compatible glaze and clay

[rebs]

hello potters! 

wondering if anyone can help. i have recently tried a glossy white (store bought but from powder) dipping glaze on black clay. the result was the whole glaze surface in tiny small ‘bubbles’. i asked my supplier and he thought there might be something in the black clay interfering with the white glaze. i can’t find any information on ingredients on the clay suppliers site about the clay. the glaze turnes out perfect on white and specked clay with the exact same firing temperature as the black clay. does this sound recognizable to anyone? i don’t make my own glazes (yet) so, it has to be a white store bought glaze. or maybe someone can explain what is the problem?  can different clays with the same firing temperature need different temperature to mature the same glaze? hope this made sense, in advance thanks a lot! 

best regards, rebecca. 

Edited November 12, 2022 by rebs

[Bill Kielb]

Yes, although there can be similar  compatibilities with respect to clays, there can also be differences as well. In general most everything is always tested first using test tiles to be sure they will work together. Clay and glaze are at the top of the “test first” list. The test is for compatibility in melt, glaze fit and then I would say aesthetics. As you explore pottery you will find that temperature is really a general guide and cones are more specific to how things melt and if they have fully melted, under melted or over melted.

Post a picture of your glaze defect and you will likely get questions about what cone were they fired to? what cone was the clay bisqued to? How long did the bisque take?……..  The defect between compatible products often points to another need.

So your description of the defect points to excess off gassing of the clay which leads me to ask, how long do you bisque and to what cone do you bisque to? Along with the usual, what cone is the published maturity of the clay and glaze?
 

 

Edited November 12, 2022 by Bill Kielb

[rebs]

@Bill Kielb

thanks a lot! i actually tried to post a picture, but got a message the file was too large and wasn’t able to shrink it.. 

i have to say i do always test my glaze. but usually i take a chance and try right away as well. this was my first time with the black clay, and i am a restless soul so the test-tile was in the same fire as the ‘test-plate’..hehe. 

 

the bisque was fired to a cone 05 with a two hour preheat and a slow program. not sure exactly how long the fire is though. 

 

the clay has a range between 1000 and 1240 degrees celsius and the glaze 1020-1200 degrees celsius. i slow glaze fired with a max temperature of 1203 degrees celsius i think, with a 15 minute hold. the guy in the ceramic shop told me to try a little lower glaze fire temperature next time because of the max temperature of the glaze, so i’ll try that. i also thought i would try a bit higher bisque temperature as well to let out more gasses. 

thanks!

[rebs]

@Rockhopperthanks a lot! didn’t think to search ‘black clay’. that’s super helpful. but then a higher bisque temperature and possibly an even slower fire may help. and i’ll research. or else hoping the dark-clay folks will chip in.  

[Bill Kielb]

3 hours ago, rebs said:

max temperature of 1203 degrees celsius i think, with a 15 minute hold.

Hmm, 1200 is cone 5-1/2 and with a fifteen minute hold probably a solid 6 or 6-1/2 so firing lower definitely something to try. Peak temperature is important in a bisque firing, but how long is super important as well to burn out all organics.  Often  especially for dark clays.

[Min]

17 hours ago, rebs said:

i actually tried to post a picture, but got a message the file was too large and wasn’t able to shrink it.. 

Try emailing it to yourself, that should automatically shrink it down then post the emailed picture.

It's the time spend in certain temperature ranges that is important to note when bisque firing dark clay bodies. Need to have time for the gaseous materials to burn out. Chart below from Steve Davis (link here to an article with it) showing a schedule to help prevent glaze faults. Between 1290F - 1650F is when you need to really slow the firing down. Also, don't overload / cram the bisque firing full, clay needs lots of airflow around the pots. This is a very long schedule, reduce or eliminate the first hold time if your pots are dry to start with.  Might be able to shorten or speed up some of the other segments depending on your results.

Edited November 14, 2022 by Min
added a link

[Callie Beller Diesel]

The red clay I use likes a slow, well ventilated bisque. I take it to cone 04, but I think the slow part is more important. The black clays I’ve tried all seem to be even more picky than my red. In my experience, a way to hedge your bets is to also do a slow first part of your glaze fire, similar to the description in Min’s post.

[Hulk]

Air flow/ventilation (adequate oxygen) almost certainly important!
Hence, powered kiln ventilation, else, leaving the top peep out.

My black clay experience (one bag) indicates slow bisque, thin sections (not thick), and glaze fired just to target cone, and not even a wee bit more, for the fizzing/bubbling ramps up quickly!

Bisque, holding at about 1500F for a half hour or more seemed to make a big difference in eliminating bloat and reducing fizzing/bubbling.
I'll hold on the way up and down, particularly for red and buff clays.

Holding ~100F below glaze fire peak for half hour or so seems to help with the bubbles/fizzing as well, perhaps giving the glaze a chance to heal over
Drop-and-Soak Firing (digitalfire.com)

[rebs]

@Bill Kielb i’ll definitely try lowering the temperature. and after searching the forum a lot the slow part seems to be the number one tip to success with the black clay. thanks!

[rebs]

@Min

thanks a lot! also for the chart. i actually tried the e-mailing to myself (after advice from hubby) before giving up, but still same default message. hmm. 

i have been searching a lot in here and what i found is just what you say; holding at certain temperatures. and sloooow bisque in general it seems. i will definitely try that! (just need to figure out how to set a program on my kiln manually). i was aware of the need of oxygen, so at least nothing has been crammed in the bisque. thanks again! 

[rebs]

@Callie Beller Diesel

thank you!

i actually had the same problem with a red clay a while back, but asked my supplier and he told me it probably wasn’t a good match to the glaze. (the same glaze as this time). i didn’t give it any more time or try to figure it out; but then this might be the exact same problem?

the slow part (in both fires) seems to be key. but hold ‘starting’ at a higher temperature in the glaze fire?! thanks again.  

