Tara & Michael on sat 1 feb 97
Tom Buck recently posted the following variation of a "Maryland" ^5 Matt
turquoise glaze.
>The other day a matt turquoise glaze was posted on Clayart. I am
>suggesting that midfire potters not use this "Maryland" ABOVEglaze for food
>containers. It doesn't have enough silica to keep the copper properly
>bound when faced with food acids. Adapting the same Seger formula, here are
>two that are "balanced":
> C5 ox. Turquoise matt C5 ox. Green matt
> 60 Neph sy. 40 Neph sy.
> 25 Strontium carb. 25 Wollastonite
> 2 Lithium carb. 12 Spodumene
> 4 EPK 12 EPK
> 9 Flint 11 Flint
> 4 Copper carb. 4 Copper carb.
> 2 Bentonite 2 Bentonite
>Only tests will show if these yield an acceptable colour and surface on
>your claybody.
I did a test of the above turquoise glaze at ^6 in reduction . The test
came out a beautiful satin blue turquoise with medium gray crystaline runs.
Too runny for my purpose unless it is food safe. I soaked the test in
vinegar (5%) for 16 hours, as Ron Roy had suggested in one of his posts,
and the gray runs and crystals turned a much lighter gray and the crystals
became more apparent. The turquoise color did not change. I don't
understand what this means..I assume the acid chemically "freed" some of
the copper oxides in the glaze. I didn'dt keep Ron Roys post so I'm
missing how to interpret my test results.
I'd much appreciate Tom's, Ron's and others' input on this.
I'm going to try Pete Pinnell's glaze which seems to be the original glaze
that was altered to reduce the firing range. It's very similar as follows:
Pinnell Cone 6
Neph. Sy. 60
Strontium Carb. 20
Lithium carb. 1
Ball Clay 10
Silica 9
____
100
Copper Carb. 4
Thanks for any help you all can give me on this!
Michael Redwine in Gray Oregon
Tom Buck on mon 3 feb 97
Michael: If you fired this NephSy/Strontium carb glaze in reduction to
Cone 6, then you pushed the envelope perhaps a bit too much. This is a
glaze designed for C5 OX; it originated from Carlton Ball many many years
ago but he used a lead-bearing material now considered taboo. As
formulated it would be overfired at Cone 6, hence runny. And firing it in
reduction would tend to make the surface somewhat "open".
You did not say which of the two you tried, the Strontium one or
the Calcium one, although I gather it was the one with SrCO3.
BTW, the Pinnell glaze is also a variant of Ball's turquoise matt.
When you did the leach test with vinegar, what colour did the vinegar
become, or did it change colour at all?
Cheers TomB Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm
Ron Roy on mon 3 feb 97
>----------------------------Original message----------------------------
>Tom Buck recently posted the following variation of a "Maryland" ^5 Matt
>turquoise glaze.
>
>>The other day a matt turquoise glaze was posted on Clayart. I am
>>suggesting that midfire potters not use this "Maryland" ABOVEglaze for food
>>containers. It doesn't have enough silica to keep the copper properly
>>bound when faced with food acids. Adapting the same Seger formula, here are
>>two that are "balanced":
>> C5 ox. Turquoise matt C5 ox. Green matt
>> 60 Neph sy. 40 Neph sy.
>> 25 Strontium carb. 25 Wollastonite
>> 2 Lithium carb. 12 Spodumene
>> 4 EPK 12 EPK
>> 9 Flint 11 Flint
>> 4 Copper carb. 4 Copper carb.
>> 2 Bentonite 2 Bentonite
>>Only tests will show if these yield an acceptable colour and surface on
>>your claybody.
>
>
>I did a test of the above turquoise glaze at ^6 in reduction . The test
>came out a beautiful satin blue turquoise with medium gray crystaline runs.
>Too runny for my purpose unless it is food safe. I soaked the test in
>vinegar (5%) for 16 hours, as Ron Roy had suggested in one of his posts,
>and the gray runs and crystals turned a much lighter gray and the crystals
>became more apparent. The turquoise color did not change. I don't
>understand what this means..I assume the acid chemically "freed" some of
>the copper oxides in the glaze. I didn'dt keep Ron Roys post so I'm
>missing how to interpret my test results.
