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I am planning to kiln cast a solid object about the size of a human head.

I would like advice on the thickness of the walls of the mold. I want something thick enough to hold the weight of the glass without cracking, but at the same time I want to keep the overall dimensions down, to have a reasonable annealing schedule.

Any other advice about a project of this nature would also be appreciated.

Thanks.

Well, sir, I admire your courage suggesting you were going to cast something the size of a human head. Some of the folks who read that will have larger heads than others but, hmmmm, perhaps I shouldn't go there.

Actually, that isn't a big cast in terms of what some folks do. The dimensions are a function of strength while the glass is casting. Mostly, if you allow 1" on all sides and 2" on the bottom you will be well served. You'll need to do some soaking at 250, 1100, process, and I'd suggesting doubling your typical annealing schedule to allow the mold to stabalize at each temperature.

Henry Halem has a great book on these issues and I highly recommend it.

Hope this helps.

Jerry

Thanks for the advice.

Yes some heads are larger than others. But the head I am casting is actually a full size human skull. Death is said to be the great leveller.........

Anyway, I have got to the point of making waxes from a rubber mold that in turn was made from a model.

We took some liberties with the skull: The jaw bone is attached and not a separate piece, the eye sockets are shallow, and of course the model had to be filled with plasticine in a few places to make a solid object.

I got to this point several monthes ago, and the project stalled, as I researched mold materials and other technical issues. I am primarily a glassblower, but did some kiln casting at school.

Any recomendations on annealing and cooling would also be appreciated.

How much will the glass weigh? How is the mold configured for introducing the glass, i. e., large or small funnel-type opening? Or rather, how much glass will be exposed to the kiln after firing.

The skull sits on a short artificial neck (as real skulls have no neck), and this neck sits on a base that is 4 inches square and about 1.5 inches high. This base is the opening on the top of the mold where the glass will be dripped in from a flower pot.

So a 4" by 4" area of glass will be exposed at the top of the mold.

I calculate that I need about 16 pounds of glass to fill the mold.

I am of course worried about the mold breaking and making a big mess. So I am also wondering if I should put some fibreglass in the mold mix, and/or surround the outside of the mold with chicken wire.

I appreciate the help I have got so far - Thanks!

First off let me say I've hijacked Ellen's name and have forgotten to add mine.
As far as your mold is concerned I would add some refractory cement to the mix. This adds to the finishing needed but really increases the mold strength at high temperatures. I'm not a fan of additives to molds unless completely necessary and I haven't found very many that really need it.

I'm a liitle ahead of myself. What are you planning to use for mold material? And what kind of glass? And what kind of kiln?

marc (Ellen's tech guy)

I am planning to use a 50-50 mix of pottery plaster and silica for the mold, perhaps with some added fiberglass or ceramic fibre.

It will be fired in a ceramic kiln with a ramp type controller.

I was planning to use Fenton FHC cullet, but this could change.

If I added cement to the plaster, what would you recomend?

I would use 40 parts plaster/ 40 parts silica/ 20 parts refractory cement.
Since your a blower, you may be familiar with different refractory cements. Try to find one more mortar-like. Uniform particles. But when it comes down to it, I just bought whatever was easy.
I haven't cast Fenton what is the annealing temp?
The ceramic kiln should be fine. Is the controller attached or seperate?

I would use 40 parts plaster/ 40 parts silica/ 20 parts refractory cement.
Since your a blower, you may be familiar with different refractory cements. Try to find one more mortar-like. Uniform particles. But when it comes down to it, I just bought whatever was easy.
I haven't cast Fenton what is the annealing temp?
The ceramic kiln should be fine. Is the controller attached or seperate?

marc (Ellen's tech guy)

The annealing temperature is about 935 F, perhaps slightly higher for a casting.

Fenton is basically just soda lime glass with a few added fluxes, but no lead.

The kiln controller is separate.

I realize that adding a castable cement would make the mold a lot tougher, but what happens after the casting is finished? How do you get the mold off, and what does it do to the surface of the glass?

Also what about putting chicken wire around the outside of the mold?

If you have any suggestions on the wall thickness of the mold, I would appreciate it. Jerry suggested one inch on the sides, but with 16 pounds of glass I am wondering if this is strong enough.

With the confidence given by one year experience in casting, that is 4 successful pieces, I suggest:


Cast the mold in a saggar if possible. That is make a box of
clay and fire it. Then cast the mold into the box. The ceramic outer surface can take the tensile stress caused by 8
ponds of glass.

