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Posts posted by Titi

  1. 32 minutes ago, Hulk said:

    Per my limited experience, a hold at peak - adding heat work - may cause more fizzing. More heat work might clear things up for you, hmm. Are you firing same glazes, applied about same thickness, to other clays in same firing? Seeing same bubbling? In other words, any evidence that the problem is the clay vs. the glaze.

    Looks like very small bubbles, which mostly heal over, but leave an orange peel look - excepting for the four (or so) larger holes, which may be a dry spot (where the glaze didn't wet the clay fully going on), or a larger chunk of something in the clay off gassing, or?

    I found dropping and holding to help. No doubt you've already found links to Tony Hansen's articles, e.g. Drop-and-Soak Firing (digitalfire.com) and Glaze Bubbles (digitalfire.com); keep on it!

    Keep good notes on each case...

    I was just answering you on my post!

    Yep! I found that schedule and was surprised! is it a 27hour firing??

  2. 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!


  3. On 3/31/2020 at 4:54 PM, Lesley Anton said:

    Ah wonderful!!!

    I feel like I now have some awesome tools with all this great info. Tom, thank you!!! (and thank you Glazenerd!) I am a newcomer (not to clay but to Cone 6 firing/black clay/this forum, so I apologize that this is repetitive for those veterans on here. My clay history is cone 10 reduction so due to a recent move across the country, the situation has it that I needed to jump over to cone 5/6 oxidation.

    I have done a ton of searches on this forum but if you don't have the right combo of wordage things don't pop up.  

    Thanks again! 

    I'm having the EXACT same issues with the PRNI clay, having little pinholes in my glaze firing :( Using only Slow fire cone 04 and slow glaze at cone 6

    Let us all know how it went! I particularly want to know.... I was thinking holding top temp on Bisque leaving peeps out, then glaze fire hold 20 mins.. I'm a newbie, never have I ever made my own firing program and I'm afraid to do it wrong ... 

  4. 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!

  5. 19 hours ago, GEP said:

    About 1/4 inch thick (6mm) for dinner and salad plates. For larger things, I'll go a little thicker. Keeping them flat is mostly about technique, handling them without too much stress, allowing them to dry evenly,  and firing them on a kiln shelf that is free of any lumps or warping. 

    and you dry them on drying racks or plaster?

  6. 31 minutes ago, GEP said:

    All of the above answers are correct. Having said that, I make my plates without foot rings, because I want to produce them as fast as possible. It can be done. I've never made a spinner, but do occasionally produce rockers, which get sold as seconds.

    Yeah, me too to make them fast, how thick do you make them?

  7. 1 hour ago, Hulk said:

    Hi Titi!


      limit unglazed portion to just the ring - the rest of the underside glazed, hence stronger, easier to clean, etc.

      limit contact surface - easier to make flat (so the pot doesn't rock) and easier to make smooth (so the pot doesn't scratch surfaces)

      provides a place for fingers to catch - easier to handle

      allows for precise thickness - better balance, lighter, closer match to the rest of the pot, etc.

      looks cool

      cut away the least conditioned part of the pot, the layer against the wheel head (or bat) is most likely to start cracking (per above)

      increase durability - ring provides mass and structure at the base, where impacts are likely


      takes time to cut foot rings

      takes more time to glaze

      generates trimmings


      plates are a special case, bein' flat an' all...

    Other other:

      We've had a week of rain and wind here!

    very useful, more pros...



    too special.. Sometimes I just want to give up making plates :/ 

    Other other:

    here only lots of rain!

  8. 46 minutes ago, LinR said:

    If you don't put a foot on them you can get what I call spinners.  The middle of the plate end up being lower than the edges.  This is also a problem of drying but a foot ring gets rid of the  problem altogether.  Lin

    Yep! that has happened! haha I thought the reason could be that the plate was a bit thin... and do you recommend a minimum size of how much tall the ring should be?


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