New Metal-Drip Method (soon to be New & Improved!)

In our previous aluminum casting session, and even in the earlier pewter ones, we often encountered this problem where the pour hole for the Venus is too narrow and won't allow the material to get in. Even when making wax copies this can be a problem. The void fills up with hot gas, creating an air pocket, and then the metal can't get in.

(A Venus attempt gone very wrong.)

I wanted to make an aluminum Venus and turned the issue around in my head for a time, thinking that maybe the mould needed a vent to allow the air to escape, but could this really be necessary for such a small form?? etc. Another issue was how to keep the mould hot while the aluminum melted in the kiln, in order to get a really good casting - not to mention not cracking the mould through thermal shock.

(A fine example of the difficulty of casting into a small opening.)

And then one day it hit me! What if we made the mould with a spot up top where the aluminum could sit and then melt and drip into the mould directly? So the mould and the metal are at the same temperature and everything becomes one! Cosmic, poetic and perhaps even efficient!

So here's the wax Venus with a new clay funnel at her base, to hold the ingot.

I was very pleased to have come up with this idea, only of course later I realized that this was already a known technique. Now, I didn't really think I'd invented it, that'd be a bit presumptuous -- me, discovering something that no one else thought up over 5000 years? But it didn't matter. I was happy that my mind had stumbled upon it, as I bumped and jiggled these images of molten metal and inverted voids around inside for a time.

(For now we see through a plastic, darkly...)

So, I made three plaster moulds. Two below show the clay plugs still in the mould, and the other is just the mould with wax inside, ready for burning out.

And then I did the wax burnout.

150°C was too hot. Too much stinky smoke.

I let it go for about 4 hours, allowing the wax to melt and drip out very slowly. The first two hours or so at 125°C and then up to 135°C degrees. That worked well, as the wax was melting but not really smoking. They all came out very well.

(A little bit of clay there to take out, but otherwise, perfect.)

I realized, though, that I had forgotten to put sand in the plaster mix! So I was a bit worried about putting them in the kiln to + 600° degrees C. But, well, I did anyway.

(Two with aluminum sitting up top, one to go.)

(And now, in the kiln, ready for cooking.)

And then I fired it up. Nice and slow, as slow as can be. Within half an hour or so, the pyrometer went up to 370°, but I knew that the plaster itself wasn't at that temperature yet. So I let it sit there for another hour or so and then I closed up the kiln vent a bit and opened the gas on the burner, and the temperature rose to 420°. Slowly, after about two hours total, I let it get to about 580°. And then, 2.5 hours in to it and letting the temp rise slowly, I could see the aluminum was starting to melt.

(You have to look very closely, but it's a molten blob.)

The pyrometer said 803 degrees C, but I know that aluminum melts at 660°, so the material itself couldn't have been much hotter than that.

I let it sit there for another 30 minutes or so, to make sure everything was melted and filled in evenly. And then I turned off the kiln, and covered it all up so it would cool down slowly.

The moulds did crack, One of them, badly, but the other two were all right. The plaster certainly needed sand, but they held together well enough, amazingly enough.

The verdict? The drip method works...

...but not perfectly. It turns out there is still an issue with venting.

Look from the knees down - it didn't fill in fully. And I believe the shape of that defect is the shape of an air bubble, sitting there at the top of the form (when it's upside down).

It could be that the aluminum sitting up top blocked the air from escaping when it melted, because it was covering up the hole. So the metal dripped in but at some point, as the hot air was rising up the empty space, it got trapped by that ceiling.

As always, it's not really about success or failure, because the thrill, inspiration and poetry (and sometimes, even insight!) is in the Doing. And anyway, look at this mini-Rodin Danaïde I found, as I cracked open a mould!

(Danaïde, by Rodin.)

 
(The Danaïdes were condemned to spend eternity attempting to fill perforated jugs with water. A task not unlike trying to cast a Venus in aluminum when you don't really know what you're doing.)

One solution could be to just make a vent for the mold, once and for all. But another solution could be to measure the amount of aluminum needed for the figure, and put only that amount in at the top. That way there shouldn't be any extra metal to create a lid that will block the air from escaping.

Right?

Who knows? Not me. But I think I'll try both ideas next time. And let me close with some words by Mr. Rodin himself:

"Patience is also a form of action."