[Hawk]

I am running into a problem when I fire some pots. The glaze inside the bowl area is sometimes bubbling up. I am thinking that perhaps the glaze is too thick or at the very least is being applied to thickly. Thanks for any ideas.

[Lucille Oka]

It is quite possible that that is the reason. Also you could be firing too hot and the glaze is boiling. You may have to reduce the temperature of your glaze fire and/or add hold time. Do some testing make a few small test bowls, label them with the glaze you are using and the number of coats to be placed on each with underglaze. Apply thin, thick and thicker coats. When you see the results you can determine how much glaze is suitable for your work.
Other potters may have alternate opinions.

[pricklypotter]

Hawk, on 02 July 2012 - 05:14 PM, said:

I am running into a problem when I fire some pots. The glaze inside the bowl area is sometimes bubbling up. I am thinking that perhaps the glaze is too thick or at the very least is being applied to thickly. Thanks for any ideas.

If it's thick and runny it could be running down the side of the bowl and pooling too thick at the bottom. break a bowl that bubbled and take a look - if this is indeed the case, the glaze will be very thin up top and very thick at the bottom. If it's like that you can thin it down a bit. more hold time could let all the gasses that want to bubble leave, but it could also encourage the running down by adding heat work. I've had this problem recently, bowls were the worst offenders because the shape funnels everything down into a narrow bottom, mugs were better. It was the dregs of that glaze bucket (very thick), now that it's been remade it's fine again.

[Marcia Selsor]

Are you dampening the bowl before you glaze it?
It could be sucking up the glaze too fast on the inside. Then the outside would still go on nicely.
Just a thought.
Marcia

[~janie]

Marcia, are you saying that I should dampen the bowl before I glazed it? Would it help to apply the glaze to the outside of the bowl, graduating the coats at the bottom? I have seen the glaze pool right at the edge of the wax on the bottom. Looks ugly.

[JBaymore]

Another potential factor if the bubbling is located in mostly the very bottom of the bowl, is that it is attributible to what is known as "thermal lag" in the kiln.

As the kiln is heating up, the mass of wares needs to have time to absorb the heat energy from the flames and hot gases or the rediant electric elements. Objects that are more "massive" than others or are further away from the effects of heat transfer take longer than other areas to become hot.

The floor area of a bowl is "shadowed" from effective radiant heat transfer as well as is a bit of a stagnant zone for convective transfer. Add to this fact that for many of us, there is a tendency for bowls to be (ahemmmmmm ) a bit thicker there than what might be the case in the rest of the wall section of the bowl. Then add in the mass of the footring too. And the final and real contributor is the fact that the bowl footring is sitting on the very LARGE thermal mass of the kiln shelf.

So while the cones or the pyrometer is measureing the general temperature of the area near the probe or the heatwork where the cones are located, it is possible that the bottom area of the bowl is a bit behind the rest of the load whre these measurements are taking place.

This means that the glaze in that area might still be going through the "fining out" process... where gasses are still being evolved in the glass and are slowly moving through the mass of taffy-like glass toward the surface whre they eventually break and disappear. When the kiln shuts off you might be catching the glaze not quite to the maturity phase of things.

best,

.................john

[yedrow]

John makes some excellent points as usual.

Another thing to consider is that if your bottoms are real thick they will absorbe more water and have a thicker layer of glaze than the walls. As John said, the thermal mass of the thicker parts plus the thermal mass of the shelving (conducting away heat) lower the actual temperature of that part. There can be as much as 212 deg. difference between the bottom of a medium sized pot and its rim (unless you are using advancers, etc.).

[Hawk]

Thanks you have given me some things to think about.