My first home-firing!

Sad to say, that is the sound of glaze crazing. You will need to use a lower-expansion glaze formula, or a different clay body. The crazing may be very fine, and hard to see without a lens. Most people try to avoid crazing on food or beverage contact surfaces for sanitary reasons, although there is some controversy about whether it is an actual hazard.

Don't open the lid until the pieces are cool enough to handle. Opening it early is one way to provoke crazing (or worse). My preference is to arrange for it to cool over night, then I feel like a kid on Christmas morning!

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

Congratulations!!!!

Do not hurry the cooling process. It is not good for your pots or your kiln. Your kiln manual might have mentioned this and recommend what temperature you should open the kiln at. Yes it is painful to wait (and I confess that I always peak in before I open the lid all the way. If you can touch your pots with bare hands, it is cool enough. I know many who prop the kiln open to hurry the kiln cooling. I do not. I do keep my fan running and have the top peep hole open once it is below boiling point (100C).

Thanks. I will just have to lengthen my "firing" time, then. I don't want pots crazing.

Marianne

This will be a flower pot, and it is low-fired. So I will have to check it thoroughly to see if it holds water, then. I can't see any crazes, but will look closer and do the water test before putting the pot on my good table :-)

For next time, would firing at a higher or lower temperature maybe relieve this problem?

As we have a third of the price for power at night, I have to run the kiln overnight, which then means that I probably have to wait until at least the next evening. But I WILL wait! No point letting impatience ruin my pieces!

Thanks Bob

Marianne

Hi Marianne, I'm not sure if this applies to glazes in general but I once had a crazing problem when using a transparent glaze on a white firing earthenware body. After a short while crazing would apear and though I tried altering the glaze recipe the problem returned. When I asked a number of proffesional studio potters (at Rufford's 'Earth and Fire', an annual show for internationally renouned potters in Nott's UK) they all said "fire higher". I try to resist the urge to open the kiln until it's well and truly cooled to room temperature but I know how hard that can be, oh the temptation! Andy

Andy,

The crazing problem with White Earthenware clays is to do with the Bisque temperature; UK white earthenware clays need a high Bisque, like

1150oC, in order to achieve enough body shrinkage (their firing range starts much higher than almost anything else), after that you can glaze at what ever temperature you like. Red, Porcelain, and Stoneware clays are OK at 1000oC. Bone China is a totally different animal (!!). This is UK specific; I have no experience of clays outside this Island.

I have yet to meet a Reduction Celadon glaze that doesn't craze, though I expect someone on this list will say *I've got one*, in which case they truly possess the Holy Grail!

Slow cooling is a great help, difficult to achieve with low thermal mass kilns unless you fire down, so glaze fit will always be a lurking problem. Others on this NG are much better qualified to give us the current thinking on reasons for crazing. Come in please David Hewitt!

Steve Bath UK

In article , plodder writes

Thanks Steve, useful information to have; I wonder, does this higher bisque have much effect on the body's uptake of glaze.

I had similar problems with Potclay's Buff stoneware/earthenware when used with low fire transparent glazes that should fire to about

1040/1060 degrees C; if I fired these to 1100 degrees C the crazing problem was cured ( I must admit though, it's some time since I have checked older pots and I suspect some crazing would be found).

There often seems to be a kind of 'sods law' with glazes; when you want them to craze they won't, when you don't they do. Andy

Hi Andy

Sometimes firing too high will change the color qualities of the glaze. I have one that goes middle- and light-beige for me, but goes blue if you fire it too long. Not an ugly effect, but not the marble-like effect I like to get.

The temptation of opening too early is HARD on me, but knowing that I may ruin a piece I have spent hours on by opening that lid too early - well - I manage to wait! hehe!

Marianne

In article , plodder writes

No, because pure white earthenware is not a naturally occurring clay in the UK; what you get is a concoction, a recipe developed (I think) quite a long time a go. At 1150oC it has the same degree of porosity as a stoneware or red earthenware has at 1000oC.

That clay is actually a stoneware clay, as are all so called

*stoneware/earthenware* clays, they are called that because the target customers are Schools! Earthenware glazes will never truly fit them; their shrinkage rates are incompatible. You can reduce the crazing by a higher bisque, but the penalty is lower porosity. A thinner coat of glaze may also help.

Absolutely, that's why glazes and glazing is such an absorbing subject, though I must admit not for me; I am mathematically challenged so have great difficulty calculating them; when I run out of fingers and toes I'm stuffed!

Steve

Hi Steve, I also have poor powers of calculation but try my best; what a great idea, I never thought to take my socks off! But really though, I have had many a struggle with these buff stoneware's in the past, both as student and teacher/tech and the one transparent glaze that seems to fit well (across the board) is that used by Mike Eden, it fires best around cone 03, is tested as 'food safe' and seldom gives any problems (the only disadvantage that I have found with it is a reluctance to produce a good translucent green with copper carbonate).

My education continues, and now I've taken off the socks, well the sky's the limit. By the by Steve I believe I may be working with an old friend of yours!

Andy

Hi Marianne, sad to say you are so right about the colour changes but I have sometimes made the compromise (not being an absolute purist), you can make pleasant or interesting discoveries this way, so it's nice to play around with the odd piece. Keep up the resistance and steel yourself against early opening.

Andy

Dear Andy,

Sadly you will only get a really good translucent green with a simple glaze containing Fritted Lead firing around 1080, which of course will never be food safe!

Who?

reply to that off list please, doing what my address tells you to do!

Steve Bath UK

In article , plodder writes