properties of mold materials

[Judi Charlson]

I have found this discussion very interesting and thought I would add the
formula that I have used for over 40 castings that have been sucessful.It is the one given to me by Irene Folic at Corning several years ago.
I make the mold in many layers.
For the layers that are to capture fine detail and touch surface of the glass:
one part water
one part plaster
one part silica
1/2 part talc (added last)

For the outer layers:
one part water
one part plaster
one part silica
one part grog

I have added vermiculite to outer layer at times to make mould lighter.
I also wrap wire (usually twice) around outside .But I have never had a mould break. Small cracks? Yes.
Also, I try to air dry for a day or two.
Then I usually dry for 12 hours in kiln at 200 degrees (longer if wax has been steamed out of mould or if the mould is really big or fresh.)
Usually the mould breaks cleanly away from the sculpture, but sometimes the inner layer wants to stick to the glass. I am not at all sure why this happens.
Judi

[David Paterson]

Judi:

What types of glass do you generally cast with? And what surface finish do you get?

And how hot do you generally fire? There is some agreement that going over 1500 F is really hard on a mold.

Also, are the measurements by weight or volume?

[Bert Weiss]

I have found this discussion very interesting and thought I would add the
formula that I have used for over 40 castings that have been sucessful.It is the one given to me by Irene Folic at Corning several years ago.
I make the mold in many layers.
For the layers that are to capture fine detail and touch surface of the glass:
one part water
one part plaster
one part silica
1/2 part talc (added last)

For the outer layers:
one part water
one part plaster
one part silica
one part grog

I have added vermiculite to outer layer at times to make mould lighter.
I also wrap wire (usually twice) around outside .But I have never had a mould break. Small cracks? Yes.
Also, I try to air dry for a day or two.
Then I usually dry for 12 hours in kiln at 200 degrees (longer if wax has been steamed out of mould or if the mould is really big or fresh.)
Usually the mould breaks cleanly away from the sculpture, but sometimes the inner layer wants to stick to the glass. I am not at all sure why this happens.
Judi

Judi

Do you mean 1 part by volume or by weight? There is a significant difference.

[Lauri Levanto]

1. Do you prefire the molds before casting? That way clay in outer layers could be fired to sintrate and add strength.
My point is that at lower temps the plaster binds, and clay
(kaolin) serves as refractory, at higher temps the plaster breaks down but clay starts to bind.

I have tried paper clay as the outmost layer. It is difficult to dry without breaking.

2. Is it really alumina that sticks to glass at higher tems?
I expected kaolin to be the villain.

-lauri

[Judi Charlson]

Bert,
I measure by volume.

David,
I use BE glass. I usually use various size frit for open face moulds.
To keep color where I want it to be I usually make the mold deep enough that I do not have to add by melting in flower pot. I have found that adding to fill mold to thickness that I want moves colors around.
I also use claer frit to separate some colors.
I fire to 1600.
The finish I get is usually between matt and a dull shine.

Lauri,
I never prefire moulds.
I do not know what paper clay is.
I think the inner layer sticks to glass when I use powder frit and binder
on part touching mould. After soaking with dental clearner it usually comes off. I have sandblasted some pieces where there appeared to by a surface scum .wher it stuck. I think some colors cause this(jade green)
Judi

[ellen abbott]

Lauri

When we do our bowls, oftentimes, the mold is so wet that it drips when moved from table to kiln. If the mold is more than 3 - 4 days old we soak it in water before we start filling. We do some other pieces in dry molds. I treat them the same in the kiln. I make one monolithic mold - no face coat - sometimes in two or more parts. Some of the larger diameters are done with an inner and out shell because they take more material than I like to mix at one time. The inner part being fairly thick. I just try to keep the moldmaking as simple as possible.
At the temperatures we use, kaolin won't bind. Its like trying to make glass with pure sand. Very high temperatures are necessary. Thats why fluxes are added. To facilitate the chemical reactions. Those fluxes and colorants become part of the chemical composition of the glass. When the glass heats up it becomes chemically active. It is fluxing the silica. The hotter and longer this fluxing occurs the more it has a chance to work.

Hot glass is sticky. If you get it hot enough it will stick to anything. Its part of its nature.

Folks

All mold formulas can be fired successfully. How the mold finishes the firing is entirely dependent on the model. Some models are harder on the mold to complete the piece. Our bowl molds are dynamic at temperature. The core is pushing down into the base. These molds are more likely to fail than ones that remain stationary.

marc