Fired to recommended cone but not vitrified?

[out to sea]

Hi Everyone,

I have been using two clays from Kentucky Mudworks, when fired to cone 6 Iceman is listed with an absorption rate of 1.4% and Brown Bear with an absorption rate of .2%. I use witness cones when I fire to make sure I'm hitting the right temp. But I find that even on glazed pieces both of these clays will cause a tissue to wrinkle when I leave a pot with water sitting out for a day. I've got to do absorbtion test but my hopes are not high at this point. 

Any thoughts on why these clays would be weeping when fired correctly? Has anyone else had this issue?

Thanks!

[Min]

If you went to cone 6 and the pieces are not inordinately thick (over 1/4") they shouldn't weep, especially the Brown Bear. I'ld suggest doing your absorption test, do a couple samples for each body, then get back to the supplier with your results. Even with zero glaze on the pots they shouldn't leak with those absorption rates. They might just tell you to fire a bit hotter but that could cause a bloating problem with the Brown Bear given that it's a dark manganese body.

Welcome to the forum.

[out to sea]

Thanks Min! I will give the absorption tests a try. And I might try to refire the previous work a little hotter. 

[PeterH]

2 minutes ago, out to sea said:

Thanks Min! I will give the absorption tests a try. And I might try to refire the previous work a little hotter. 

A query about heat-work and refiring. What is the effect of simply refiring to the same cone?

[Min]

1 minute ago, PeterH said:

What is the effect of simply refiring to the same cone?

This will add heatwork also, part of the reason glazes can get quite runny when refired and clay can get bloats from the body breaking down and offgassing into a sealed surface.

[out to sea]

Would you suggest firing to cone 6 again then? I'm not too concerned about the glazes running, most of this batch of work is only glazed on the inside, with underglaze paintings on the outside, but I would like to avoiding bloating of the Brown Bear.  

 

It just occurred to me that the only cup I tested that didn't weep was one Speckled Brown Bear that actually had been fired twice. So maybe for my next batch I should shoot more for ^7 from the beginning  and cross my fingers nothing bloats.

I'll put some unglazed cups in this re fire and test absorption on those still.

 

[Callie Beller Diesel]

I work with a clay that needs to go somewhat hotter than the manufacturer recommends to get a better absorption rate. It too can get bloaty if you take it too far beyond the recommended temperature, but I found a drop-and-hold-to get the hotter cone approach gives the best of both worlds. Try both to see which works best for you.

[out to sea]

Great! I willl do that.