[rebs]

@Hulk

thank you!

my kiln is not vented (but in a big vented space). previously i have only fired with all peepholes open with good results. this was actually my first ever fire where i closed both (side) peepholes after a ‘cold spot’ on a bowl my previous glaze firing.. but holding longer at the top temperature (with the peepholes open) during the glaze fire would maybe prevent this? or else how do one prevent this with all peepholes open? 

my pieces are relatively thin i think, hand built and about 0,4/0,5/0,7 cm (it differs a bit)

looks like i need to save them up and fire only my black pieces (alone) together. ..and figure out how to make a ‘black-clay-program’ on my kiln.

[rebs]

20 hours ago, Hulk said:

glaze fired just to target cone, and not even a wee bit more, for the fizzing/bubbling ramps up quickly!

@Hulk

this might be a stupid question, but what is target cone? 

 

[Hulk]

Hi rebs,

By "target cone" I mean the heat-work you intend to complete.
We could just say "peak temperature" - however, not exactly the same as heat-work.

Heat-work is achieved through time and temperature.
Look at a pyrometric cone chart.
Note that the temperature required to bend the cone is higher when the rate is higher.

...where Cone 5 is achieved at 2118 °F when the rate is 27 °F/hour (slow).
At the higher rate, 108 °F/hour (fast!), 2167 °F is required.

I shoot for a 2112 °F reading on my pyrometer, no hold, for that drops a Cone 5 cone reliably.
My kiln doesn't heat particularly fast.
My pyrometer might be a bit off as well; what matters, I believe, is repeatable results - good results, preferably!!
Verifying with cones can be very worthwhile.

From an Orton cone chart:
"Cones do not measure temperature alone. They measure heatwork, the combined effect of time and temperature. The role that heating rates have on the endpoint temperature is observed to be that the temperature required to cause a cone to bend will be higher for faster heating rates and lower for slower rates."

The black clay I've used (Aardvark Clay's Cassius Basaltic* - now "Obsidian") is labelled as a "Cone 5" clay.
I found the clay fired nicely at Cone 5, however, when fired just a bit higher - where the 5 has bent past the 5 o'clock position - the problems compounded!!

I'm using a white stoneware that is advertised as a "Cone 5" clay. It tolerates firing to Cone 6, no problems.
I've used a red stoneware that doesn't tolerate firing over Cone 5½, if that - better at Cone 5.
The red stoneware I'm using now will tolerate Cone 6, no problems.

Some clays can tolerate firing a half cone higher; some clays can tolerate firing even higher yet.

Your black clay may be sensitive to over firing.
Check with cones, keep detailed notes!
If you are depending on the controller+pyrometers only, I would like to suggest that you check the actual heat-work with pyrometric cones.
Set cones at each level, note the heat-work achieved and correlate to the results you see.

* (Formerly Cassius Basaltic) Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5.

Edited November 16, 2022 by Hulk
a bit more detail!

[Rae Reich]

5 hours ago, rebs said:

previously i have only fired with all peepholes open with good results. this was actually my first ever fire where i closed both (side) peepholes after a ‘cold spot’ on a bowl my previous glaze firing.. but holding longer at the top temperature (with the peepholes open) during the glaze fire would maybe prevent this? or else how do one prevent this with all peepholes open? 

Usually, peepholes only remain open in the early part of the warm-up, until all moisture has been driven from the wares, then the plugs remain in place for the remainder of the firing. 

[Callie Beller Diesel]

On 11/16/2022 at 5:38 AM, rebs said:

but then this might be the exact same problem?

Almost certainly. It could be worth going back and playing with it. But I’m biased. I do a lot of white glaze on dark clay, because I like the contrast.

If you want to narrow it down properly, change only one variable in the firing cycle at a time. Try the slower bisque, and see if that makes some or any improvement. If that helps but doesn’t fix all of it, you could then look at glaze firing holds. 

[rebs]

@Hulkthank you so much for the thorough explaining of the cones and heat work and the chart. (puh, that stuff is best read when you are well rested i think, hehe.) i have understood that holding can give a result as if it is fired higher, but how much is a different story. this is really helpful. i have been meaning to buy some pyrometric cones, and now my supplier was out, but will definitely do! thanks again!

[rebs]

@Callie Beller Diesel  agreed, the contrast black clay/white glaze is really nice.  

thank you! that’s smart. i will try slooowing the bisque first then; hopefully that will make a difference.

[rebs]

@Bill Kielb @Min

i figured out the shrinking of the photo.  it doesn’t look like ‘regular’ pinholes does it? 

[Bill Kielb]

Looks fizzy, under melted, over melted (toward drying up). I would definitely get the extended bisque going, I think everyone has mentioned time now helping with burning things out. You can spend lots of time in the 1500 - 1700 degree range and not worry so much about going into cone 04, 03, 02 direction which is usually an issue with holding at peak. The last 200 - 250 degrees should be in the 100f per hour range just to ensure even heating and a reasonable amount of heatwork to hit your planned cone. Bisque products are sintered so not necessarily a fluxed reaction.

I would suggest trying an extended bisque on test tiles and for the test glaze firing truly hit the desired cone for clay and glaze, not exceeding the maximum cone for either. If that provides a mostly good result then a drop and soak can help heal any minor imperfections that might remain. Right now there are too many unknowns, maybe get rid of them one at a time holding other things constant such as cone fired to, and note the level of improvement.

Edited November 21, 2022 by Bill Kielb

[Callie Beller Diesel]

Drop and hold does work for bisque too, if you feel like it hits temperature too fast.