Phyliss Ward on wed 5 jan 00
I made a set of dishes for myself that I, as a novice potter, am quite
proud of. They came out of the kiln this morning and tonight we ate
chili and rice on them. When I went to rinse them off I noticed some
discoloration had occurred! Ugh! Can somebody please explain this to
me? I used a recipe directly from Richard Zakin's book "Electric Kiln
Ceramics" which says it is good for food surfaces. However I added 3%
copper carbonate to make it a lovely green. Here is the recipe:
soda feldspar 44
ball clay 6
dolomite 12
gerstley borate 12
zirconium opacifier 26
copper carbonate 3
I used Kentucky ball clay and Zircon G for the opacifier
--
Phyliss
pward@lightspeed.net
http://www.bodywise.com/consultants/bpward
Cindy Strnad on thu 6 jan 00
Phyliss,
Your copper is leaching badly. Copper is not an especially stable mineral to
add to glazes. It likes to sneak out at the first chance. The acid and
moisture in your food gave it that opportunity. Chili, as you know, is an
especially acid food. Good test. Most folks use vinegar, however tomatoes
will work.
Sorry to be the bearer of bad news. We've all learned hard lessons. Next
time, test a sample of the glaze by filling a small test bowl half-way with
vinegar overnight. See if there is a difference in color come morning. This
isn't as sophisticated a test as can be performed for you at a lab, but it
will give you a good indication as to whether you even have something you
want to try sending to a lab.
You could try soaking your plates in muratic acid for a bit (can't give you
specifics, as I've only read about this trick) to leach out the copper that
isn't bound. This may even up the color, and then you could use the plates
for dry things such as breads or crackers, and so on. Or as decorative
pieces.
Cindy Strnad
Earthen Vessels Pottery
Custer, SD
Ron Roy on thu 6 jan 00
Hi Phyliss,
You did not say what cone you fired to - Zakin says it's a cone 6 glaze.
It is low on silica if you look at it without the zirconium silicat and the
KNaO is more than I would put in a liner glaze - and then you added copper
carb at 3% which is rather high and is contributing to the low resistance
to acid attack.
If you would like to do some testing to make it a durable glaze I'll do the
recipes if you will fire them and send them to Alfred to have them tested.
Anyone else what to get involved?
RR
>----------------------------Original message----------------------------
>I made a set of dishes for myself that I, as a novice potter, am quite
>proud of. They came out of the kiln this morning and tonight we ate
>chili and rice on them. When I went to rinse them off I noticed some
>discoloration had occurred! Ugh! Can somebody please explain this to
>me? I used a recipe directly from Richard Zakin's book "Electric Kiln
>Ceramics" which says it is good for food surfaces. However I added 3%
>copper carbonate to make it a lovely green. Here is the recipe:
>
>soda feldspar 44
>ball clay 6
>dolomite 12
>gerstley borate 12
>zirconium opacifier 26
>
>copper carbonate 3
>
>I used Kentucky ball clay and Zircon G for the opacifier
>Phyliss
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Alison Hamilton on fri 7 jan 00
Ron,
I put Phyllis' glaze into Insight (without the zirconium silicate) and
got this:
NC4 SODA FELDSPAR... 44.00 59.46%
BALL CLAY........... 6.00 8.11%
DOLOMITE............ 12.00 16.22%
GERS BORATE 9/97.... 12.00 16.22%
========
74.00
CaO 0.37* 10.46%
MgO 0.23* 4.62%
K2O 0.06* 2.73%
Na2O 0.18* 5.71%
Fe2O3 0.00 0.28%
B2O3 0.15* 5.34%
AL2O3 0.32 16.29%
SiO2 1.82 54.57%
COST/KG 0.69
Si:Al 5.69
SiB:Al 6.18
EXPAN553.16
If this is a cone 6 glaze, would you explain why you say
> the
> KNaO is more than I would put in a liner glaze
If I did this correctly, I would have thought that the KNaO fit within
cone 6 limits - what are you looking at that makes you say it is more
than what you would want in a liner glaze?
