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vitrification; functional use of cone 6 stoneware

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OffCenter    82

How do you know if ^6 stoneware is vitrified? Is it safe to use stoneware to ^6 in oven, coffee mugs, or casserole dishes?

 

 

Throw a small cylinder. Do not glaze it. Fire small cylinder. Put small cylinder on a sheet of paper. Fill small cylinder with water. Leave small cylinder over night. Next morning move small cylinder and look very carefully for any sign of dampness on the paper. If there is no dampness then that clay is vitrified at the cone you fired to. If there is even the slightest hint of moisture where the cylinder was on the paper then that clay is not vitrified and you should test further to see if firing it to a higher cone will mature the clay. But, do not make functional pottery out of it and do not think you can stop the leaking with glaze.

 

The answer to your second question is yes if the stoneware matures at cone 6.

 

Jim

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JBaymore    1,432

A more subtle test.......

 

Make a small object out of said clay that will weigh at least 100 grams. Fire it to the appropriate cone, completly unglazed.

 

Immediately after unloading the kiln, weight the object on an accurate scale like a triple beam balance (to an accuracy of plus or minus 1/10 gram).

 

 

Place it in a pan of water that covers the object, and boil it in the water for a few hours (maintaining the water covering the piece).

 

Remove it from the water, immediately dry off the surface well and weigh it again.

 

The percentage of weight change is what is known as the "Apparent Porosity" number for the clay body. To be considered fully vitreous it should be less than a 1% change.

 

best,

 

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

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BRL    0

How do you know if ^6 stoneware is vitrified? Is it safe to use stoneware to ^6 in oven, coffee mugs, or casserole dishes?

 

 

Throw a small cylinder. Do not glaze it. Fire small cylinder. Put small cylinder on a sheet of paper. Fill small cylinder with water. Leave small cylinder over night. Next morning move small cylinder and look very carefully for any sign of dampness on the paper. If there is no dampness then that clay is vitrified at the cone you fired to. If there is even the slightest hint of moisture where the cylinder was on the paper then that clay is not vitrified and you should test further to see if firing it to a higher cone will mature the clay. But, do not make functional pottery out of it and do not think you can stop the leaking with glaze.

 

The answer to your second question is yes if the stoneware matures at cone 6.

 

Jim

 

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BRL    0

How do you know if ^6 stoneware is vitrified? Is it safe to use stoneware to ^6 in oven, coffee mugs, or casserole dishes?

 

 

Throw a small cylinder. Do not glaze it. Fire small cylinder. Put small cylinder on a sheet of paper. Fill small cylinder with water. Leave small cylinder over night. Next morning move small cylinder and look very carefully for any sign of dampness on the paper. If there is no dampness then that clay is vitrified at the cone you fired to. If there is even the slightest hint of moisture where the cylinder was on the paper then that clay is not vitrified and you should test further to see if firing it to a higher cone will mature the clay. But, do not make functional pottery out of it and do not think you can stop the leaking with glaze.

 

The answer to your second question is yes if the stoneware matures at cone 6.

 

Jim

 

 

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BRL    0

How do you know if ^6 stoneware is vitrified? Is it safe to use stoneware to ^6 in oven, coffee mugs, or casserole dishes?

 

 

Throw a small cylinder. Do not glaze it. Fire small cylinder. Put small cylinder on a sheet of paper. Fill small cylinder with water. Leave small cylinder over night. Next morning move small cylinder and look very carefully for any sign of dampness on the paper. If there is no dampness then that clay is vitrified at the cone you fired to. If there is even the slightest hint of moisture where the cylinder was on the paper then that clay is not vitrified and you should test further to see if firing it to a higher cone will mature the clay. But, do not make functional pottery out of it and do not think you can stop the leaking with glaze.

 

The answer to your second question is yes if the stoneware matures at cone 6.

