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I have been having a strange issue with my Cone 6 Black Stoneware Clay from Standard (266). It gets these big cracking bubbles upon glaze firing. I bought this clay to marble with Porcelain (Standard 365) because it had a similar shrink rate. The marbled pieces came out SUPER bubbly, really horrible. I thought it was just poor clay preparation on my part because of the mixing of the two. However it has been happening with pots I make solely out of the Black Stoneware. I prepare this clay the same way I do all my other clay bodies and never really have this issue otherwise. Has anyone else had this problem with this clay? Any advice?

 

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Gas bubbles in the 266. The glaze is maturing and sealing the surface before the gas bubbles from the clay body are released. Most common remedy is to hold top temperature during bisque to allow more of the Sulphur and other impurities to burn out. Also, stack the 266 loose in the bisque to maximize surface area to promote burnout of impurities.

 

Not an uncommon problem with 266 and similar clay bodies.

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I had the same problem.  I like the colour of the dark body PSH- black clay, so I made a slip out of it and I use it with my regular Plainsman cone 6 clay. So far- so good. I bisque to 04 and I have an old bisque kiln with just a kiln sitter so I couldn't soak it at 04. The slip seems to be a good solution but I have also just made some things out of the dark clay without glaze and that works too. My profile picture is of unglazed black PSH and a cone 6 porcelain mixed together.

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Actually you are dealing with bloating. Follow Bruce,s recommendation first and see if that solves the problem. Bloating is caused by a couple of things; even though both are related. High iron/magnesium/ titanium clay ( better known as dark brown clay) has high levels of impurities. If the above recommendation does not fix the problem, then you will need to change your glaze firing schedule. From 2050f, up to 2230f becomes more critical in your glaze fire.

Nerd

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  • 2 weeks later...

If none of the above works, try a one hour soak at around 800C (1470F) during the bisque firing. If organic matter in the clay is the problem, that is when it burns out (and is also before almost anything else in the clay starts melting or ceramic change starts happening). Make sure the kiln lid is cracked or all bungs are out, since the organic burnout process needs plenty of air to get completed.

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  • 2 years later...
On 5/22/2017 at 2:04 PM, dsgeater said:

I had the same problem with Laguna B3 Brown, which is almost black when fired in oxidation.  I was advised to change firing from C6 to C5 and no more bubbles or cracks.

I've been firing to cone 5 and still getting bloating. I've been using a skutt 818 with no kiln vent (just leaving lid propped and top peep out). i slow bisque to cone 04. I've been trying to figure this out for 3 years!! Any other details on your firing methods?

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1 hour ago, demeter said:

I've been firing to cone 5 and still getting bloating. I've been using a skutt 818 with no kiln vent (just leaving lid propped and top peep out). i slow bisque to cone 04. I've been trying to figure this out for 3 years!! Any other details on your firing methods?

Are you using a digital or manual kiln? A manual kiln will be difficult to slow down at the high end of a firing to achieve better burnout. Have you put cones in the kiln to verify that you're getting a true cone 5?

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2 hours ago, demeter said:

I've been firing to cone 5 and still getting bloating. I've been using a skutt 818 with no kiln vent (just leaving lid propped and top peep out). i slow bisque to cone 04

Slow bisque schedule probably isn't slow enough. With dark clays containing high amounts of carbons to burn out the firing really needs to slow down between 1290F through to 1650F, this is the range in which the inorganic carbons burn out. If you go through this range too quickly there will be pockets of carbons that haven't burned out; the clay walls will begin to seal over from the fluxes in the clay and the trapped gasses cannot escape therefore the clay bloats. Bloats might (probably) won't be visible after the bisque fire but the walls of the pot will be sufficiently sealed off to prevent gasses escaping during the final part of the bisque and then again during the glaze fire. Keep the peeps full open until you've passed through 1650F, don't bisque stacked pots inside each other, they need room around them to allow the gasses to fully escape. The other temperature range to not go too fast through is between 300-600F when the organic carbons burn off, 200F/hour through this range should be fine. If you have a digital kiln then program your own bisque schedule, if you use a manual kiln then have a look at the colour of the light inside the kiln to make a rough guesstimate as to what the temperature will be. 

