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New to Casting - need help with Sterling Silver

Hi there,

I am new to this community and I joined because I need advice. In the last five months I started casting. I have been successful at casting brass but am finding issues with casting sterling silver. I follow the same steps that I do with brass - but keep finding flaws in my castings when going to polish. Cracks, little holes.

Here is the process I am following.

ultravest investment - following recommended ratios
vacuum bubbles out, fill flask, vacuum again.
let sit for at least 2 hours
put in kiln -
full speed up to 300 degrees for 4 hours. pull tray with wax out, close kiln.
ramp up 225 degrees per hour until 1350, hold for 6 hours
full speed down to 1000 degrees, hold for three hours till ready to cast.
melt sterling to 1825 (i have used straight sterling casting metal and also sterling sheet metal scrap with no solder etc. and casting media)
pull flask, place in vacuum, turn on vacuum then pour metal.
continue vacuum for 90 seconds.
pull flask and let rest with charcoal block on top of button for 15 minutes then quench.

any advice would be greatly appreciated.
thank you :slight_smile:

I would think that. You would do well to use some flux on the melt. The copper in the sterling can (will) give you some oxides that need to be stopped and if any form, skimmed off before pouring.

JD

Like Matt’s casting flux? I have that -
I was speaking to a tech at rio and he said I didn’t need to use flux with a graphite crucible. When you say use flux - how much do you mean? And do you just sprinkle it in?

Isn’t your melt temp a bit high? Graphite crucibles don’t need flux, but depending on how fast your pour is, you could be catching a lot of oxide that way if it’s too slow. I always follow the stream with the torch. Also, the vacuum should be on BEFORE you set the flask. You set it edge down on one side then tip it so you get a good seal. Make sure your flask has about an 1/8” or 5mm clearance short of the flask edge so it seals. Unless you’re using one that drops into the machine, in which case your seal is formed at the flange.

You don’t say how big your castings are. That makes a difference in the burnout. I always followed a longer cycle for complete burnout. I taught University and College courses and Student pieces are notoriously unpredictable in size and construction.

The other critical area is how you sprue and gate the piece. This is very important to prevent porosity and vacancies in the piece. In fact that can make a huge difference on some pieces. There are some good articles here on Ganoksin on the subject.

Ruthanne

1 Like

Hi Ruthannie,

ooo thanks for the great response.

I originally was melting the sterling at like 1739 degrees. After talking to Scott at Rio grande’s technical support he said I should try 1825. What temperature do you go to?

I have a furnace and haven’t used my torch while I pour - i can try that though the next time.

I have a vacuum that you insert the flask into - I have been using perforated flasks.

My castings are 1/3" x 4mm and 1/2" x 4mm circular flat discs.

I’ll look into more articles on sprues and gates - although I have had consistent success casting brass.

thank you and happy holidays!!
Jill

…the vacuum should be on BEFORE you set the flask.

With a flask being placed on top of a pad, then sure, turn on the vacuum first. But not with a perforated flask with rim being placed into a cavity.

The suction will pull the flask in instantly with incredible force. It is like the reverse of a car piston firing or the reverse of a bullet going off in a gun barrel and seemingly as forceful.

I did this once and was afraid the flask slamming to such a hard stop would crack the investment. It didn’t, but… After that eye-opening slam I always insert the flask first and then turn the vacuum on.

Neil A

agreed. i was not planning on that :slight_smile:

Do your discs have any detail on them? You might find simple Delft clay casting does the job…Rob

nope but the volume I want to produce will not be good for delft clay.

Are you able to cast sterling in that - would be cool for some other smaller projects. I have a whole set up and thought that was just for lower temp metals?

Cast sterling, fine and 14K all the time.

oooo exciting…

I was talking about setting a flask on top of a vacuum table. The best way to get a good seal is to set it down on one edge and then tilt it down to make contact and seal. If you have a drop in vacuum flask, that’s a completely different story.

If you are doing disc shapes that are sort of flat, are you arranging them so they are angled in the flask. Otherwise air bubbles can get caught underneath, that wouldn’t cause vacancies, it would leave round balls on the surface.

Your metal is clean? No sharpie, no grease, no glue, etc. in fact I wouldn’t even reuse metal that has major porosity problems from one casting to another if I can’t nail down the problem. I had a student once who just kept “reinfecting” her castings with porosity because she kept reusing scrap. Early on, she had used some pieces that had solder and she didn’t know it. The problems perpetuated from one casting to the next.

One thing I found out over the years, was that in some cases, pieces need a sprue or gate extending beyond where they logically end. For example, we were trying to cast some very open lacey pieces and they kept coming out partially cast. It didn’t matter how many sprues there were and where. Finally, in some research, I figured out to add small extensions at the intersections of the ribs. Everything cast fine after that.

Sprue button- are you calculating enough mass for the button so there is plenty of metal as the the pieces cool? This should be 25-30% of the total weight including the sprue system.

Are you attaching your main sprue line to the heaviest part of the piece? Do sprues allow the metal to flow straight in without constriction or backflow?

Bear in mind that even the most professional casting houses battle with porosity. Spruing and gating are a really big deal when pieces are designed and can make all the difference in success.

Good Luck!

How about posting a picture of the defects and a tree sprued before investing. When you pour the silver are you seeing it dance in the pool?