=3Eis up there. And as I have a tendancy for using alot of a glaze when I =
find one
=3Ethat I like, cost is a factor.
=3ECasting about looking for a cheap source of Tin, the best I have been =
able to
=3Efind is =247.00 a lb.------ still high but loking within grasp.
=3EI am thinking of doing a line blend with the tin to find the lowenst =25
=3Ethat will
=3Estill give the results, also maybe some substitution of Mason stain or
=3EZircopax.
=3E
=3EHas anyone out there in clayland experimented withthis glaze or have a =
good
=3Esource of Tin, or for that matter any insights into this glaze. Any help =
would
=3Ebe greatly appreciated.
Hope you are not planning to use this on functional ware. There is nowhere
near enough alumina or silica to make this a durable glaze. I will
personally guarantee this will leach Barium and any other poison in this
glaze. Even on the outside of pots it will change with washing. Beware
which mason stains you use - - some will have toxins which will come out of
this glaze.
On the other hand - as a decorative glaze on none functional clay .........
Just make sure any ware which may be used for food is sold with a warning.
You can take out the 5 Barium and replace it with 2 more Whiting (that
means the new glaze will have 12 whiting instead of 10. I am guessing the
glaze will look the same and it will be a cheaper fix than the strontium.
If you go with a Strontium sub then 3.5 is the right amount.
I strongly advise anyone not to use this glaze with the Barium in it.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
long gretchen on tue 25 feb 97
Regarding the Cream breaking to Red, Ron Roy responded that this glaze did
not have the right balance of Alumina and Silica to make it durable.
I hate to ask the same question others have, but what exactly IS the
correct balance of Alumina and Silica to make this (or any) glaze
durable (and/or safe). I have been working in clay for many years but have
never analized this. (I don't have a glaze calc program.)
Thanks,
Gretchen
Ron Roy on thu 27 feb 97
>----------------------------Original message----------------------------
>Regarding the Cream breaking to Red, Ron Roy responded that this glaze did
>not have the right balance of Alumina and Silica to make it durable.
>
>I hate to ask the same question others have, but what exactly IS the
>correct balance of Alumina and Silica to make this (or any) glaze
>durable (and/or safe). I have been working in clay for many years but have
>never analized this. (I don't have a glaze calc program.)
>
>Thanks,
>Gretchen
Hi Gretchen,
I've been trying to think of how to answer this in a concise way - if this
answer is not complete enough just ask away.
It is not the balance of alumina to silica which is the criteria but rather
the amount of each along with the other oxides in a glaze. If there is too
little SiO2 in the glaze it is easily attacked by acids. If there is not
enough Al2O3 the glaze will likely be degraded by alkalies.
We use "limit formulas" to check the level of each oxide in a glaze. These
limit formulas are arrived at by testing glazes. When enough glazes are
tested for durability we can then draw certain conclusions and
generalizations about what must or must not be included in order to have a
stable glass. These same "limits" can also be used to make unbalanced
glazes when durability is not important. For instance if we know we need a
greater amount of calcium in a glaze to get a certain kind of surface we
can use the upper limit for CaO as a starting point. What this means in
reality - if you put more of an oxide in your glaze (glass) than can be
held in suspension during cooling then it will precepted out and add
certain qualities to that glaze. Hi calcium transparent mattes are an
example of a glaze "oversupplied" with calcium.
One set of limits I use (with B2O3 included with the fluxes) for a cone 6
glaze says the Al2O3 must be between 0.275 and 0.650.
The SiO2 should be between 2.40 and 4.70
In the cream breaking red glaze the Al2O3 is 0.14
The SiO2 is 1.36.
As you can see this glaze is not a little short of both but a lot short of both.
There are some glazes which seem to be OK when compared to one set of
limits but are "out" of others. It is an area I have questions about - I
for one, wish some one would do some work in this area - we, as a group
could use the information. I'm up to my neck in dilatometer experiments so
I don't have the time. Anybody have a friend with access to a lab?
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Lawrence Ewing on sat 1 mar 97
BARIUM GLAZE RESEARCH
Those of you who are following the Cream breaking Red/reds at ^6?
discussion may be interested in the research into the stability of barium
in glazes being carried out by Janet DeBoos in Australia (reported in
the latest Ceramics Technical Vol.3).
This research is so far clearly showing the importance of having
sufficient alumina and silica in the glaze to involve all of the barium
flux in the glass structure. The required quantity of alumina and silica
is directly related, as Ron has pointed out, to the firing temperature of
the glaze.
One of the fascinating facts emerging from Janet's research is the
dramatic reduction (5 - 10 fold) in barium release resulting from just
one cone increase in the firing temperature.
Lawrence Ewing
Dunedin
New Zealand.
Ron Roy on thu 6 mar 97
>----------------------------Original message----------------------------
>BARIUM GLAZE RESEARCH
>
>Those of you who are following the Cream breaking Red/reds at ^6?
>discussion may be interested in the research into the stability of barium
>in glazes being carried out by Janet DeBoos in Australia (reported in
>the latest Ceramics Technical Vol.3).
>
>This research is so far clearly showing the importance of having
>sufficient alumina and silica in the glaze to involve all of the barium
>flux in the glass structure. The required quantity of alumina and silica
>is directly related, as Ron has pointed out, to the firing temperature of
>the glaze.
>
>One of the fascinating facts emerging from Janet's research is the
>dramatic reduction (5 - 10 fold) in barium release resulting from just
>one cone increase in the firing temperature.
>
>Lawrence Ewing
>Dunedin
>New Zealand.
Just a word of cauction here. The cones Lawrence is talking about are 10
and 11. I would not think the same improvement would be found between, for
instance, cone 6 and 7.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Lawrence Ewing on fri 7 mar 97
>>----------------------------Original message----------------------------
>>BARIUM GLAZE RESEARCH
>>
>>Those of you who are following the Cream breaking Red/reds at ^6?
>>discussion may be interested in the research into the stability of barium
>>in glazes being carried out by Janet DeBoos in Australia (reported in
>>the latest Ceramics Technical Vol.3).
>>
>>This research is so far clearly showing the importance of having
>>sufficient alumina and silica in the glaze to involve all of the barium
>>flux in the glass structure. The required quantity of alumina and silica
>>is directly related, as Ron has pointed out, to the firing temperature of
>>the glaze.
>>
>>One of the fascinating facts emerging from Janet's research is the
>>dramatic reduction (5 - 10 fold) in barium release resulting from just
>>one cone increase in the firing temperature.
>>
>>Lawrence Ewing
>>Dunedin
>>New Zealand.
>
>Just a word of caution here. The cones Lawrence is talking about are 10
>and 11. I would not think the same improvement would be found between, for
>instance, cone 6 and 7.
>
>Ron Roy
>Toronto, Canada
>Evenings, call 416 439 2621
>Fax, 416 438 7849
Ron,
Thanks for that important clarification. Could you expand on your
observation relating to the different results which might be expected at
lower temperatures? I note in Janet's research report that she suggests
further research should be undertaken at the lower temperatures.
Lawrence Ewing
Dunedin
New Zealand.
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