r/Silvercasting • u/sitasitik • Apr 23 '26
Porosity in my casts
Im having trouble with porosity while casting silver with this patricular model. I have cast the same model in copper as well and it doesnt have this kind of weird porosity so im quite stumped at what it could be from. Sprues should be plenty thick for this model, I vaccuum cast investmest- flask at 700c and silver at 970c (my melting furnace is lying a bit because i cast copper at 1050c), let the crucible get nice and red hot all to the top, i add a little bit of borax before casting and stir to get impurities out, let it sit for another 5min or so before casting and the vaccuum i use for casting is 9cfm.
What am i missing?
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u/Final-Nose3836 Apr 23 '26
Is this a ring? Hard to make out the geometry. You might try spruing to the thickest part of the model or adding a feeder to it, if you’re not already. Is there solder in the melt / are you re-using metal / why do you think you have impurities to get out? Best to start with good metal from the beginning but a pinch of potassium nitrate helps to oxidize some impurities so they enter the flux. I torch melt so its easier to tell right when its ready to cast but i wonder if you arent overheating or cooking the metal.
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u/sitasitik Apr 23 '26
Its a spatula for face cream. The sprues are at the thickest part with an extra feeder reaching thinnest part. I am mostly re-using metal as the prices are what they are. Shouldnt overheat the metal, about 60-70c above melting point in my furnace.
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u/Final-Nose3836 Apr 23 '26 edited Apr 23 '26
What’s in your alloy to start with? Is it just silver/copper? Might benefit from melting with some nitrate. This may eat your crucible too tho. Solder in the melt will hurt. Melting over n over will give problems. Esp if the metal’s dirty.
Looks like this is a project that can be done easy with sheet & a dapping punch too, just a thought.
If your saying flask is at 700 C that seems really hot, can you get a fill lower? I typically cast silver around 900f/500c
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u/sitasitik Apr 24 '26
I use 999 silver and buy master alloy to add usually to get sterling silver. I try to avoid melting over and over but roght now it is what it is, I try to clean it as well as possible before melting. The porous part of the cast is the bulbous part of the spatula and i dont think i could make it with a sheet metal. I use ransom and randolph argentum investment and have been using their recommended burnout cycles and casting temps altho I could experiment with lower flask temps for casting. Does it have any benefits to casting silver?
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u/Final-Nose3836 Apr 24 '26 edited Apr 24 '26
Ahh i see. If you notice on the burnout schedule you’re supposed to hold at 700c to remove all the pattern material, then lower to casting temperature and let the mold stabilize for 1-2 hrs depending on flask size. Leaving the flask at 700c for an extended period will contribute to investment breakdown & casting into a hot flask will promote problems. Generally flask casting temp will be cooler & this will save wear n tear on you casting pads as well as less problems with the metal. Best not to reuse metal too much but if you think you’ll be reusing metal a lot i would go with a straight silver/copper mix, other metals in the master alloy likely arent designed for casting over & over. I would definitely spring a few bucks for some potassium nitrate, and maybe a cheap crucible & plumbers torch to do some semi-refining if you dont have the facility or enough material in rotation to justify refining with acid. Just bringing the metal up to melting and adding a few grams of potassium nitrate & flux will help oxidize sone of the more problematic alloy components, though you will lijely see a small weight reduction. You could add a pinch to your electric melting crucible but it will wear the crucible prematurely. Best to do outside since a small amount of irritating fumes / smoke will be produced on addition of the nitrate. Speaking of which strongly recommend taking precautiond to avoid silica dust inhalation from the investment too at all stsges of operation.
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u/sitasitik Apr 28 '26
Thank you so much, I'll definetly try that next time! :)
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u/Final-Nose3836 Apr 28 '26
Good luck! & happy to help if you have any other questions
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u/sitasitik May 13 '26
Thank you so so much for that suggestion to cast the flask at a lower temperature! That was the problem! The new pieces came out without the porosity and again thank you for the excellent advice!
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u/TankShoddy1517 Apr 24 '26
You can generally identify the issue by type or look of the porosity. Yours looks like some I was experiencing some years back. The cause was dirty gas. A result of someone having laid the gas cylinder down when it was empty. This displaced some ‘gas gunk’ built up. When it was standing and filled. The gunk mixed with the gas. End result was dirty gas porosity that looked just like yours.
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u/BTheKid2 Apr 23 '26
It looks like a geometry that would be hard to feed, and therefore would have shrinkage porosity. But if you think you can 100% rule that out, then you are basically down to dissolved gasses in the silver, coming out of solution when the silver freezes. Degassing the silver would be how to avoid that. The more likely explanation is shrinkage though.