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Black slag

What is the black graphite looking slag that is formed when sterling silver is melted with borax? Is there a way to remove it from the surface of the sterling silver? It appears to be indestructible!

The easiest way to remove it is while the silver is molten by using a graphite or fused silica stirring rod to skim off the slag.

If you mean that the black, glassy material is on the cast piece, try boilng water soak to remove it.

Yup, I will bet it is removable by boiling or even just a hot water soak. Then scrub it with dawn and a brass brush if that is appropriate? I do this frequently to success.

Don Meixner

Hey! Thanks for your replies. I will try your recommendations. I have this same post over on the stack exchange chemistry forum where I have added images and new information. Just last night this stuff ate a hole through my crucible and then ate a big hole through the bottom of my Ney furnace and 2.25 Ozt sterling silver ended up somewhere down inside the refractory lining of the furnace. It’s a long story, too many images and typing to repost here so if you’re interested you can go to the thread over there.
Here is the URL. If you have something to add please post here or if you have an account there post over there, either way, I would love to hear what everyone has to say. Thanks!

Oh and, I had been using a vacuum furnace for melting without flux so I have no idea what the hell all this black stuff is. If you hear in the news that Cold Springs Nevada turned into a giant sinkhole YOU’LL KNOW WHY!! HAHAHA! :rofl::rofl::rofl:

Cast or soldered, if a piece has any kind of glassy flux residue on it, I will leave it in rinse water for as long as possible. In stead of removing it and probably some metal with a file or abrasive wheel, it just dissolves in the water. I do sometime have a pit where it used to be that has to be dealt with. A good soak is a god thing. Change your rinse water often…Rob

James are you treating your crucibles with with a layer of boric acid or borax before you use them? Just asking as I don’t cast anything but ingots. My crucibles get coated with boric acid and I have never had that problem before as your photos show.

Don Meixner
---- James Kincaid wrote:

Hi Don,

No, I figured that was done to make the crucibles last longer so I just sprinkled a minute amount of borax on the sterling silver casting grain before placing in a room temperature furnace. Do your crucibles stay looking like that clear glass coating?

I’ll have to try boiling and or soaking in tap water and see what that does. Thanks for the sugestions.

Hello Don & Rob…this thread is going out to the both of you… I watch you guys on this site all the time, although I don’t know either one of you personally but I believe you both have bought something from me in the past, you guys are brilliant with your knowledge and what you do along with everyone else on this site. What this has caused me to think about is possibly setting up a Centrifugal Casting unit in my backyard and to try my hand at casting. I definitely have the right casting machine, because I manufacture them, I just have to find a school nearby that teaches centrifugal casting and finally learn how to use one of my machines. I know that I can get a good deal on the machine… because I know people. If you can guide me to a place that teaches casting, I sure would appreciate that… Thanking you in advance… Regards, Richard Lucas.


Nope, mine get pretty groddy I think the crucibles are pretty porous and need some kind of treatment to make them less so. Honestly I do this because that was what my Dad did. I have never done a search on breaking in the new crucible because it worked and it was intuitive to me. I am sure there must be something about this in the archives.


---- James Kincaid wrote:

Put your borax in a new salt shaker and start to coat the crucible with it once it is hot. This takes a while, but try to coat all of it. The borax will also fly around the area in which you are working. You might want to do the coating process in the garage or someplace where the mess won’t be a problem. My crucibles are real nasty looking, but they work. General wisdom is to keep separate crucibles for gold and silver. This is because you leave little droppings behind and they might contaminate the next melt if it is a different metal. I know that there are metallurgists among us who might be able to provide more of the why to what we do. I just know what works for me…Rob

Richard…thanks for your kind words and great pedals. The ganoksin archives are a good place to start looking for how to use your casting arms. You also live in an area where you are much more likely to find formal training than Don and I can find up here in the wilds of Central New York where it is currently 12 degrees and about a foot of snow on the ground. So far it has been a mild winter. Do a search on line for jewelry schools in your area. Also look for rock clubs, they may be a resource for less formal training. Good luck…Rob

Thanks Richard, very generous words. I find the foot switch to be a best investment.

Give this place a look. The classes seem reasonably priced but perhaps you have something trade and barter is acceptable.

Don Meixner

---- richard lucas wrote:

General wisdom is to keep separate crucibles for gold and silver. This is because you leave little droppings behind and they might contaminate the next melt if it is a different metal.

I find sterling silver to generate more dirty oxides which leave a nasty residue at the bottom of the crucible. However, it does not have a negative effect on the casting. My gold crucible, even those it is used 1/10th as often, still is very clean.

I thought it was because of my graphite stirring rod, but when I had to season a new crucible, I used a fused quartz rod and the crucible still got grimy.

One other story - I have tried to melt down sterling silver sweeps from my workbench and found that if I just tried to melt it ‘as-is’, I would get a gooey (nice technical term) slag that appears to be a combination of rubber/silicone wheel debris and other undesirable compounds. The silver would bead up in little balls, surrounded by the hard crusty goo. It was very difficult to remove and with the small amounts of silver, some of it got tossed into the garbage.

So I learned a lesson never to melt bench sweeps again without further refining steps. :wink:

Sweeps, polishing duff, grinding swarf, chains of unknown origin, old crucibles and other nasty stuff all go back to Hoover and Strong. Word of caution; don’t send it in an old box from the liquor store unless you cover anything that identifies the product that previously occupied the box. Apparently the post office will think that you are shipping some sort of alcoholic beverage and refuse to deliver it. My local post office had no problem with this, but the one on the the end did and sent it back. I missed out on a very favorable spot price…Rob

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