> Anyone else what to get involved?
are you sure about that? :)
Alison Hamilton
Trout Lake
Dorset, Ontario
Tom Buck on fri 7 jan 00
Ron:
Here's a bit more on the glaze that released copper on contact
with an acidic food (chilli).
Let me add a thought: As cited the glaze has 26% zirconium
silicate opacifier which is 2+ times the amount of this material
(Zircopax, others) typically used as a whitener in a base clear. Why so
much ? Seems unwarranted.
ZrSiO4 (or ZrO2.SiO2) is a crystalline material that is most
stable in a glaze melt since it doesn't melt until 2500 oC. So the fine
powder is simply dispersed throughout the alumino-silicate melt (liquid)
at say C6 (1230 oC) and will affect the surface of the cooled glaze.
Add in 3% Copper carbonate which converts to black copper oxide (CuO) and
this will add to the difficulties of the base clear holding in solution
and/or dispersion such an amount. The CuO is on the surface and will be
leached by acidic foods.
Perhaps one could look at this type of glaze as if one were making
toffee or fudge. Just the right amount of sugar and flavorings and you get
toffee; put in more components and you end up with fudge, a much softer
material.
Happy Year 2000 Ron. keep well. Peace. Tom
Tom Buck ) tel:
905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Phyliss Ward on sat 8 jan 00
Thanks Ron - And you are correct - I did fire to cone 6 ox. I'd be happy to
try some modified recipes and send them for testing. Just let me know what I
need to do - I'm new at this ; - )
Also, it may be a few weeks before I can get started as I just had foot surgery
yesterday and have to stay in bed (with trusty laptop) for a few days then take
it easy for a while.
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Hi Phyliss,
>
> You did not say what cone you fired to - Zakin says it's a cone 6 glaze.
>
> It is low on silica if you look at it without the zirconium silicat and the
> KNaO is more than I would put in a liner glaze - and then you added copper
> carb at 3% which is rather high and is contributing to the low resistance
> to acid attack.
>
> If you would like to do some testing to make it a durable glaze I'll do the
> recipes if you will fire them and send them to Alfred to have them tested.
>
> Anyone else what to get involved?
>
> RR
>
> >----------------------------Original message----------------------------
> >I made a set of dishes for myself that I, as a novice potter, am quite
> >proud of. They came out of the kiln this morning and tonight we ate
> >chili and rice on them. When I went to rinse them off I noticed some
> >discoloration had occurred! Ugh! Can somebody please explain this to
> >me? I used a recipe directly from Richard Zakin's book "Electric Kiln
> >Ceramics" which says it is good for food surfaces. However I added 3%
> >copper carbonate to make it a lovely green. Here is the recipe:
> >
> >soda feldspar 44
> >ball clay 6
> >dolomite 12
> >gerstley borate 12
> >zirconium opacifier 26
> >
> >copper carbonate 3
> >
> >I used Kentucky ball clay and Zircon G for the opacifier
> >Phyliss
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
Ron Roy on sat 8 jan 00
Hi Alison,
It's within limits for assessing melt but don't use those numbers for
estimating durability - KNaO makes glazes more suseptable to acid attack
and there is 0.29 of KNaO - you still have the B2O3 in unity - Take it out
and resave your MDT so it will stop coming up like that.
I try to keep the KNaO down around 0.2 when I can. All it means is - you
want to make it as dirable as possible no mater what other things you add -
and copper is not something to add to a liner.
I'll get to your last one later tonight.
