Bubbling glaze, re-firing and now more bubbles

[AnnaVela]

I made three large mugs, glazed inside with store bought Amaco Potters Blue Rutile (cone 5-6). I use mid fire speckled clay, busque fired to cone 04.

After glaze firing to cone 6 a few bubbles appeared on two of three mugs. Inside only as I used a different glaze outside. I patched them with a bit of glaze and re-fired to cone 5.

I did Re-firing on a slower mode, I have old manual kiln, loaded just bottom shelf with re-glazed vessels. Put it on low for 2h, med 2h, closed the lid and peep holes and firing on high till the end. Cooked naturally overnight.

Well, the two troubled mugs are now covered with bubbles in addition to the previous few. Any recommendation how to fix them? I want to try to re-fire them for the third time. Maybe try a different glaze on top? 

[liambesaw]

It'll get more and more brittle the more times you bring it to cone 6.  I'd toss them in the broken pile and start over.  Very rarely have I ever had anything get "better" on a refire.  Most problems get worse.

[AnnaVela]

My friend has her eye set on those that’s why I’m trying to fix them. I’d try one more time and maybe toss them after that ))

[liambesaw]

18 minutes ago, AnnaVela said:

My friend has her eye set on those that’s why I’m trying to fix them. I’d try one more time and maybe toss them after that ))

Bubbles tend to get bigger because it goes from a bubble to a crawl.  But no harm in re-firing them again!  You made them once, you can totally make them again

[Babs]

Try grinding the bubbles, put glaze onto those spots. 

What is your firing schedule?

This could greatlyaffect result.

Slow last ramp,  80degC/hr say,  lower top temp and put in a hold of 15-20 min. may help. Then as Liam says.

Looks like glaze is getting bit thick so only apply glaze to blisters.

Position in kiln can affect glazes too. I'd suggest middlee of kiln.

Are your cones saying not overfiring though those blisters related to  body speckles?

[AnnaVela]

19 hours ago, Babs said:

Try grinding the bubbles, put glaze onto those spots. 

What is your firing schedule?

This could greatlyaffect result.

Slow last ramp,  80degC/hr say,  lower top temp and put in a hold of 15-20 min. may help. Then as Liam says.

Looks like glaze is getting bit thick so only apply glaze to blisters.

Position in kiln can affect glazes too. I'd suggest middlee of kiln.

Are your cones saying not overfiring though those blisters related to  body speckles?

My reply got lost somehow yesterday ))

I did grind the bubbles and ready to go. I have 100% manual kiln without thermocouple, so I’m not sure how to slow the last ramp and hold, how to know when to start etc. I use witness cones and check the approx.color of the kiln inside to check how firing goes. 
Kiln fires pretty exact, I didn’t notice any overfiring based on how witness cones bend.

I now gave another idea. What if I I use low-fire glaze on bubbles and fire them to cone 06? I have some low fire glaze remains, so could use a bit. 
considering that mugs were already fired twice to cone 6 and 5, the clay is vitrified and won’t have issues with leaking. What do you think?

 

 

[Callie Beller Diesel]

I think your glaze was possibly overfired to start with, which would also explain why the bubbling got worse the second time. Some glazes are better at cone 5 than they are at cone 6. I'm sorry, but I think adding a low fire glaze and firing a third time is only adding more flux to the mix, and will add to your problems rather than help. There's also a good possibility that the first glaze will be only partially melted at cone 06, and be an even bigger mess.

 You are officially at the point where it's going to be less work to remake them than to keep trying to rescue these ones. If it's any consolation, we've all been there!

[glazenerd]

I see a dark or red bodied clay from the pictures. Every potter has their definition of pinhole; but I would put those in the blister category. I can see bare clay in the center, with raised rims; both signs of gasses escaping under pressure. I suspect these were fired at a medium,or fast speed during bisq from .1250 up to 1750F. Which leads me to believe this is off gassing from iron disulfide, that will produce this type of blistering. 

