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Hello! Hope you all are doing great! 

I recently started using Black Clay PRNI from SIO ceramics it fires cone 5-7

In my glaze tests everything went fine with the ones I'm using ( i know most glazes don't match with black clay) I'm talking specifically about Obsidian Celadon and Deep Fire Brick by Amaco. Yesterday I unloaded my kiln and both pieces using those glazes had pinholes. Slow Bisque fire cone 04 - then Slow Glaze fire cone 6

Do you think refiring them could solve those pinholes?

What do you recommend in this cases? or how to avoid them?

I read about firing schedules, specially on bisque to get rid of the impurities of the clay. But don't know how to! I would LOVE to have someone that teaches everything about ceramics in my country (ECUADOR) but here we don't have much professional potters that could teach. So I have to read and do and learn from mistakes. I love this community!

Also read about adding a 10 minute Hold to the glaze fire could help.

 

PS: I really LOVE how black clay looks, but I'm feeling a little disappointed with how hard it is to handle and solve it's problems, I know its about practice practice practice.

Thank you!

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Hi Titi!

One (twenty five pound) bag of cone 5-6 black clay experience have I. Several problems:

Bloating - extended bisque cleared up the bloats (avoiding thick spots helps as well, trying for five to six mm or so - less on small pieces). I had been pausing at 752, 1063, 1500F already, so held at 1500F longer, about an hour, and held it there again on the way down.

Bubbling - extended bisque helped the bubbling - somewhat. Some glazes clear bubbles better than others; the clear that's working well on the red clay also cleared bubbles very well on the black clay.

Crazing - wasn't until I had just about finished the bag o' clay that I noticed the crazing in the clear  (the one that clears bubbles well) - tiny lines. 

Other - some of the coloured glazes came out just beautiful on the black clay (Aardvark Clay Cassius Basaltic, now called Obsidian), some frothed up or otherwise mainly failed.

Other II - sufficient air/oxygen is important; there must be some exchange during bisque and glaze fire. I have a fan powered vent system that pulls a small stream of air out the bottom, which then mixes with ambient and is pushed outside via a dedicated vent pipe. Leaving a peep or two open might do.

Other III - like the red and buff clays I'm using, the black definitely misbehaved when fired much over about cone 5.5 or so - more bubbles!

Other IV - dropping from peak glaze temp 100F, then holding that temp for half hour, and controlled cool (elements on "low") to 1850F before shutting off the heat is also helping.

Not likely I'll be buying any more black clay, just not interested (for now); it's expensive and finicky! Just shout if you'd like me to post the glaze recipes that worked for me on Aardvark's black clay...

Likely you'll get some more responses as the day wears on! Post back your results, 'k?

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9 hours ago, Hulk said:

Hi Titi!

One (twenty five pound) bag of cone 5-6 black clay experience have I. Several problems:

Bloating - extended bisque cleared up the bloats (avoiding thick spots helps as well, trying for five to six mm or so - less on small pieces). I had been pausing at 752, 1063, 1500F already, so held at 1500F longer, about an hour, and held it there again on the way down.

Bubbling - extended bisque helped the bubbling - somewhat. Some glazes clear bubbles better than others; the clear that's working well on the red clay also cleared bubbles very well on the black clay.

Crazing - wasn't until I had just about finished the bag o' clay that I noticed the crazing in the clear  (the one that clears bubbles well) - tiny lines. 

Other - some of the coloured glazes came out just beautiful on the black clay (Aardvark Clay Cassius Basaltic, now called Obsidian), some frothed up or otherwise mainly failed.

Other II - sufficient air/oxygen is important; there must be some exchange during bisque and glaze fire. I have a fan powered vent system that pulls a small stream of air out the bottom, which then mixes with ambient and is pushed outside via a dedicated vent pipe. Leaving a peep or two open might do.

Other III - like the red and buff clays I'm using, the black definitely misbehaved when fired much over about cone 5.5 or so - more bubbles!

Other IV - dropping from peak glaze temp 100F, then holding that temp for half hour, and controlled cool (elements on "low") to 1850F before shutting off the heat is also helping.

Not likely I'll be buying any more black clay, just not interested (for now); it's expensive and finicky! Just shout if you'd like me to post the glaze recipes that worked for me on Aardvark's black clay...

Likely you'll get some more responses as the day wears on! Post back your results, 'k?

Hi!! Thanks again for the detailed info, it helps me a lot.

I found this firing schedule  https://digitalfire.com/schedule/plc6ds  

"Other IV - dropping from peak glaze temp 100F, then holding that temp for half hour, and controlled cool (elements on "low") to 1850F before shutting off the heat is also helping." ------- Is this the one you are talking about here? If it is, its 27 hour firing :O ... Oh boy!

1468869566_Capturadepantalla2021-03-04ala(s)11_18_30p.m..png.acc7d82ddf39a3862278c4f5b965c96b.png

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Hi Titi,

Looks like the "Time" column is cumulative (total) to that point, e.g. Step 1, takes about 2:37 to ramp to 250F at 108/hr and hold for sixty minutes.

Typical total glaze fire time in my kiln is just over ten hours(per my notebook*). I'm not holding under the boiling point at the beginning, for I'm ramping right up to 200F the night before, and leave the fan on all night. The next morning, it's still well over 130F. I'm also not doing the optional soak on the way up. However, the last 400 degree rise is a bit slow, as my elements show their age; also, I'm holding for closer to an hour at -100F, and then slow cool to 1850F before shutting down.

*Notes can be helpful! I keep a notebook for times and temperatures (each firing) and how the cones came out (each shelf). Notes on results I'm keeping in a separate notebook; for each firing, how the clays/glazes came out, on which shelves, what I liked, didn't like, and so on.

Edited by Hulk
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Hi Titi:

Black clay does have impurities, and most likely uses sodium flux: which adds to the pinhole issues. A slow his cycle is necessary: running at 108F (44C) an hour climb from 1250 to 1800F (1000C).  Looking at your glaze program: extend your hold  time on step 3 to 20 minutes. Sodium fluxes off gas vigorously at the top end of the firing cycle. 

Tom

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