RR
>----------------------------Original message----------------------------
>Ron,
>
>I put Phyllis' glaze into Insight (without the zirconium silicate) and
>got this:
>
>
>NC4 SODA FELDSPAR... 44.00 59.46%
> BALL CLAY........... 6.00 8.11%
> DOLOMITE............ 12.00 16.22%
> GERS BORATE 9/97.... 12.00 16.22%
> ========
> 74.00
>
> CaO 0.37* 10.46%
> MgO 0.23* 4.62%
> K2O 0.06* 2.73%
> Na2O 0.18* 5.71%
> Fe2O3 0.00 0.28%
> B2O3 0.15* 5.34%
> AL2O3 0.32 16.29%
> SiO2 1.82 54.57%
>
> COST/KG 0.69
> Si:Al 5.69
> SiB:Al 6.18
> EXPAN553.16
>
>
>If this is a cone 6 glaze, would you explain why you say
>
>> the
>> KNaO is more than I would put in a liner glaze
>
>
>If I did this correctly, I would have thought that the KNaO fit within
>cone 6 limits - what are you looking at that makes you say it is more
>than what you would want in a liner glaze?
>
>
>> Anyone else what to get involved?
>
>are you sure about that? :)
>
>
>Alison Hamilton
>Trout Lake
>Dorset, Ontario
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Alison Hamilton on mon 10 jan 00
Ron,
This is simply becoming 'curiouser and curiouser'.
> It's within limits for assessing melt but don't use those numbers for
> estimating durability - KNaO makes glazes more suseptable to acid attack
> and there is 0.29 of KNaO - you still have the B2O3 in unity - Take it out
> and resave your MDT so it will stop coming up like that.
I thought you included B2O3 in unity for cone 6 glazes (because it acts
as a flux at this temperature?). If this is correct, then why are you
suggesting to take it out of unity?
And, do you have different limits for all of the oxides re: melt vs.
durability? What are they and where did they come from? I hadn't
realized that there were different limits for melt and durability.
> and copper is not something to add to a liner.
There are an awful lot of green glazes out there with copper carbonate
in them as colourant. Are you suggesting that none of them should be
used on the inside of a functional pot? So, even if you recalculate
Phyllis' glaze, you would say that it shouldn't be used as a liner
glaze?
just when i thought i was starting to understand something about
glazes..... :(
Alison Hamilton
Trout Lake
Dorset, Ontario
Ron Roy on mon 17 jan 00
Hi Alison,
So many questions and so little time - seems my lot in life.
I used to use B2O3 in unity with fluxes - John Hesselberth pointed out to
me that it was wrong to do so and I agree with him. The limits I use for
glazes with boron in them are different from other limits but they are
still for assessing melt - not durability.
Sending glazes to Alfred for testing is what will help us come to some sort
of limits for durable glazes - If you do have any glazes tested make sure
John Hesselbert has the recipe, cone fired to, how long and the lab
results.
Some glazes are better at keeping copper in than others - but copper is
hard to keep in. Seeing colour changes is a sure sign the glaze is
unstable.
We all want simple answers and the more we get into this the more we all
have to realize - no matter how much we want simple answers - it will not
change any of the science.
If you don't want to have your glazes come apart during use - keep the
inside glaze as free of colourants as possible or have your glazes tested.
I wish someone would see how much copper you could add to my black glaze
before it started coming out - it's just a matter of leaving out the cobalt
and iron and adding 1, 2, 3, 4 and 5% copper and having the tests done.
RR
>----------------------------Original message----------------------------
>This is simply becoming 'curiouser and curiouser'.
>> It's within limits for assessing melt but don't use those numbers for
>> estimating durability - KNaO makes glazes more suseptable to acid attack
>> and there is 0.29 of KNaO - you still have the B2O3 in unity - Take it out
>> and resave your MDT so it will stop coming up like that.
>I thought you included B2O3 in unity for cone 6 glazes (because it acts
>as a flux at this temperature?). If this is correct, then why are you
>suggesting to take it out of unity?
>And, do you have different limits for all of the oxides re: melt vs.
>durability? What are they and where did they come from? I hadn't
>realized that there were different limits for melt and durability.
>> and copper is not something to add to a liner.
>There are an awful lot of green glazes out there with copper carbonate
>in them as colourant. Are you suggesting that none of them should be
>used on the inside of a functional pot? So, even if you recalculate
>Phyllis' glaze, you would say that it shouldn't be used as a liner
>glaze?
