Blistering on Stoneware

I'm still learning, but I solved my blistering problems with a 20 minute soak at ^6 and then a 10 minute soak at 100 degrees lower on the way down. I guess it gives the gases time to get out. No blisters since then. I do have an electronic control, thank heavens.

Tom

I'm still learning, too, after 30 years, and am having a similar problem with a gloss glaze at cone 10--blistering on undersides of plates. What I think is happening is that the shelves retain heat longer, and heat the pot bottoms more. In some instances what Tom says might apply.

Firing cooler would help prevent the glaze from boiling, or try reducing the whiting or other flux. I've found it helpful to brush on a bit of more refractory glaze in the areas likely to be affected before dipping the pot in the glaze which causes it. I've also found that it's likelier to happen when the glaze gets used up closer to the bottom of the bucket (at which point the formula may be changed from being more of the heavier constituents). In this case mixing up a fresh batch can help. Also, unless this glaze you're using has some special properties, you might do better to change to a different recipe with some different materials--some glazes can be too fussy to keep working with... I think I may be facing that myself. Raw materials change from shipment to shipment--so a once reliable glaze can go belly up. Brad Sondahl

re: rafractory glaze: Are you suggesting the same glaze but with the addition of refractory materials?

This is worth considering. It would certainly be easier than trying to analyse the effects of different firing temps. Thanks.

I'm thinking that it may be a combination of glaze thickiness and proximity to radiant heat source (such as the elements or, as you suggest, reflection or radiation from the shelves).

When glazing, I dip pots upside-down and allow them to drain this way. There is always a heavy layer around the rim. Although the rims tend to drain off pretty well in the fire, the are beneath them tend to have the heaviest layer if glaze.

Althought his may also be true of the lining, I don't get blistering in here, and that may be because the lining doesn't get any direct radiant heat.

I am tending more and more toward this solution. My concern with this is: Soaking would allow formed bubbles to break and heal. However, what if the glaze continues to boil at the soaking temp?

Sorry. I just realized that you are soaking at 100 degrees lower in your cooling cycle. This would answer the concern about boiling during the first soaking,

Sorry. I just realized that you are soaking at 100 degrees lower in your cooling cycle. This would answer the concern about boiling during the first soaking,

Then try to soak at a lower temp. That's just a guess, of course. It's all a learning process and experimentation. I'm just now getting into mixing my own glazes. So far so good.

No, I actually meant a glaze you have which doesn't exhibit the problems cited--such as a white or clear glaze. Brad

Hi, I've only been at this ceramics game for fifteen years so i'm also still learning. I don't know if this will be of any help but; I recently had similar problems with a low fire(1100c) glaze and found the problem to be a too rapid firing.

That makes sense in that the offgassing from the body that can cause the blisters is more likely to be finished or greatly reduced with a slow firing--it's also a reason for a higher (cone O5 at least) bisque. In my case, however, it was happening even before I changed the elements, when the kiln was firing much slower, and doesn't seem to be worse for the faster firing new elements gives it. Brad

Man, Brad, that is a WONDERFUL site you have. Very, very helpful. Great tips.

Thanks,

Tom