Making REALLY fine shot

Hi all! I'm a new member and have been reading the forum with great interest & wanted to share an experience I had when casting.

Reading about shotting metal to aid digesting it brought to mind an experience I had while casting metal a while ago. I had burned out a small flask to vacuum cast a heavy 22k gold band for a special order and had a flask blowout. I could tell something had gone wrong as soon as I poured the metal and saw it all disappear in to the flask instead of filling up and forming a button. I pulled the flask off the vacuum casting machine expecting to find lump of gold that should have been my casting and when I looked inside to retrieve the metal, it was nowhere to be found! I broke open the flask- no metal there- back to the vacuum machine, didn't see it, starting to get a little nervous about where exactly this half ounce or so of gold had got to- eventually I found it as a very fine sand of brownish oxidized gold in the bottom of the well of the casting machine. I can only surmise that something about the way the plaster flask failed and the pressure of the air over the vacuum had managed to spray the molten gold into a mist that froze into gold sand before reaching the bottom of the casting chamber.

I wonder now about repeating that experience to intentionally shot metal into sand like grains for refining- wonder if anyone else has ever had something like that happen or knows of a way to replicate the process?

I once did a little research and testing with making zinc powder somewhat like that.
I used a piece of graphite precision drilled so the hole was dead center. I then attached that flat disk to a motor that spins 30,000 rpms. When you drop metal into the center of the disk the centrifugal force will sling the molten metal to the edges of the disk where it will disintegrate into tiny pieces. It makes great shot, but it wasn't fine enough to make the fine size powder i wanted.

Preparing Metal for Digestion
Re: 99.5%? Is it possible?

Dave

Interesting, I'm trying to picture the setup- did the graphite disc have walls machined into it to contain the zinc shot? I'm imagining something like a 2part mold:

or were you spraying it out into a collecting chamber of some kind?

The sand I produced wasn't exactly powder, really more like the consistency of hardware store play sand, the biggest grains well under a millimeter, I would guess under about 0.4mm or so. It was a visible mix of particle sizes but all tiny little round spheres. I'm sorry now that I didn't examine the plaster mold more closely before destroying it. I do recall that the underside had a bubble shaped indentation about a centimeter across, obviously not a defect from the investing process but appearing like an ellipsoid cavity, like conchoidal spalling where the plaster broke through. I wonder if that shape might have played a role in spraying the metal. I didn't notice if the path of the defect was straight through the mold, it could have had followed some internal crack.