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Casting Safety

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Dick Morris:
A couple of thoughts/notes on casting safety.

When I put the flask back in the electric foundry furnace yesterday after pouring out about half of its contents, this is what I found. There is a vertical crack in the upper rim. After pouring the rest of the contents I noticed that crack extended down the side. If you see this dispose of the crucible as soon as possible.

I've had a crucible fail when full of about 2KG of bronze in it. The only result was a mess to clean up, but it could have been a lot worse.

A couple of things I do to lessen the hazard. I use two pairs of tongs when lifting and pouring a crucible from the electric foundry furnace. One to lift it about half way, then the second to help left it the rest of the way and give support when tipping the crucible so all the stress isn't on the rim of the crucible.

I'm posting a couple of photos of my casting area. Note the plywood next to the table. In case a crucible fails the wood blocks the splash of hot metal from hitting your legs. (Mine could actually be a bit higher.)   

A7er:
Are the crucibles graphite?

Dick Morris:
Yes, graphite.

Alyn Foundry:
Hi Dick.

From your photo it appears to me that the crucible is almost a “ dead fit “ in the furnace opening, is that the case? This could be the cause of the premature failure. We always allowed a good clearance between the furnace walls and the pot. The tongs could then be placed at roughly the middle point for lifting out.  Lifting from the top can put quite a heavy stress through the material.

Are you heating your tongs prior to the lift? Cold tongs can cause a seriously drastic change in temperature ( thermal shock ) which can also cause cracking.

I’m envious of the electric furnace, is it an Induction variety? We used gas and air for the higher temperature metals and the noise was incredible. Luckily we didn’t have any neighbours close by when we were casting.

 :cheers:  Graham.

Dick Morris:
The crucible is a loose fit and is the one that fits this generic type of Chinese foundry. They are on Amazon as having a 3KG (of gold) capacity. It's low tech and uses a resistance coil. I keep a spare crucible on hand, but usually they just erode to the point where they need to be replaced. This was something new in about 100 melts. The initial lift from the furnace needs to be by the grooved flange at the top of the crucible. As the crucible can be fairly heavy when full of bronze, I use a second set of tongs to support it. It's possible that cooling by the tongs is a problem, but I don't think so. They are about 5/16 rod that fits around the crucible rim on an extended set of plyers.

This type of furnace is convenient and doesn't have the exhaust products that a gas furnace has. I'm in Alaska and can't use a foundry outside for five to six months of the year.  However, it's slow. An initial melt in bronze to 1090c takes about an hour. 

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