A Quick Mass-Finishing Experiment

I did some casting recently, and finally remembered to chase a little experiment.

Lost wax casting is a bit labor- and material- intensive. It quickly becomes worthwhile to prep and cast many pieces in one go, rather than individually. But as a result you now have many pieces that need cleaned up all at once.
For example:

This casting run resulted in 17 pieces (plus 3 buttons that I can melt down and reuse). What you see in this photo are all of those pieces just after casting. I've cut them apart and done a little bit of scrubbing in a bucket of water, but that's it. The extra dark pieces at the top right are still wet.

All of the pieces are covered in a dark grey layer called fire scale, which is oxidized copper from heating the metal and probably carbon from the mold. On top of that in places there is dirty white investment (the molding material) still clinging to grooves and textures. When scraped out, the investment becomes a powder. You have to wear a respirator to protect yourself while cleaning, and it tends to cover everything. Plus sanding 17 pieces full of curves and grooves and textures, by hand..!
There is a much better way.

For mass finishing, I throw everything into a rubber barrel (actually a rock tumbler) with abrasive media. Tumbler media are plastic or ceramic pieces with a controlled grit, like sand paper, and they're sold in different grits, shapes, and sizes. Mine are blue pyramids, equivalent to 220 grit sand paper. Tumbling is a very gentle method, so while 220 would be an aggressive grit with a sanding stick or dremel, its very light for tumbling.

Usually I set this to rolling at the end of casting day and let it run all night. But recently I got to thinking, and asked myself how long it needs to run. What's the minimum? 🤔

So this time I set a 1 hour timer, and I pulled one piece out every hour.
(Until I forgot)

Here are some close-up photos. The first piece never went into the finisher: you can see the white/cream investment in places.

After one hour of tumbling, most of the investment and fire scale have come off already, but there is still a dark shadow on the silver and some gunk deep in the grooves.

From the back side it's easier to see that the next few hours of tumbling work to slowly remove that dark oxide layer.

So, uh, after the 4th extraction I forgot to set my next alarm 😅 The remaining pieces tumbled over night. While the piece that tumbled for 4 hours looks fairly good, the overnight pieces are just a little shinier and brighter.

Its also really important to see that the textures aren't eroded or damaged by tumbling over night. So while 1-4 hours of tumbling gets the bulk of the work done, there is nothing lost by leaving them in for 12 or more hours, and the extra tumbling time reduces the amount of hand work I have to do.

And that is why I mass-finish my cast pieces 😃
As bonus eye-candy, here are some mermaid tail pendants that were also part of this casting run, after cleaning in the finisher. On to the next step!