Jump to content
Sign in to follow this  
Hawk

bubbles

Recommended Posts

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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 B);) ) 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

Share this post


Link to post
Share on other sites

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.).

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×