Degassing coppers is usually done with phosphor copper shot. But it's done with just a tiny amount to 100 pounds of copper and if I recall correctly, overdoing it can be worse than not doing it at all. Tricky to get the right dose with just a few pounds of copper. If we had some scrap parts that were already made of our target alloy to use (even just to get the melt started with), it likely might have come out better.
Arrgh, I hate porosity. New alloys and new patterns don't always come out perfect on the first try. Even before seeing these pics my bet on the source of the porosity would be mainly oxide films generated while we were melting the copper. Could be some core gases involved too.
Most porosity is usually either related to air/gas bubbles and/or localized shrinkage nucleated on oxide bifilms floating around in the melt. So they say.
Copper is very prone to oxidation while molten. We tried using a charcoal cover (which mostly just seemed to make a mess), and I had the furnace running a reducing flame which should have helped some, but that may not have been enough. Next time I alloy bronze, I'll try using a molten glass cover on the melt to shield it from oxidation.
The offset pouring basin and tapered sprue used SHOULD have prevented any (most) air bubble entrainment. The runner and gating system SHOULD have prevented any (most) of the turbulence during filling, therefore stopping too many more oxide films from being generated there inside the mold. Even if the sizes of my hand cut runner and gate were off a little from what they should have been, tin bronze is supposed to be somewhat forgiving. What remains is the oxides that came out of the crucible. Rule #1 is start with clean metal. So that's our #1 suspect.
I'm hesitant to blame the porosity on core gases too much as those types of defects often cause big blow holes in the cope side of the casting, these are more like small pits. However, in my photos from the other day you can see that the vent on the in-gate side (the fat end of the nut) of the coreprints got blocked by a bit of bronze that got around the end of the core, and therefore would not have been effective at venting core gases.
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So it's possible the gases from the core didn't vent very well. And we weren't sure how much epoxy was in the core to begin with. The porosity does look to be a fair bit worse directly above the core too. Hmm...
If you want to try again, perhaps with a slightly bigger pattern for the added shrink/machining allowance, I'm game to do another session and we can try a few things differently. Like that glass cover. Or else we might be able to clean up the metal a bit just by remelting what we already poured and skimming off more of the crud that floats up instead. Bronze should melt much cleaner than the pure copper did. But every melt does introduce new oxides, so that can really only be taken so far. We could also try less binder in the core.
Jeff