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  • 1 month later...

Hi all, this thread is so helpful, thank you!  I have been working with Sio2 PRNI Black Stoneware and having issues with bubbling, cratering glazes. (cone 5/6 ox) The same glaze tests on their Black Ice Porcelain are perfect but this clay is so soft and very hard to throw large scale work. So, I am understanding that the PRNI has high Fe, Co and Mn content (as per the manufacturer) so might the recommendations above (slow bisque firing/holds through 300-600F and 1290F - 1650F for avoiding bloating also apply to the glaze finish?  (bubbling, cratering) and maybe a hold at the top cone 5/6 temp to allow for bubbles to settle? They noted to use glaze recipes without zinc, but I did test some and these tests also bubbled. There is sadly scarce advice on this on the internets. I tried Cassius Basaltic for a bit but bailed, way worse issues and also no help from the internet or Aardvark on solutions aside from 'try these few commercial glazes that we sell' (don't want to use commercial glazes). I'll of course try the advice from this thread on my next test run myself but wondered if any additional thoughts on this. Thanks! 

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If you’re making large scale work, by which I’m assuming you mean pieces larger than 5-10 lbs of clay, you might have to add even more time to the soak in the bisque. It takes longer for more mass to even out temperature wise. 
When I was first figuring out how to fire even just my red clay, someone gave the advice that if you focus on firing the clay properly, many glaze flaws will be resolved in the process. This is particularly true if the glazes in question don’t bubble, pinhole or blister on other clays. 

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fwiw, have one set of test tiles and a few small test pieces in Cassius Basaltic; the same clear glaze that's working for red clay lays down smooth on the Basaltic as well, "Wollastonite Clear" (Bethany Krull) ...some of the coloured glazes look good as well, some froth up and come out crusty. Next glaze fire will include a few more pieces...

I'm firing a modestly paced bisque - wouldn't call it slow - however, significant pauses at the three temps (per GlazeNerd) 752, 1063, 1500F

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Hi Lesley!

Yeah (runnin' out to studio to check firing notebook ...person I bought antique kiln from advocates keeping a log, aye that), about 30 mins. Manual kiln is fitted with a decent pyrometer; as temp reaches next critical temp, I'm twiddling the switches to keep it near there for half hour to forty minutes, yep! Excepting foot ring - which isn't glazed anyway - most walls are 4-6 mm (dry) or so; I'd go a bit longer for thicker stuff. Oh, sufficient oxygen during bisque firing likely also important; if you're not fitted with a fan, perhaps leave out a peep or two?

Perhaps my bisque fire is a bit slow, haha! I've shortened the run some by coming up to 200F the night before, leaving the kiln fan on all night, then cranking right through and past 212F the next day, as I'm assuming all wet water has been driven off ("bound" water still present, of course).

The source post, hrmm ...aha, copied from another thread:

Detailed article:

https://community.ceramicartsdaily.org/topic/17903-critical-firing-temperatures/

Compilation of links:

https://community.ceramicartsdaily.org/topic/20132-slow-bisque-kiln-help/

Tony Hansen's article:

https://digitalfire.com/4sight/glossary/glossary_glaze_bubbles.html

 

 

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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! 

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Kentucky Mud recommends a very slow ramp from 5 to 7.

I accidentally did this with 266 in a manual kiln and got no or very small bloats.

Seen someone fire 266 to cone 10 in a wood kiln on FB, (that stands for For Bidden.)

Standard has switched the literature to recommend a Cone 4 firing for 266.

Sorce

 

 

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  • 11 months later...
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 ... 

Edited by Titi
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4 minutes ago, Roberta12 said:

@Titi  Could you attach a picture of the piece?  Pinholes might indicate a problem with the glaze.  Large bubbles (bloating) would be an issue with the clay.  

 

Here it is!!

PXL_20210304_174144631.jpg.487eaeb5d8e53645ee1366892f0b10d7.jpg

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4 hours ago, Roberta12 said:

@Titi  Could you attach a picture of the piece?  Pinholes might indicate a problem with the glaze.  Large bubbles (bloating) would be an issue with the clay.  

so you think with a hold on the Glaze fire could solve it?

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