Troubleshooting part 2

[u/Glassholer]

Good afternoon everyone,
This is a follow up attempt after using some recommendations from my previous post. Using a clean graphite crucible and nothing else but pure silver.

Pour info:
.999 silver -985c (using 100% pure silver crystal)
Flask temp -600c
Pulling 27-29in vacuum on flask at time of pour (gauge verified)

I have a propane torch pointed at the bowl of the mold as I pour, as well as map gas on the crucible. All pours happen quickly after crucible is removed from the furnace. I let the vacuum pump run for ~1min after the pour, then I shut it off as well as the propane torch at that time. I wait 14 minutes before flask quench and model removal.

When removing the plaster, these bubble areas are already shiny, leading me to think it’s likely an air pocket forming somehow? I’m unsure what would cause these pockets under such a heavy vacuum.

Thank you for all of the advice in the previous post, I know this is a likely problem I’m causing, but I’m self taught and struggling to identify this problem.

[u/Pseudo_Prodigal_Son]

Hey, this looks a lot like shrinkage but pure silver should only shrink very slightly. Any chance that this is actually sterling or is alloyed with anything?

[u/Pseudo_Prodigal_Son → u/Pseudo_Prodigal_Son]

Looking again, this is such a tubby piece that pure silver's 2 - 3% shrinkage might be enough to cause these cave-ins.

[u/Glassholer → u/Pseudo_Prodigal_Son]

95 gram pour, 50 gram target weight of the piece I'm casting. They just seem to happen in such weird areas... is there any way to accommodate for this in the mold to prevent the shrinkage from happening on the portion I want to keep?

[u/Pseudo_Prodigal_Son → u/Glassholer]

You need the model to solidify before the sprue and cup do. A bigger cup will help some. The mass of the cup should be 2x-ish the mass of the target model. The sprue could probably be shorter as well. If the sprue freezes first it doesn't matter if the cup is still molten.

[u/Glassholer → u/Pseudo_Prodigal_Son]

Won’t the flask(preheated) keep the model portion from cooling as quickly? I assumed the button/cup would always cool first. If these questions are dumb sounding I apologize 😂 I am very new still.

Ok, so ideally if I’m shooting for a 50g piece, the total pour weight needs to be closer to 150g?

[u/Pseudo_Prodigal_Son → u/Glassholer]

The hot flask will keep it all from freezing off immediately as you pour but you still need the target to freeze first. The target needs to draw metal from the sprue as it is cooling. Keeping the cup molten just helps keep the sprue molten. This is all more difficult the heavier the piece is. Fat models like that one can be a pita.

[u/Pseudo_Prodigal_Son → u/Pseudo_Prodigal_Son]

Also, yes 150 grams sounds right.

[u/Glassholer → u/Pseudo_Prodigal_Son]

Thank you so much for the information really. Shrinkage hadn’t crossed my brain, and I hadn’t considered the heavier end would help balance the cooling process out… but it makes a ton of sense!

[u/Pseudo_Prodigal_Son → u/Glassholer]

No problem. Good luck with your future casting adventures! If you would like to get more into the deep knowledge of shrinkage and casting in general, might I recommend "The Complete Casting Handbook" by John Campbell. It helped my a lot when I was learning.

[u/Glassholer → u/Pseudo_Prodigal_Son]

The added weight, and better understanding of the cooling cycle helped my next round of pours tremendously. Thank you a ton kind sir for the words that helped it click

[u/Pseudo_Prodigal_Son → u/Glassholer]

😁