Glass not filling void in lost wax technique

[Michele-MD]

After not casting in years, I am trying to get back into it. I am doing a basic drip casting from a flower pot reservoir of soda lime glass into a mold created using the lost wax method. I am just trying to make a simple shape, a chicken size egg, pointy end down. One of the problems I am having is that the glass is not completely filling the reservoir in the mold and I assume that is because air must be getting trapped as the glass flows. I did not think a sprue would be needed for such a simple shape? Does it matter how fast the glass flows into the void? Should I either go higher or lower in temperature to change the flow rate? Would it be better to put smaller pieces of glass into the mold void rather than have it all flow from the pot?

2nd question: I am using a basic plaster/silica/water mix, all equal amounts by weight. Besides the soda lime glass I have also been casting float which, according to the schedule I have, needs to go to 1950 degrees. The molds cracked in that firing and I wonder if the higher heat is a factor and should I increase the amount of silica to make the mold stronger when firing float glass? My schedule is a slow ramp at 150 dph with a 2 hour hold at 1250 degs, then 150 dph to 1950 degs. Thanks for any advice you can offer!

[Kevin Midgley]

sprue sprue sprue and more sprue.
High speed may cause cavitation where the glass hits the mold.
Chances are you won't be able to consistently control the speed given the thermal characteristics of different molds.

Float glass? why bother unless you are wanting a devitrified look.

Eye candy here:
http://www.castglassforms.com/index.html

More but different eye candy.
http://higuchis.com/

[Michele-MD]

Thanks, Kevin, for the eye candy, beautiful work! Many years ago I heard the Higuchi's speak at Corning and bought a bag of the plaster they use, just found it in a bin the other day. I hope I can ask for some sprue advice. I know the concept, but not sure how to use one. When I search what comes up is using a sprue as another glass path but I want a vent. Maybe I am mixing up the terms? My idea was to put a wire into the wax form and then pull it out after pouring the mold. I saw a reference that they are made of wax but that seems difficult with a poured mold, maybe need to slip cast for it to work? Sorry for the dumb questions...... Michele

[Kevin Midgley]

Wire if you can pull it out. Straw will burn out as will wax.

[Michele-MD]

Straw sounds like a good plan. I meant to answer your comment about the float glass. I am making "energy" eggs, a variation on an old Chinese tradition. The idea is that the egg sits in water and absorbs negative energy which transfers to the water and is dumped. Lots of negative energy around at the moment, a covid/election project. Thought recycled glass was appropriate for this, have a bunch of tempered, and I like the pale aqua color. There has not been any devit on them but I would just blast it off if there was. But I don't like having to go so hot so that is an issue. it seemed very simple when I started but think I have managed to make it difficult. Thanks again for responding!

[FusedLightStudio]

I am using a basic plaster/silica/water mix, all equal amounts by weight. Besides the soda lime glass I have also been casting float which, according to the schedule I have, needs to go to 1950 degrees. The molds cracked in that firing and I wonder if the higher heat is a factor and should I increase the amount of silica to make the mold stronger when firing float glass? My schedule is a slow ramp at 150 dph with a 2 hour hold at 1250 degs, then 150 dph to 1950 degs. Thanks for any advice you can offer!

You don’t need to go nearly that hot. I cast float at 1625F.

I always do two-layer molds. The first layer is for detail, plaster-silica as you are doing. Second layer is the same with the addition of an equal part of grog added. This layer is stronger because of the grog. I’ve had some very small cracks in the first layer at times, but never the second layer.

[Terry Gallentine]

The two separate coats of investment are important. You need to do an initial face coat of the wax with a plaster and silica flour mix, let it set and then do a jacket coat with a plaster, silica and grog mix. I have had both face and jacket coat crack some but they don't crack in alignment with each other and the result is that they hold together well. The Bullseye Tip Sheet #8 is a pretty good resource for lost wax casting.