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Impossible Quest? Cone04 Satin Food Safe


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#1 parrotfeathers

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Posted 11 April 2010 - 05:05 PM

hello,
i am new here so...hey all!

but on topic: i have been seeking high and low for a cone 04, (give or take a cone) satin glaze for my terra cotta/ low fire clay tea bowls.
i'd prefer something that doesn't contain colorants but with some minor testing will takes oxides or stains.
so far all of the recipes i have found are...well, not intended for dishes, or in one case shivered so bad that my poor little cup shattered in my hands when i took it out of the kiln.

am i crazy? does this glaze exist? will you be my hero?

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#2 JBaymore

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Posted 11 April 2010 - 08:29 PM

On the shivering glaze..........

If the glaze is what you want except for the pesky little problem of the shivering Posted Image ....... can you adjust the body, or are you using a commercial body and don;t want to get into mixing your own? An addition of crystobalie into an earthenware body can incease the COE of the body and it might be made to fit the existing glaze.

Also.... what exactly do you mean by "food safe"? That is a very slippery definition. In the USA only lead and cadmium are actually regulated by the FDA.

best,

......................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#3 parrotfeathers

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Posted 11 April 2010 - 11:54 PM

On the shivering glaze..........

If the glaze is what you want except for the pesky little problem of the shivering Posted Image ....... can you adjust the body, or are you using a commercial body and don;t want to get into mixing your own? An addition of crystobalie into an earthenware body can incease the COE of the body and it might be made to fit the existing glaze.

Also.... what exactly do you mean by "food safe"? That is a very slippery definition. In the USA only lead and cadmium are actually regulated by the FDA.

best,

......................john



thanks, john! eh, i don't think the glaze will be worth adjusting. before the piece decided to end it all i was in the middle of thinking about loosing the recipe anyways. i was using ward's red so commercial body.
but now that you bring it up i will keep your advise in mind if i ever decide to work from scratch.

when i said "food safe" i was thinking about the melting point of the glaze being enough to seal the parts of the clay it was on. in this case a tea bowl.
heh, i never trust the so-called "standards" put forth by the fda; as a personal rule i do not use lead, cadmium, chrome, manganese, barium, (to name a few) on food ware because i honestly don't know enough about them to use them responsibly.
for sculpture yeah fine but definitely not on anything that i would put tea in.
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#4 JBaymore

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Posted 12 April 2010 - 08:47 AM

Brain screwup there parrotfeathers on my part! Sorry. I was reading "shivering" and thinking "crazing". The crystobalite addition in the body is for making the body shrink MORE as it cools....not less. Crazing is the usual problem in earthenware glazes.....not shivering.

There a re other alterations you could make to the body, but you said you are using a commercial clay.....and fixing the COE in the glaze to increase is typically easier.

If you add some higher COE fluxes into the glaze in substitution for some of the ones that are there, that will increase the COE of the glaze and (hopefully) cause it to match the COE of the body. At earthenware temps that is not usually an issue to acomplish. Mostly it is the opposite issue...... crazing when you don't want it.

Can you post the recipe? What Orton cone are you firing it to?

For "food safe" from your definition you just want a glaze that is not crazing on the body, and which is composed with as much SiO2 as an earthenware glaze can hold, in order to make the glass "hard". And using a limited "pallate" of available fluxes that are considered the safest. If it is fully fused and not crazed, it will be impervious to the penetration of water and the like.

Watch out for "delayed crazing"......... which means the fit is close....but not exact. Test the fit using a thermal shock test to see if the glaze remains un-crazed even if iot comes out of the kiln with not obvious crazing. And use a loupe to look at the glaze surface, ...... or maybe use some india ink to stain any craze lines so that you can see if theyr are there more clearly.

best,

....................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#5 Icyone2

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Posted 12 April 2010 - 12:26 PM

what is the small studio way to do a thermal shock test? I've had pieces that did not show crazing for 2 years before.

#6 JBaymore

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Posted 13 April 2010 - 09:08 AM

what is the small studio way to do a thermal shock test? I've had pieces that did not show crazing for 2 years before.


For your tests, make sure that there are varying thicknesses of application of the glaze. Thicker areas will typically show crazing before thinner areas.

Take a small room temperature glazed test cup. (BTW.....should be done with more than one cup for accuracy.) Examine it with a loupe for crazing. Once you have established that there is none, boil some water. As soon as the water is boiling, and hopefully as it is still at a slight boil...... pour it into the test cup. Let it sit for a fixed amount ot time (1 minute?). Empty out the hot water. Dry it off and then use the loupe to check for crazing.

Repeat this test over and over. If the glaze truly fits the body, you should be able to repeat this until you get totally bored and not see any crazing develop.

If you eventually are getting crazing, reformulate the glaze (molecular calc) and make a new set of test cups. Then do the above test again, but count the "cycles" until you see crazing. The test cup with the most number of cycles before you see crazing is showing you the correct direction to take the COE. Use that to guide you to hitting the COE for the body.


To "speed up" the results.......... you can run a series of freeze / hot water cycles.

Take an empty test cup, place it in the freezer. Let it stay there a standardized amount of time....... or overnight. Remove it from the freezer. Chack for crazing. Then do the boiling water bit. Dry it off and refreeze. Repeat.


Both of these are also testing the clay body. It will show up any issues there as well. Expect the possibility of a failure of the body and boiling water going everywhere when you least expect it. Counting the number of cycles of freeze/thaw/boil until the body fails is a relativer and personal means of establishing a "strength" level of the body. Not able to compare to ASTM numbers .... but close enough for most studio work.

best,

.................john

best,

.................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#7 parrotfeathers

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Posted 13 April 2010 - 02:20 PM

john, will post recipe later.

just wanted to quick reply to the craze test.

you can also do it in kind of a ghetto way...this is how i test all my dishes.

put the cup, bowl, etc through the dishwasher. in between cycles make some coffee, (or even grape juice works...anything dark with a tendency to stain) and leave it in the cup overnight.
dishwasher again. within a couple cycles if there is crazing you will see coffee stains in the cracks.
www.parrotfeathers.etsy.com

#8 JBaymore

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Posted 13 April 2010 - 03:06 PM

john, will post recipe later.

just wanted to quick reply to the craze test.

you can also do it in kind of a ghetto way...this is how i test all my dishes.

put the cup, bowl, etc through the dishwasher. in between cycles make some coffee, (or even grape juice works...anything dark with a tendency to stain) and leave it in the cup overnight.
dishwasher again. within a couple cycles if there is crazing you will see coffee stains in the cracks.


I use india ink for that same purpose.

best,

.............john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com




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