Glaze bubbling due to atmosphere?

[tomhumf]

I love this glaze but it can go bubbly during reduction. I suppose it must be something to do with the oxygen levels because it usually happens underneath footings, or at the bottom of thinner, taller cups. I used to fire it in electric kiln and never got any bubbles. 

I don't want to change my firings as my other glazes work well with my methods. 

Can you see anything in the recipe I could change to reduce the bubbling, without totally changing the look of the glaze? 

I'm firing to cone 7

 

				Turquoise
wollastonite				15
calcium borate frit				13
China clay				25
quartz				15
soda feldspar				35
titanium dioxide				7
cobalt oxide				0.1
copper carb				1

[tomhumf]

[tomhumf]

Outsides and sides inside always look fine

[liambesaw]

Kind of looks like heavy iron crystallization

[tomhumf]

Hmm, would the copper encourage that? Maybe I can use a bit less

[liambesaw]

I think the inside is just more heavily reduced? Not sure

[tomhumf]

Right, it's hard to see from photo but the surface on these bits is really pitted, like the glaze has been bubbling or something. Would that be due to too much reduction? 

 

[Magnolia Mud Research]

I would back off on the titanium.  The recipe does not have iron.

[PeterH]

28 minutes ago, tomhumf said:

Right, it's hard to see from photo but the surface on these bits is really pitted, like the glaze has been bubbling or something. Would that be due to too much reduction?

Would pressing a bit of clay/plasticine onto the pitted area and then photographing that be clearer?

[Min]

Thermal lag? Is it worse in the cooler parts of the kiln? I agree with MMR that the titanium could probably be reduced.

[tomhumf]

16 hours ago, Min said:

Thermal lag? Is it worse in the cooler parts of the kiln? I agree with MMR that the titanium could probably be reduced.

Hmm, not really sure. It seems to just be on the bits that are getting the least air. I'll do some tests with less titanium. I guess I could substitute some for zirc sil. 

 

 

[C.Banks]

I'm sure you've considered it but I'd suspect glaze thickness or your clay body which looks speckled?

[tomhumf]

Hmm, I don't think it's glaze thickness. I use it on a lot of wider open items and it's fine. On the underside of plates it's ok outside footring but bad inside the footring.

It is a speckled body, I don't really want to change that though. Hopefully I can tweak the glaze to fix. 

[Bill Kielb]

18 minutes ago, tomhumf said:

Hmm, I don't think it's glaze thickness. I use it on a lot of wider open items and it's fine. On the underside of plates it's ok outside footring but bad inside the footring.

It is a speckled body, I don't really want to change that though. Hopefully I can tweak the glaze to fix. 

here is a thought
Are you doing an early body reduction? Body reductions actually help your clay and glaze play better later in the firing. Areas that are confined will often receive the most reduction so your evidence appears to point to poor performance where the reduction is traditionally highest. Proper body reduction often makes this  issue diminish considerably. Typical decent body reduction starts about 1600 degrees, lasts for about 45 minutes  and is a heavy reduction to get the iron in the clay to play nicely later in the firing.

[neilestrick]

Could be that it's just not getting as hot at the bottom of the cup, especially on the inside. Is the firing in the gas kiln faster than the electric?

[Sorcery]

I find the more protected parts to be less reduced, but I'm not convinced reduction has anything to do with it.

Are the bottoms thinner or thicker than the rest?

Seems more like that thermal mass, being protected, is remaining hot for longer, so the glaze is still bubbling when it finally cools.

Sorce

 

[Bill Kielb]

1 hour ago, Sorcery said:

I find the more protected parts to be less reduce

Inside of covered bowls, wares in sagers nearly always more reduced. 

[C.Banks]

1 hour ago, neilestrick said:

Could be that it's just not getting as hot at the bottom of the cup, especially on the inside. Is the firing in the gas kiln faster than the electric?

if this is the case a soak might help

or it could make it worse but then you'd have more clues anyway

[tomhumf]

I do start reduction around 1600, and I'm pretty happy with how my firing cycle goes at the moment for most of my work. Even this glaze turns out good 95% of the time.

I think I'll just do some tests reducing the titanium then look at other options if that doesn't work. 

I read this on digitalfire.com , hopefully it's ok to quote :

". 6% is pushing the edge of how much titanium should be in a recipe. Any more, or cooling too slow, could transform the surface into a mass of white crystals (which would be rough and non-functional)."

[Bill Kielb]

5 hours ago, tomhumf said:

I do start reduction around 1600, and I'm pretty happy with how my firing cycle goes at the moment for most of my work. Even this glaze turns out good 95% of the time.

Heavy body reduction very often helps the iron in your clays to play better later. It’s a good technique to be aware of.

[curt]

I would be looking at the calcium borate frit.  What chemistry?  Could be a low melter making trouble with the other glaze ingredients...

[tomhumf]

I tried some tweaks to the recipe for this. In the end I've found changing the firing schedule has greatly improved the original recipe. I have added around 30mins going up between 1100C and 1200C, soak for 20mins when the cones drop, then downfire for 20mins. So around 1 hour extra I'm total. The reduction levels are around the same as before. 

[Bill Kielb]

Good to hear. Holds at the top can often make things worse with respect to bubbles. Firing down  has a decent history of helping things heal So maybe include just the fire down part as a test without the hold. (Just a thought) Just curious what this glaze looks like at its best. Have a picture?