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Low fire glaze on stoneware bisque


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Hi there,

I've been asked to make some plates which presumably will have to be a stoneware clay and fired high in order to be food safe.

However the glaze they want is a eathenware glaze usually fired at 1060.

Q. Is this possible to combine stoneware bisque with eathenware  glaze?

Thanks, Julia 

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I have used a low fire clear commercial glaze on cone 10 stoneware (and porcelain) fired to cone 10 reduction without any problems.  Also used that clear glaze as a “base glaze” with added oxides for high fire tests.  
I suggest you make a test run with the stoneware in something similar in shape and thickness to confirm.  
 

low fire glazes are fully melted at or near bisque temperature of stoneware and the open pores of bisqued stoneware soak up the glaze like a sponge.  
LT

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Your clay wouldn't be vitrified unless you fire it to the cone it vitrifies at.   It would be hard to glaze a  vitrified pot and then you might have bad glaze fit and liquid food particles could seep into hairline crazing of  the glaze.   You need to do some testing,  the crazing can occur several months after the firing,  take your time on the test you don't want anyone getting food poisoning  from your pots.       Denice

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It is definitely possible to have chemically and structurally sound pottery for food use that is made from earthenware clay. It involves doing your due diligence, testing the compatibility of your clay and glazes, and knowing your chemistry and physics. Every clay and glaze combo has its own set of difficulties, and you have to decide which problems you’d prefer to solve.  
Combining low fire glazes and high fire clay wouldn’t be my first impulse, but as MMR has said, it is possible. What glaze is it that the client likes so much, and is there an analogous one at high temperature? Which material do you have the most familiarity with?
 

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I don't think you should. The earthenware glaze matures at low temp and the stoneware clay vitrifies at the higher temp. There are reasons to fire both at the right temp. The stoneware will not be durable if its under fired and the low fire glaze will likely just run off the pot and/or in the case of your plates puddle and look like crap if you fire it too high. I would worry about testing though because of my kiln shelves. Maybe put them on a big throw-a-away platter or something.

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Hi everyone, thanks for you're replies and help, I dont know how to reply individually 

I guess I have a few tests to do ahead of me ! 

Just to be clear, the plan was to bisque the stoneware at the correct temp , then glaze fire with my glazes at the correct temperature which would be 1060.

Unfortunately  no there isn't a similar stoneware glaze available. 

Thanks Callie, I think I'll ask my supplier if they have an earthenware clay that would be suitable for food use then I can use my glazes.

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yeah doing that will leave the plates essentially at bisque strength and I would not sell or give those to someone to use under any circumstance. Bisque firing is to make the pottery fired enough to handle throughout the rest of the process. Some people skip it entirely but I have never heard of anyone doing only the bisque. It will be brittle.

If I were you and determined to use the glaze then I would switch to low fire clay that would be fully vitrified at the low temp or do what magnolia suggested and just fire that glaze to the higher temp and see if it works but what you are I think saying would in my opinion anyway be the worst approach. Interested in seeing if someone disagrees. 

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  • 2 weeks later...

Just curious, how could they know what glaze they wanted, when they have not seen what it looks like on the stoneware clay? Assuming they picked a test tile, why not use the same clay used on the test tile? It is unlikely to look the same on a stoneware body. 

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