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Glaze cracking boiling water


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This would indicate that the coefficient of expansion of the fired clay and fired glaze are not close enough or compatible enough  and delayed crazing will likely occur in items using this clay and glaze. It generally means adjust the glaze chemistry for a better fit or in the case of a commercial glaze use a different or more compatible one. Most mechanical or other solutions such as glaze thickness  alleged will not really fix this and the combination will eventually craze.

Most folks would consider this not suitable for food products and this would be a glaze defect when used with this claybody. Food and bacteria will accumulate in these cracks and the cup will be weaker than a non crazed equal. There are folks that consider crazing to be aesthetic and ok to use though. Most potters out of conservative caution will not sell crazed products for potential food use.

Here is a good read IMO on glaze defects. The safety issues are summed up in paragraph seven.

https://digitalfire.com/glossary/crazing

 

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If you make your own glaze,  you need to make up test samples of some different formulas.   Purchased glaze cannot be easily change,  trying a different brand might help.   It is probably the clay,  if you are firing  C5/6  glazes  you need a clay that is  C5/6  not one that can fire to  7,8,9,10.    Denice

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12 hours ago, ashhorth said:

Firing 

I think I’m gonna have to contact the supplier as I can’t actually find info on what cone they fire to!

@ashhorth, are these commercially made mugs that come glazed then you do the decorative work on? Could you give a bit more information on your process?

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Okay, so the clear glaze doesn't fit the clay. For commercial glazes it's really a matter of trial and error to get one that fits. I'ld start with contacting the supplier of the bisque and asking which clear glaze they recommend. If you use any green underglazes it's best to use a zinc free clear. Zinc can mess up any underglaze colours that contain chrome and turn it an ugly brown. Are you using cones to verify the glaze is getting to the correct cone? Underfiring can cause a glaze to craze too.

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From Bisque Direct's mug page https://bisquedirect.com/bisque/mugs.html

Earthenware Clay - Glazing and Firing.

The recommended glaze firing temperature is 1000˚C to 1080˚C and soak (hold) 30 minutes + for all our Earthenware bisque items.

For tableware items firing 1030˚C to 1080˚C produces a more durable product.

Which is repeated on their dipping glaze page https://bisquedirect.com/glaze/clear-gloss.html
They offer three glazes all firing "Approximately cone 06 to 05." Also gives Bullers ring ranges for each glaze.

 

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