Air entrapment

[Terry Gallentine]

I am sifting a mixture of System 96 Frits on a large (24" diameter) flat mold with low relief impressions and I am having some air entrapment problems. I am firing rather slowly so the mold temp doesn't lag behind too much and so that there is time for some organics to burn out well but I am still having a problem. When the temp reaches 1350F the frit starts to fuse together and by 1400F it has fused together enough to form a solid layer. It is at this point that I start to see air forming blisters between the mold and the glass. The blisters are large and will grow until they burst. Is the air expansion coming from air in the mold that as it expands is released into the area between the glass and the mold? Any ideas?

[Bert Weiss]

What is the mold made of? if it is a cast refractory, they can outgas. You might try placing it on a bed of sand, so the gas has an easier path to escape by.

[Terry Gallentine]

Hi Bert--The mold is made of a mix of calcined kaolin, plaster, silica sand and silica flour (an adaptation of the Rousseau mix). I have entertained the idea of a layer of large grain silica sand in between the mold and the kiln shelf to give an escape route for air in the center of the mold and your suggestion means that I will definitely test it out. I have also considered cutting a shallow grid in the bottom of the mold but the sand layer would be easier and weaken the mold less.

[Bert Weiss]

The other solution is to hold at 1350 or 1360 for quite a bit longer. This will keep the glass stiff enough to not blow any bubbles. It may not yield the look you want though. Let us know if the sandbed works.

[Terry Gallentine]

Will do.

[Peter Angel]

Terry do you have a photo of the bubbles that you can post? Pete

[Terry Gallentine]

I will take some pictures in the morning and post them.

[Kevin Midgley]

fire multiple thinner layers so air does not get trapped. The air insulates the glass below.

[Bert Weiss]

fire multiple thinner layers so air does not get trapped. The air insulates the glass below.

Kevin, the refractory mold sends gas out. When the glass is fluid enough, it blows bubbles. I had this problem @1990, casting inch thick bas relief panels. I gave up before I solved the problem. Years later, I learned the sand under the mold mitigation. This allows and easier path out for the gas. With a very thin layer of glass, this might not work as well as with a thicker layer of glass. In that case, prefiring the refractory mold could help.

[Terry Gallentine]

I think Bert is right about the thickness of the layers. I tried a thin layer and it produced more small blisters while a heavier layer produced fewer larger blisters. I plan on trying the sand layer because I think the answer is in allowing the air to escape before it pushes the blister up. The mold was prefired to fusing temps and held together well before I did the fusing firing.