Firing schedules to fix offgassing of ash glaze (^6 ox)

[Northern Pots]

Hi everyone, 

I've been tweaking a ^6 ash glaze for some time now and when it works, its doing pretty much what I want it to: thick, iron brown glassy drops with a slight rivulet effect and beautiful crazing. Unfortunately on the areas of glaze build up I get these awful white scabs or bubbles. I'm assuming they're caused by offgassing of the glaze. 

I'm already using a Cone 6 with a  30 min soak, and a drop and 45-60min hold firing pattern, taking the firing to around cone 8. I'm loathe to soak at the lower temp any longer for the sake of my elements, and dropping to a lower top temp with the same pattern didn't seem to help the bubbles, it just made the glaze a little drier looking. 

Does anyone have any experience of this type of bubbles coming from their glaze? They look so different to the normal blisters and pinholes that people seem to experience that I'm not sure what to try - they seem to occur where the glaze drips or where its been double dipped.

I'm wondering whether a longer soak at the dropped temp or a slow cool down to around cone 04 might do it, but I'm wary of sacrificing the glazes gloss. 

Any suggestions welcome, this glaze has been in the works a while and it seems so close to working, just need some help getting it over the line I think!

Recipe 

Wood Ash, unwashed      56

Kaolin                                        19

Silica                                           14

Frit 3124                                  11

[Northern Pots]

On 3/14/2021 at 5:30 PM, neilestrick said:

In order to get the same results from my 3 kilns, which are all different sizes and cool at very different rates, I do a controlled cooling of 175F/hr from peak temp (cone 6) down to 1500F. Cooling like that from the peak rather than crashing down a little first does add a little bit of heatwork, maybe 1/2 cone. My glazes like that, though. In terms of seeing a difference in your glazes due to slower cooling-crystal growth, etc- most of that takes place from about 1900F to 1450F, so crashing down is fine.

From this post @neilestrick could assume that a slow cooling to a temp above 1900f / 1040c shouldn't unduly matt my glossy glaze? Thank you 

[neilestrick]

2 hours ago, Northern Pots said:

From this post @neilestrick could assume that a slow cooling to a temp above 1900f / 1040c shouldn't unduly matt my glossy glaze? Thank you 

Correct. And it doesn't have to be anything super slow. 175F-200F/hr cooling rate would slow it down enough to mimic the big kiln and not affect things. Of course, nothing is definite in ceramics so test it before jumping in full on.

[Northern Pots]

2 minutes ago, neilestrick said:

Correct. And it doesn't have to be anything super slow. 175F-200F/hr cooling rate would slow it down enough to mimic the big kiln and not affect things. Of course, nothing is definite in ceramics so test it before jumping in full on.

Thanks Neil. I'll test and report back. Haven't been able to find any glossy, drippy ash glaze recipes for ^6 online so I'll share once this one is working smoothly.

[GEP]

Based on my somewhat limited experience with ash glazes, this glaze looks underfired in the places that are not rivuleting or crackling. The foamy stuff might not be offgassing, but simply unmelted glaze. Perhaps increasing the cone 6 fluxes would help?  

[Northern Pots]

17 hours ago, GEP said:

Based on my somewhat limited experience with ash glazes, this glaze looks underfired in the places that are not rivuleting or crackling. The foamy stuff might not be offgassing, but simply unmelted glaze. Perhaps increasing the cone 6 fluxes would help?  

Gep thank you, I thought the same, but on the more extreme examples you can see these white foamy patches turn into big bubbles that you can pierce with a needle tool! (Maybe the unmelted glaze producing bubbles later in the melt)

 

I will bump up the frit in the recipe and see if you are right! 

[GEP]

I see, then here’s another possibility to consider. Sometimes bubbles in glaze are caused by overfiring by a lot. The bubbles are a sign of boiling, not offgassing. So the answer might be to dial down on the melt. In this case, the other areas of the glaze look too dry because the glaze ran too much and left behind only a  thin a layer of glaze in those areas. 

[neilestrick]

On 9/17/2021 at 10:27 AM, GEP said:

The foamy stuff might not be offgassing, but simply unmelted glaze. Perhaps increasing the cone 6 fluxes would help?  

Too me the milky areas look like there's not enough silica in the glaze for the calcium to work on. It would also explain all the other things going on with bubbles and such. It's probably got a very narrow firing range, and slight changes in firing temp cause it to behave differently, hence sometimes it's good and sometimes it's not. I would increase the silica until you find the sweet spot where it stabilizes but still runs.

[Northern Pots]

3 minutes ago, neilestrick said:

Too me the milky areas look like there's not enough silica in the glaze for the calcium to work on. It would also explain all the other things going on with bubbles and such. It's probably got a very narrow firing range, and slight changes in firing temp cause it to behave differently, hence sometimes it's good and sometimes it's not. I would increase the silica until you find the sweet spot where it stabilizes but still runs.

This sounds likely. I've already line blended silica from 0 - 14 in 2% increments and the milky patches did get better each time. I'll keep upping it.

Would adding feldspar provide a more balanced variety of oxides to form a glass and stop it misbehaving? Im not sure I can afford to keep packing a kiln with glaze tests to find a narrow temp range where it works.

4 hours ago, GEP said:

I see, then here’s another possibility to consider. Sometimes bubbles in glaze are caused by overfiring by a lot. The bubbles are a sign of boiling, not offgassing. So the answer might be to dial down on the melt. In this case, the other areas of the glaze look too dry because the glaze ran too much and left behind only a  thin a layer of glaze in those areas. 

The schedule at the mo is firing up to ∆6, holding half an hour, dropping to 1140C, holding for hour, which is why I was dubious at slow cooling to fix blisters so I think you're probably right here! 50% ash and 11% frit should be more than enough for a good melt at almost ∆8 right...

Maybe reducing the soak and lowering the drop temperature would stop the boiling.. 

Too many variables!! Determined to make this glaze work though. 

[Northern Pots]

Update, 

Ive just laid out the line blends I did with silica, on 2 different stoneware bodies  - not only do the milky patches decrease with increased silica, but on the terracotta/stoneware blend (which usually is quite refractory for glazes and makes glazes dry), there are almost none of these foamy patches at all. 

Perhaps the more refractory body is preventing the boiling effect. 

[Northern Pots]

I followed both your advice and got some great results - I added silica and reduced the firing temp. Lovely runny ash glaze that looks like it could be straight from a gas kiln. Will send pics later on. Thank you both