 

Jim

 

 

 

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BRL    0

How do you know if ^6 stoneware is vitrified? Is it safe to use stoneware to ^6 in oven, coffee mugs, or casserole dishes?

 

 

Throw a small cylinder. Do not glaze it. Fire small cylinder. Put small cylinder on a sheet of paper. Fill small cylinder with water. Leave small cylinder over night. Next morning move small cylinder and look very carefully for any sign of dampness on the paper. If there is no dampness then that clay is vitrified at the cone you fired to. If there is even the slightest hint of moisture where the cylinder was on the paper then that clay is not vitrified and you should test further to see if firing it to a higher cone will mature the clay. But, do not make functional pottery out of it and do not think you can stop the leaking with glaze.

 

The answer to your second question is yes if the stoneware matures at cone 6.

 

Jim

 

 

 

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BRL    0

Thank you for your answers and suggestions. One more question. Why do handles of some mugs get very hot in the microwave after heating for 30 seconds?

 

Happy Thanksgiving to all!

BRL

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Mark C.    1,806

Thank you for your answers and suggestions. One more question. Why do handles of some mugs get very hot in the microwave after heating for 30 seconds?

 

Happy Thanksgiving to all!

BRL

 

 

This is an easy one to answer-Those mugs are made from iron bearing clays like some stoneware's-there is enough iron in clay so pot heats up .

Microwaves heat iron and it can be bad news to put meat in them-as far as iron clays its ok but the clay becomes hotter than say an iron free clay like porcelain.

Mark

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TJR    359

Thank you for your answers and suggestions. One more question. Why do handles of some mugs get very hot in the microwave after heating for 30 seconds?

 

Happy Thanksgiving to all!

BRL

 

 

We had our Thanksgiving in October as usual. But thanks anyway. Happy Thanksgiving to all the Americans out there.

TJR. In Canada.

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JBaymore    1,432
Why do handles of some mugs get very hot in the microwave after heating for 30 seconds?

 

 

Another potential factor is that the clay body is not totally vitreous...... and over time in washing and soaking in dishwater and such.... a small amount of water gets into the clay body from the (likely) unglazed foot area. (Or possibly via crazing in the interior surface glaze.) The water migrates within the pores in the clay over time. It is not a lot of water... just a tiny little bit. The moisture can easily migrate back OUT of the clay surface in the general foot area. But in the handle which has glaze all around the surfaces and only adjoins the unglazed foot area at the bottom end, it is a bit more "trapped" in there...so it is slower to get out. So the water molecues heat up in the microwave from the energy... as does the handle.

 

 

best,

 

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

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Some companies will have the numbers for you (if you mix your own clay, test, as said above):

 

scroll down to 'fired absorption', defined as John said, and see.

 

^6 is NOT inherently less vitrified than ^10, as the numbers show:

 

http://www2.plainsmanclays.com/data/M390.HTM vs http://www2.plainsmanclays.com/data/H440.HTM

http://www2.plainsmanclays.com/data/M325.HTM vs http://www2.plainsmanclays.com/data/H550.HTM

http://www2.plainsmanclays.com/data/P300.HTM vs http://www2.plainsmanclays.com/data/P600.HTM

 

the numbers are more or less alike, and UNLIKE, for example,

 

http://www2.plainsmanclays.com/data/L212.HTM which is not even close to vitrified even at ^2 (but is usually fired to ^02-04!).

Red earthenware bodies seem to fare better and are as vitrified, at ^2, as ^6 stonewares are at ^6 but I don't know if this applies across the board.

 

Ovenware has more to do with thermal expansion properties, again, supplied above, in some cases. The alpha-beta quartz inversion (573C = 1063F) is a concern with every clay body but home ovens don't reach temperatures that are even close. The cristobalite inversion is of more concern (220C = 428F, a hot oven setting reached routinely in home ovens) but not all bodies have that. It would show as a bump around 220C on the thermal expansion curve.

 

Again if unsure, test.

 

David

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