If the saggar is not possible, then reinforce the mold. With fiberglass (borosilicate type) test first that it does not melt in your casting temperature. If you have to use wire, kanthal
or steinless steel can be used outside the mold. A chicken wire mesh must be baked into the mold, otherwise it is oxidized to powder.

Make a separate splash layer fx. plaster and kiln wash, to get a non-sticking surface.

Do you have proper air escape? Does it make an air pocket
under the jaw or behind the skull when it is turned upside down. Air channels you can make of straws, or simply
use a cotton thread. The cotton burns off and leaves
a channel behind.

If you make a flower pot cast, the risk of air pockets is greater. If you fill the mold with frit, you get more bubbles
and may need to add more glass.

During annealing, the mold is a heat reservoir, and the glass tends to cool thry the open top. You cancounteract by
using only top heating elements in annealing, OR
by covering the opening with a fiber blanket.

The latter can be done when the temp is a bit over
annealing, say 1000F. Open the kiln and quikly inset the
blanket. As a blower you know the safety factors.

Finally -have you considered to blow the skull into a preheated mold?

-lauri
All above is based on numerous different failures. Consider it and make your own decisions.

David Paterson wrote:Any recomendations on annealing and cooling would also be appreciated.

I've done very little casting -- and certainly nothing that size.

I was taugh that when you work out your annealing schedule you need to include the width of both glass and mold.

Assuming 8" for both glass and mold you'd be looking at almost 6 days of annealing cool down if you were working with BE and following their guidelines.

- Paul

My two cents.

I agree with the advice of using a thin face or splash coat. Then add a thicker coat with your refractory cement and or fiberglass additive. I've heard recently, though haven't yet tried it myself, that a handful of cellulose fiber thrown in will give the mold a porous structure that allows channels for water to burn out without causing cracks.

If you really want to support the mold against catastrophic failure, build it square, brace the outside with pieces of kiln shelf and brace the pieces of kiln shelf against the walls of your kiln.

All that being said, why not cast a simple sphere as a test so you don't waist weeks of model making? This stuff takes time and experience.

Sorry David your post came in late and I had to work some today.

The refractory cement gives it strength while at casting temperature. After the mold cools down the silca and plaster should be very weak. A face coat would also work but I don't like those. I try to cast using standard plaster casting techniques. Besides, everybody got their own face coat in addition to mold formulas. I've used chicken wire before. I doesn't have to be baked in and it won't burn up either. After casting you have to cut it apart with a wire cutter to demold.
Casting is complicated because there is lots of steps. The steps don't have to be overcomplicated. I try to keep it simple. I have tried all the complexities that I've run across and I haven't found any that affect the quality of the glass in a positive manner.

I would make the mold 1 1/2 - 2" thick.
Don't use a square if you don't have to. A cylinder works better for mold drying and annealing.

I know I keep forgetting to sign these posts Sorry. marc

Marc:

Thanks very much for all your help. I now know basically what I am going to try on my first attempt.

One question. If I add 20% refractory cement, what should I expect in terms of the surface finish of the glass? I have a sandblaster, and could easily clean the casting up that way, but would prefer not to, and am hoping to get a matte watery effect.

David Paterson wrote:One question. If I add 20% refractory cement, what should I expect in terms of the surface finish of the glass? I have a sandblaster, and could easily clean the casting up that way, but would prefer not to, and am hoping to get a matte watery effect.

If you make a splash coat (as described earlier in the thread) the refractory cement investment mixture won't be touching the glass.

David
Surface quality is dependent on a bunch of different factors too, all related to time and temperature. You can't get a good surface without finishing. The only way is to cast at the lowest possible temperature for the shortest amount of time using the fewest pieces of glass. I don't think your going to be able to cast that much glass that way without some surface reaction. I haven't done the flower pot thing. How do you keep the clay from breaking the glass when it cools down?
I want to add a disclaimer to all this. I've probably cast succesfully 500 times. I've probably done over 800 casts. I consider myself very sucessful.

Ellen - your work continues to blow me away.

Ellen:

Thanks for all the help.

I checked your website out. WOW!

If I get a successful casting, I will email you a picture along with a description of all the technical stuff I did.

Again, Thanks.

as for strengthening the mold - i've seen the liebenskys doing a second mold to support the original one - this is how they make their big castings