big bubble dilemma

[jerry keller]

It looks like a volcanic explosion caught in action! The secrets of the ancients rising up to meet modern day glass art ...?
I've tried to cast a 15x21" piece of 1/4" starfire glass in a plaster/silica mold at 350-1250, hold 30, 60-1280, hold 30, then 9999 to 1450 ( and also to 1525), hold until I"m satisfied w. the cast (visual discretion).
The mold has vines on the bottom, the size of a small rope, and nothing on the top half of the mold. There were several small holes I drilled into the plster on the first try - still got a 7" diameter, and very beautiful, bubble. Like a setting sun on the horizon. Great, if it would fit into the grand scheme of the design...
I figured it was OK to use the mold again, which had broken apart from the first cast, and would have a lot of air that could move through the cast for gasses, and got the same result on the second try, at a higher temp (1525). Actually got some more bubbles after about 10 minutes (3 in total).
The mold is sitting on the bottom of the kiln, on sand, and bricked in on all sides. Should I move it off of the floor, so it may not be as much of a heat sink? I also used a pea-sized piece of glass in the corners to let the glass drape, so as to not catch air.
I'm beginning to run out of ideas....may have to turn to prayer....! .any help would be wonderful.
Jerry

[charlie holden]

You don't say how thick the mold is, but in any case, imagine the temperature at the center of the mold where the mold and the sand meet. The heat has to get through the glass and the mold to warm that spot up. Meanwhile the sand is even colder and the kiln bricks underneath are colder yet. The air that is there, whether it has moisture in it or not, is going to expand as it heats. Better that it expands as much as it can before the glass gets very soft.

A rule of thumb is that 1/4 glass in a kiln during heat up is about 15 degrees F and 15 minutes behind the temperature on your thermocouple. Plaster is a better insulator than glass so it probably hasn't caught up after a 30 minute hold, to say nothing of the sand and kiln floor.

I would do two things: Put the sand and mold up on a ceramic kiln shelf and add a long soak around 1050 - 1100 F. (I don't know the softening point of Starphire). If you have to use a fiber shelf or can't move the mold off the floor, then you will have to soak there even longer. The difference between 1250 and 1280 is not significant in my book. Just soak longer at 1250. Get as much slump out of the glass as you can before it really gets liquid.

Maybe try some cellulose in the plaster mix next time for porosity.

ch

[Lauri Levanto]

Thinking thedictionaru. I have thiought casting
is from frit - directly or with flowerpot, and sheet glass is slumping.
Do I interprete the terms right?
-lauri

[charlie holden]

Thinking thedictionaru. I have thiought casting
is from frit - directly or with flowerpot, and sheet glass is slumping.
Do I interprete the terms right?
-lauri

Slumping almost always means the deformation of sheet glass. Other terms sometimes used are sagging or draping. It usually means heating the glass just to the point of changing its overall shape.

The term casting is used in the USA fairly loosely. In architecture they often call textured sheet glass "cast". As a general rule you can say that cast glass is fired hotter than slumped glass with the intention that it take on the shape and surface detail of a mold.

The amount of heat is more important than the type of glass.

ch

[Lauri Levanto]

I'll update the dictionary entries to your definitions
-lauri

[charlie]

Thinking thedictionaru. I have thiought casting
is from frit - directly or with flowerpot, and sheet glass is slumping.
Do I interprete the terms right?
-lauri

Slumping almost always means the deformation of sheet glass. Other terms sometimes used are sagging or draping. It usually means heating the glass just to the point of changing its overall shape.

The term casting is used in the USA fairly loosely. In architecture they often call textured sheet glass "cast". As a general rule you can say that cast glass is fired hotter than slumped glass with the intention that it take on the shape and surface detail of a mold.

The amount of heat is more important than the type of glass.

ch

it's also the process. for instance, you can get both cast or extruded acrylic sheet.