>
>just when i thought i was starting to understand something about
>glazes..... :(
>
>Alison Hamilton
>Trout Lake
>Dorset, Ontario
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Barney Adams on tue 18 jan 00
Ron,
Would it make sense to add copper to a base glase and have it tested?
Would that be a rule of thumb for stable for other colorants bearing in mind
the amounts are reasonable?
Barney
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Hi Alison,
>
> So many questions and so little time - seems my lot in life.
>
> I used to use B2O3 in unity with fluxes - John Hesselberth pointed out to
> me that it was wrong to do so and I agree with him. The limits I use for
> glazes with boron in them are different from other limits but they are
> still for assessing melt - not durability.
>
> Sending glazes to Alfred for testing is what will help us come to some sort
> of limits for durable glazes - If you do have any glazes tested make sure
> John Hesselbert has the recipe, cone fired to, how long and the lab
> results.
>
> Some glazes are better at keeping copper in than others - but copper is
> hard to keep in. Seeing colour changes is a sure sign the glaze is
> unstable.
>
> We all want simple answers and the more we get into this the more we all
> have to realize - no matter how much we want simple answers - it will not
> change any of the science.
>
> If you don't want to have your glazes come apart during use - keep the
> inside glaze as free of colourants as possible or have your glazes tested.
>
> I wish someone would see how much copper you could add to my black glaze
> before it started coming out - it's just a matter of leaving out the cobalt
> and iron and adding 1, 2, 3, 4 and 5% copper and having the tests done.
>
> RR
>
> >----------------------------Original message----------------------------
> >This is simply becoming 'curiouser and curiouser'.
> >> It's within limits for assessing melt but don't use those numbers for
> >> estimating durability - KNaO makes glazes more suseptable to acid attack
> >> and there is 0.29 of KNaO - you still have the B2O3 in unity - Take it out
> >> and resave your MDT so it will stop coming up like that.
> >I thought you included B2O3 in unity for cone 6 glazes (because it acts
> >as a flux at this temperature?). If this is correct, then why are you
> >suggesting to take it out of unity?
> >And, do you have different limits for all of the oxides re: melt vs.
> >durability? What are they and where did they come from? I hadn't
> >realized that there were different limits for melt and durability.
> >> and copper is not something to add to a liner.
> >There are an awful lot of green glazes out there with copper carbonate
> >in them as colourant. Are you suggesting that none of them should be
> >used on the inside of a functional pot? So, even if you recalculate
> >Phyllis' glaze, you would say that it shouldn't be used as a liner
> >glaze?
> >
> >just when i thought i was starting to understand something about
> >glazes..... :(
> >
> >Alison Hamilton
> >Trout Lake
> >Dorset, Ontario
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
Ron Roy on sat 22 jan 00
Hi Barney,
I think it is the wrong oxide to use because it also makes glazes more
suseptable to leaching. Better to you an oxide that does not have the
reputation for letting acid in.
I feel the right way to go about this is find durable glazes using iron or
cobalt and then go to copper to see if there is more of a problem.
In fact it may not be necessary to use a colour at all - why not test for CaO.
If a glaze leaches almost no CaO and you then start adding colours you will
find out much more about what the colours do or don't do.
I hope John Hesselberth will comment on this.
If you want to do some glazes and feel you could use some help just let me
know Barn.
RR
>----------------------------Original message----------------------------
>Ron,
>Would it make sense to add copper to a base glase and have it tested?
>Would that be a rule of thumb for stable for other colorants bearing in mind
>the amounts are reasonable?
>
>Barney
>
>Ron Roy wrote:
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Barney Adams on sun 23 jan 00
Thanks Ron,
I've started working on new glazes and I tend not to use much
color oxides as I like the natural colors. Eventually when I lock down
the recipes I want to send them in for testing. I tend to use your ^6 black
as a main staple.
