Air entrapment
Moderator: Brad Walker
-
- Posts: 190
- Joined: Mon Jul 19, 2004 2:08 pm
- Location: River Falls, WI
- Contact:
Air entrapment
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?
-
- Posts: 2339
- Joined: Tue Mar 11, 2003 12:06 am
- Location: Chatham NH
- Contact:
Re: Air entrapment
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.
Bert
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
-
- Posts: 190
- Joined: Mon Jul 19, 2004 2:08 pm
- Location: River Falls, WI
- Contact:
Re: Air entrapment
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.
-
- Posts: 2339
- Joined: Tue Mar 11, 2003 12:06 am
- Location: Chatham NH
- Contact:
Re: Air entrapment
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.
Bert
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
-
- Posts: 190
- Joined: Mon Jul 19, 2004 2:08 pm
- Location: River Falls, WI
- Contact:
Re: Air entrapment
Will do.
-
- Posts: 186
- Joined: Fri Oct 31, 2008 9:26 am
- Location: Newtown, Sydney, Australia.
Re: Air entrapment
Terry do you have a photo of the bubbles that you can post? Pete
Peter Angel
http://peterangelart.blogspot.com/
A bigger kiln, A bigger kiln, my kingdom for a bigger kiln.
http://peterangelart.blogspot.com/
A bigger kiln, A bigger kiln, my kingdom for a bigger kiln.
-
- Posts: 190
- Joined: Mon Jul 19, 2004 2:08 pm
- Location: River Falls, WI
- Contact:
Re: Air entrapment
I will take some pictures in the morning and post them.
-
- Posts: 773
- Joined: Mon Mar 10, 2003 11:36 am
- Location: Tofino, British Columbia, Canada
Re: Air entrapment
fire multiple thinner layers so air does not get trapped. The air insulates the glass below.
-
- Posts: 2339
- Joined: Tue Mar 11, 2003 12:06 am
- Location: Chatham NH
- Contact:
Re: Air entrapment
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.Kevin Midgley wrote:fire multiple thinner layers so air does not get trapped. The air insulates the glass below.
Bert
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
-
- Posts: 190
- Joined: Mon Jul 19, 2004 2:08 pm
- Location: River Falls, WI
- Contact:
Re: Air entrapment
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.