[Bubbles In Black Stoneware]

1 hour ago, Callie Beller Diesel said:

Yes, it will remain bloated. Clay bloats when it gets hotter than the top recommended temperature, and some of the ingredients are starting to break down, which starts releasing assorted gasses. Because the clay isn’t as porous anymore, those gasses get trapped and form pockets inside the wall of the pot, which is bloating. Adding more heat only makes it worse, and the pots will get very brittle.

The reason a slow bisque helps with darker clays is because they have a lot of secondary clays in them, which tend to have a lot of impurities that need more time to burn off. If they’re not given enough time in the bisque, those materials will continue to burn off in the glaze. Because glazes begin to melt and fuse at lower temperatures than the clay body does, the glaze can seal in gasses that still need to escape. This is especially true if the glaze fire is set to move through bisque temperature range quickly. 

If you want to have a look at what a bloated clay looks like in cross section, I had a kiln malfunction earlier this spring that lead to a load being VERY over fired. If you go into the link in my signature here to my instagram feed, at the top tap on the circle marked NOOO! I talked about how it worked in my Stories

Ah! So it’s overfired you are saying that causes the clay to bloat not just because the bisque is not slow enough, ventilated so the impurities are not thoroughly burn out? Or both occurs that causes the bloating and glaze flaws? I have read and followed the recommended bisque procedure provided by Aardvark but somehow the outcome from my last two firings are the same and I don’t quite understand!

[Bubbles In Black Stoneware]

11 hours ago, Callie Beller Diesel said:

1260 C is cone  9, or a hot 8 depending on the firing speed. Do you know how fast the firing was programmed for, and if the person firing the kiln used cone packs to verify the end temperature?

I’m assuming you’re using Black Mountain from Aardvark, because that’s the cone 10 black clay that comes up when I google it. I just want to verify that, because your location states you’re in Hong Kong, and I want to make sure there’s not another one with a similar name that came from somewhere closer. If it is the Aardvark stuff, they do specify in its description on their website that it needs lots of oxygen in the bisque, and that is true of many black clays. They need a longer, well ventilated bisque because stuff needs more time to break down and escape so it’s not causing problems in the glaze.

Yeah I’m using black mountain from Aardvark and it’s alright sometimes but not everytime. I’m firing it at a communal studio so often I’ll have to stick to their firing schedule if not I’ll have to reserve the whole kiln which costs me a lot. Anyway for my last firing it should be set to 1240c with a 10 mins hold at the top temp so it’s around 1250 ish. I understood about the lots of oxygen this clay needs so did not stack them already but I guess I have not try the slow bisque method yet so it may makes the difference? Also do you know if once the clay is bloated it will remain bloated even if I refire it? 

Many thanks!!

[Bubbles In Black Stoneware]

Thanks! I bisque at 900C and then glaze fired to 1260C and the thing is there are pieces in this same firing that came out fine! No bubbling in the cups but in all the bowls! They are on the same level of shelves too so I don’t understand why.. also a lot of bloating in the pieces

[Bubbles In Black Stoneware]

Here’s a picture of it 

[Bubbles In Black Stoneware]

Hi! I’m so glad that I’m not the only one experiencing the dark stoneware problem and thank you for all the helpful advices! Currently I’m using black mountain and throughout the years I have very unstable results with this clay, bloating, glaze crawling or blistering (I’m using a simple white tin glaze that sometimes works fine but most of the time isn’t) and it is very frustrating. I tried not stacking them giving them space during bisque, tried firing it at 900 and 1000c and problem still occurs! Haven’t  tried holding the temp during bisque yet as I’m firing it at a communal space but hopefully someone here can solve the the mysteries! 
 

Mariane