Not sure if bisqueware pots were fired hot enough

[Claybot]

I thought my clay was 06 earthenware. I made some pots, let them dry, painted them with 04 underglaze and fired them at cone 04. The bisqueware pots seem okay and normal.

 

However, I'm new to firing clay and when I bought the box of clay at a ceramics supply store a couple months ago, I think I may have mistakenly asked for stoneware instead of earthenware. I threw away the box that the clay came in, so I can't check, and now I'm concerned that the clay was stoneware and my pots should've been fired at a much higher temperature, like cone 5.

 

The pots make the typical "tink" noise when I flick them with my fingernail, if that means anything. And they seem hard like they should be.

 

Is there a way to tell if my pots have been fired to the proper temperature? If the clay was stoneware, would the pots still be fragile?

 

I also have several bags of clay that were given to me. They are probably cone 06 earthenware clays, but I don't know; I'm just guessing. The person sometimes used stoneware clay, I recall. Is there a way to check these bags of clay to determine if they are earthenware? Do I make a tiny pinch pot and tap on it kinda hard? What would you do with unknown clay?

 

Thanks for any responses.

[neilestrick]

The only way to know for sure is to fire it up to cone 6 or 10 and see what happens. Fire a bit of the clay in a small bowl made out of high fire clay, in case it melts. If it does melt, then you've got a low fire clay.

[Lucille Oka]

Neil is correct in this test.

I will add taking the clay samples up to C5 in one test firing, then in another test firing go to about C8, if these go well, in the last test firing go to C10. In this final test it is also a perfect time to test shrinkage and your glaze application. All of the tests should be done on 'protected surfaces' as Neil suggests.

 

Did you also throw away your receipt? Vendor numbers maybe on it and the supplier can help with information on the clay you purchased.

[Claybot]

Thanks for the helpful replies.

 

By the way, I'm going to start marking the cone on my bags of clay with a Sharpee before de-boxing the bags. Another potter told me she does that.

 

Also, tragically, I can't find my receipt for the clay. I searched for it before making the thread.