Blistering

[ayjay]

This bowl below has an ash glaze on it, I may have sprayed it on too thick, not sure, it didn't blister when I tested it in a previous firing. Nothing else in the kiln was blistered. 

 

On one item I used the same glaze around the rim and it's fine so I don't think it was fired too fast, but maybe it was too fast for this glaze at thickness.

 

Any suggestions for a future cure is what I'm looking for, I want to keep the runs but lose the blisters (if anything can be done with this bowl it may be worth a try, but I'm not expecting much in that respect).

 

 

 

Glaze is Val Cushing G4 Ash :- 

 

Washed ash (pine) - 50

Ger.Bor -       -   - -  -20   (I subbed Standard Borax Frit)

Whiting   -  - - - - - - 12

China Clay - - - - - -  8

Flint -  - - -  - - - -- - 10

Cob. Carb  - - - - - -  1

Cop Carb. -- -   -- - -6

 

Sprayed over a light coat of Strontium CM Warm and also a Pearly white glaze just to the central area.

 

Electric ^6 @100°C p/h <1100°C -  @ 75°C p/h <1210°C and a 15 minute hold. Natural cool down.

 

 

 

[neilestrick]

There are several possible reason for the blistering:

 

1. The Frit you used is not the same as Gerstley Borate, and therefore is not going to react the same. The only direct substitute for Gerstley is something made specifically to be a substitute, like Gillespie Borate or Laguna Borate.

2. Glaze too thick, although I doubt it

3. Glaze doesn't like the other glazes that are under it.

4. Ash glazes are quite unstable. It could be that you need to adjust the glaze. I would start by adding small amount of silica and/or alumina. Too much and you'll lose the rivulets, but you may be able to find a balance. All ash is different, so just because you used washed pine ash doesn't mean the glaze will work. The degree to which you washed it and the makeup of the wood used to make the ash will all affect the melt. Personally, I would revise the recipe to a fake ash glaze to avoid the inconsistency of real ash.

5. Kiln temp was not correct.

[timbo_heff]

or it cooled too quickly and the molten glaze froze while bubbling rather than settle down first.

Sometimes if you run a very light weight load in the kiln it will cool too quickly since it does not have the thermal mass to cool slowly.

Was it a light load?

[neilestrick]

or it cooled too quickly and the molten glaze froze while bubbling rather than settle down first.

Sometimes if you run a very light weight load in the kiln it will cool too quickly since it does not have the thermal mass to cool slowly.

Was it a light load?

 

The bubbles should settle down before peak temp is reached. If that's the case then the kiln didn't get hot enough or the glaze needs to be tweaked for a lower temp.

[ayjay]

There are several possible reason for the blistering:

 

1. The Frit you used is not the same as Gerstley Borate, and therefore is not going to react the same. The only direct substitute for Gerstley is something made specifically to be a substitute, like Gillespie Borate or Laguna Borate.

2. Glaze too thick, although I doubt it

3. Glaze doesn't like the other glazes that are under it.

4. Ash glazes are quite unstable. It could be that you need to adjust the glaze. I would start by adding small amount of silica and/or alumina. Too much and you'll lose the rivulets, but you may be able to find a balance. All ash is different, so just because you used washed pine ash doesn't mean the glaze will work. The degree to which you washed it and the makeup of the wood used to make the ash will all affect the melt. Personally, I would revise the recipe to a fake ash glaze to avoid the inconsistency of real ash.

5. Kiln temp was not correct.

1)  I use it regularly in place of both GB and FF3134, it's not been a problem before, although I appreciate that the chemistry is different.

 

2) This is still my best guess.

 

3) There's only a light (airbrushed coat) of SCM under the ash glaze, and the same is on the pic later in this post, but a much lighter coat of the ash glaze.

 

4)I'll give it a try - would 2 or 3 tests at 5% increments be about right? Should I reduce the ash by similar amounts or is that too much of a change?   Silica is straightforward enough, what's  best for the alumina? I already have  Alumina Hydrate, China Clay and Ball Clay - is there a best option?

 

5) I'm 100% confident the kiln temp was correct - no cones in this firing but I do use them regularly and they have always been spot on.

 

One thing I'd like to understand  is the actual process of when the bubbles were formed, the rivulets going down to the centre are as smooth as a babies bum, did they run before the rest blistered or after?  

 

The rivulet effect has all but disappeared from the top edge of the bowl - does this indicate that I should be firing this cooler or adjusting the melting temperature upwards somehow?

 

This goblet has the same SCM as a base and another glaze over that and then a light coat of the ash glaze, just brushed on under the rim, there's no hint of any blistering.

 

[ayjay]

or it cooled too quickly and the molten glaze froze while bubbling rather than settle down first.

Sometimes if you run a very light weight load in the kiln it will cool too quickly since it does not have the thermal mass to cool slowly.

Was it a light load?

 

It was a fairly light load, there were another two large (but shallow) items (one apiece on a shelf - and no room for anything else) and the highest shelf (at the centre of the kiln's height) was full of smaller items.  The kiln temperature drops at about 100°C per hour.

 

I know the theory of the *frozen* bubbles, but these look worse than that, there doesn't appear (to me) to be anything there to heal over with a slower cool.

[neilestrick]

In my experience, blistering is a chemistry issue, not an application issue. However it could be that the thickness in relation to the other glazes is a problem, because layering glazes changes the chemistry as they melt together. Have you applied it thick on its own to see how it behaves?

[neilestrick]

If you want to try adding silica I would do 2 or 3% increments. To add alumina alone, you'll have to do the increase in the unity formula, because adding clay will also increase the silica. Alumina hydrate/oxide is not a good way to add alumina to a recipe. But you could just try adding clay and see what happens. It's all worth a shot.