I figured if CuO is more leechable (?) then if I tested a base glaze with
a reasonable amount of CuO then any other colorant (within safe % for
each one) would tend not to leech. Not so much that I would want CuO in the glaz
but to test for worst case.
I suppose this may not be the case since the chemistry of glaze doesn't
always follow common sense, well at least my sense.
Barney
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Hi Barney,
>
> I think it is the wrong oxide to use because it also makes glazes more
> suseptable to leaching. Better to you an oxide that does not have the
> reputation for letting acid in.
>
> I feel the right way to go about this is find durable glazes using iron or
> cobalt and then go to copper to see if there is more of a problem.
>
> In fact it may not be necessary to use a colour at all - why not test for CaO.
>
> If a glaze leaches almost no CaO and you then start adding colours you will
> find out much more about what the colours do or don't do.
>
> I hope John Hesselberth will comment on this.
>
> If you want to do some glazes and feel you could use some help just let me
> know Barn.
>
> RR
>
> >----------------------------Original message----------------------------
> >Ron,
> >Would it make sense to add copper to a base glase and have it tested?
> >Would that be a rule of thumb for stable for other colorants bearing in mind
> >the amounts are reasonable?
> >
> >Barney
> >
> >Ron Roy wrote:
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
John Hesselberth on mon 24 jan 00
Hi Ron and Barney,
With the very limited data we have accumulated so far it is difficult to
be very definitive, but I've never been shy about speculating based on
limited data as long as everyone realizes it is nothing more than a
semi-educated guess. Barney, I think your approach could work, but it
may be more rigorous than is necessary if you don't plan to use copper as
a colorant. Certainly copper appears to be the most difficult colorant,
of those I've checked, to keep in a glaze. It may well follow that if a
glaze holds copper in the leaching test, it will hold other colorants
even better. It would be easy to get a couple data points on that by
testing a couple of the glazes I've posted that hold copper very well.
However, on the other hand, I would be pretty sure that some glazes that
leach copper badly would not leach, say, cobalt. So, as a practical
matter, it might be more productive to take Ron's approach. You would
probably end up with more useable glazes, but perhaps not with as many
universal glazes that you could use with any colorant without concern. I
wish we could find a universal, simple answer to all this, but I'm afraid
it is not to be. Nonetheless, Barney, I'll certainly give your idea
further consideration as I try to untangle this complex puzzle.
So much to learn, so little time.
John
P.S. As a side note to all Clayart members. I put in a proposal to
present a paper on this general subject of glaze stability at NCECA.
Even with good support from Louis Katz, Ron Roy and other Clayart
members, it got rejected. Then I proposed to lead a discussion group on
the subject. Got rejected a second time. Sigh. They said there would
not be enough interest. Oh well, that means I'll get to attend NCECA
with only the goals of soaking up as much as I can and meeting people-- I
won't have to worry about doing any talking. That I'm looking forward to.
Ron Roy wrote:
>----------------------------Original message----------------------------
>Hi Barney,
>
>I think it is the wrong oxide to use because it also makes glazes more
>suseptable to leaching. Better to you an oxide that does not have the
>reputation for letting acid in.
>
>I feel the right way to go about this is find durable glazes using iron or
>cobalt and then go to copper to see if there is more of a problem.
>
>In fact it may not be necessary to use a colour at all - why not test for
>CaO.
>
>If a glaze leaches almost no CaO and you then start adding colours you will
>find out much more about what the colours do or don't do.
>
>I hope John Hesselberth will comment on this.
>
>If you want to do some glazes and feel you could use some help just let me
>know Barn.
>
>RR
>
>>----------------------------Original message----------------------------
>>Ron,
>>Would it make sense to add copper to a base glase and have it tested?
>>Would that be a rule of thumb for stable for other colorants bearing in mind
>>the amounts are reasonable?
>>
>>Barney
>>
>>Ron Roy wrote:
>
>Ron Roy
>93 Pegasus Trail
>Scarborough
>Ontario, Canada
>M1G 3N8
>Evenings 416-439-2621
>Fax 416-438-7849
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
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