>
>I'd much appreciate Tom's, Ron's and others' input on this.
Ron's reply.
Hi Michael,
I don't think I gave any ways to interpret what is happening except that
some of the ingredients in that glaze are being leached out by the acid.
The same thing will happen with certain foods and may also happen with
washing.
In reading various texts to find out how to express the problem without
putting everyone on the list to sleep I "discovered" some interesting
information.
What is happening - in simple terms - some of the oxides in the glaze are
being leached out by the acid. The glaze is being made thinner and if
there are hazardous oxides in the glaze they are probably coming out as
well. Don't drink the acid or - don't eat the food. Many glazes do not have
hazardous oxides in them so they are not a problem. If the glaze is "soft"
it will dull with use (in contact with liquids which are capable of
leaching.)
The two main oxides which must be sufficiently present in glazes to make
them durable are Silica and Alumina. That's not the whole story but the
most important one. It is known that certain oxides will automatically
increase the solubility of a glaze - copper and sodium being two. Oversuppy
of other oxides can have a similar effect. Zirconium Silicate has a good
effect on durability.
The glaze in question: is short of silica according to my limit formulas -
it needs at least to be raised to 12 (instead of 9) but I don't think that
will be enough because if the high amount of Sodium and the copper. A line
blend starting at 12 silica up to 27 would be an interesting experiment.
When reading up on this I found a reference to the percent of Silica in a
glaze to make it acid resistant - 60% is a standard. That's where the 27
silica came from. In the above glaze there is quite a high % of Alumina.
Because there is a much higher amount of Alumina than normal for this
temperature there must be a correspondingly higher amount of silica to get
60%. The "right" amount of Alumina makes for good alkali leaching
resistance. I would be interested to see if this was the case - does the
glaze stand up to repeated dish washing?
Well now that I've put you all to sleep I'll just get on with my diatometry.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Tara & Michael on wed 5 feb 97
I want to thank both Tom Buck and Ron Roy for their help with this glaze.
I has turned into a rather fun quest.
Ron wrote the following:
>
>The glaze in question: is short of silica according to my limit formulas -
>it needs at least to be raised to 12 (instead of 9) but I don't think that
>will be enough because if the high amount of Sodium and the copper. A line
>blend starting at 12 silica up to 27 would be an interesting experiment.
Ron I'm going to do as you suggested above and do a line blend. However,
I'm firing in the range of cone 6 at 3 o'clock and cone 7 bending in
reduction. Is this going to put me out of the range of your calculation?
I've also put one of the tests in my dishwasher so it can wash and wash !
For those who have not been following this thread her is the glaze in question:
>>> Tom Buck C5 ox. Turquoise matt
>>> 60 Neph sy. .
>>> 25 Strontium carb.
>>> 2 Lithium carb.
>>> 4 EPK
>>> 9 Flint
>>> 4 Copper carb.
>>> 2 Bentonite
>>>Only tests will show if these yield an acceptable colour and surface on
>>>your claybody.
Thanks again from sunny Oregon
Michael Redwine
Ron Roy on fri 7 feb 97
>I want to thank both Tom Buck and Ron Roy for their help with this glaze.
>I has turned into a rather fun quest.
>
>Ron wrote the following:
>>
>>The glaze in question: is short of silica according to my limit formulas -
>>it needs at least to be raised to 12 (instead of 9) but I don't think that
>>will be enough because if the high amount of Sodium and the copper. A line
>>blend starting at 12 silica up to 27 would be an interesting experiment.
>
>Ron I'm going to do as you suggested above and do a line blend. However,
>I'm firing in the range of cone 6 at 3 o'clock and cone 7 bending in
>reduction. Is this going to put me out of the range of your calculation?
>
>I've also put one of the tests in my dishwasher so it can wash and wash !
>
Hi Michael!
What a refreshing attitude.
I suggest you try it - it's the only way to know. If it needs adjustment we
can take out some Al2O3 and at the same time replace the EPK with some ball
clay for better floatation. If you want to do them at the same time just
let me know and I will design a line blend for you.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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