[AnnaVela]

1 hour ago, glazenerd said:

I see a dark or red bodied clay from the pictures. Every potter has their definition of pinhole; but I would put those in the blister category. I can see bare clay in the center, with raised rims; both signs of gasses escaping under pressure. I suspect these were fired at a medium,or fast speed during bisq from .1250 up to 1750F. Which leads me to believe this is off gassing from iron disulfide, that will produce this type of blistering. 

What would be your solution to avoid it in future? I’m surprised that one of three mugs turned out perfect and other two had bubbles. Same clay body, same application process etc. 
The only difference I could think of is position in kiln and maybe bisque sponging (maybe one was more wet/dry than the others).
The clay is light yellow with speckles. The reddish color near the lip is due to reaction with the other glaze, which was coated outside and on the lip.

[liambesaw]

Oh, manganese speckled clay will pinhole quite a lot, even if it's well bisqued.  The manganese dioxide or carbonate violently decomposes into gas and manganese, and actually is known to even spit manganese and glaze around in the kiln.  Same issue as dark clay, solution is possibly long hot bisque to 04, although I still get pinholes on a lot of my speckled pieces

[AnnaVela]

9 minutes ago, liambesaw said:

Oh, manganese speckled clay will pinhole quite a lot, even if it's well bisqued.  The manganese dioxide or carbonate violently decomposes into gas and manganese, and actually is known to even spit manganese and glaze around in the kiln.  Same issue as dark clay, solution is possibly long hot bisque to 04, although I still get pinholes on a lot of my speckled pieces

Yes! I was surprised to see some tiny spots of glaze around the kiln. I’m using this body a lot and it does produce pinholes often, but nothing like those huge blisters. I guess this glaze is just not a good fit for this body.

[glazenerd]

20 minutes ago, AnnaVela said:

Yes! I was surprised to see some tiny spots of glaze around the kiln. I’m using this body a lot and it does produce pinholes often, but nothing like those huge blisters. I guess this glaze is just not a good fit for this body.

Anna:

given the current blister population (few) you are close to resolution. You had one clear, two pieces blister- very close. Inorganics in dark/red body clays burn out from 1250-1750F. Program a firing segment at 108F climb between these temperatures; or just select slow bisq cycle. (Most are 108F an hour climb). If the kiln is heavily packed; open a peep hole to supply oxygen- also important.

Tom

[Joseph Fireborn]

As someone who has worked with a bazillion dark clays, almost every single one commercially on the market, find another clay body that has similar looks but is formulated better. It isn't worth the time to try to figure this bisque and results out because of the poor quality of clay ingredients. Honestly, you can search through my nightmares with bloating here somewhere. I did everything humanly possible and they would still randomly appear on my work now and then, it just isn't worth your time. Before you do search for another clay, borrow someones white clay or porcelain/white clay and fire this same glaze on it; then alongside the porcelain fire some new works with clay pictured and see if the dark clay bloats and blisters and the white/porcelain clay doesn't. If it doesn't then you instantly narrow down that isn't an overfiring issue but a clay issue, or a clay/glaze issue combined. Either way this is the place to start before you start modifying all your bisques and firing schedules without simply trying this first.

[AnnaVela]

26 minutes ago, Joseph Fireborn said:

As someone who has worked with a bazillion dark clays, almost every single one commercially on the market, find another clay body that has similar looks but is formulated better. It isn't worth the time to try to figure this bisque and results out because of the poor quality of clay ingredients. Honestly, you can search through my nightmares with bloating here somewhere. I did everything humanly possible and they would still randomly appear on my work now and then, it just isn't worth your time. Before you do search for another clay, borrow someones white clay or porcelain/white clay and fire this same glaze on it; then alongside the porcelain fire some new works with clay pictured and see if the dark clay bloats and blisters and the white/porcelain clay doesn't. If it doesn't then you instantly narrow down that isn't an overfiring issue but a clay issue, or a clay/glaze issue combined. Either way this is the place to start before you start modifying all your bisques and firing schedules without simply trying this first.

Sounds great! I actually have a few different clay bodies, so I will test the glaze with all of them. I really like the look of speckled clay, but it’s very unpredictable, so I guess I’ll continue my search For a perfect speckled body.