foundry question

[Nobody casts pure copper. It takes a lot of heat, and oxidizes badly. If you want back-up plates for welding, why not just get some copper plate and cut it up?]

Andrew Werby

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Hi Roger - There's also a lot of information about how to build your own foundry on the web. Google for "home foundry" - a lot of hobby machinist types have made really nice tutorial web pages about how to build a furnace, sand cast, etc. You can get a small foundry set up for about $200 (propane). You might try starting with Aluminum for casting. Cheap, melts easily.

- Jud

| > Roger in Vegas | > Worlds Greatest Impulse Buyer | | [Nobody casts pure copper. It takes a lot of heat, and oxidizes badly. If | you want back-up plates for welding, why not just get some copper plate and | cut it up?] | | Andrew Werby |

So that reminds me of a question I've had for awhile. What's the best places to get copper plate, and what are the sizes and shapes we should be looking for? Are there sizes that are more versatile than others? What is the common ways of securing them in place while welding? Obviously magnets won't work, so what's best?

Ping Tim Williams.

I'm not convinced your plan is well conceived. If you've never worked with molten copper, you're in for some unpleasant surprises, one of which is the casting temperature is relatively high, over 2,000 F. It's a bitch to work with in that it often comes out quite porous. It loves to solder to almost anything it contacts, to add insult to injury. I can't help but think you'll spend a lot more money trying to make your copper pieces than you'd pay for some scrap buss bar at your local recycling place, but if you're hell bent on melting copper, you'll need a decent furnace, fuel to fire it, a proper handling tool, as well as a reasonable crucible. One made of steel won't do well unless you cover it with a wash, otherwise the copper will dissolve a percentage of the steel and ruin the copper to some degree. You also risk dissolving through your container if it's very thin. I'd suggest a graphite clay crucible, or even better, a silicon carbide model. Bottom line, unless you'd like to do some casting in the future, you'd be far better off not trying to cast your own plates. The cost of getting set up is prohibitive if that's your only purpose.

Assuming you get so far as to actually melt your copper, you could probably use a heavy channel with plates welded on the ends for a mold, but it will need some mold dressing so the copper won't solder to it. You won't achieve flat castings, but they'll be close.

Foundry supply houses sell a mold dressing, and would be the source for a decent crucible and a handling tool. You'd probably have to build your own furnace, although they sell them, too.

I used to melt scrap copper to cast bars for silver recovery. It was less than pleasant, and I had the equipment at hand.

Harold

You'll need a silicon carbide crucible and a furnace capable of about

2100deg. I've melted copper scrap using a propane furnace. It takes a long heating time and leaves a lot of dross or oxide. The pieces of scrap (in my case, copper tubing) seem to hold their shape due to the outer layer of oxide. The copper inside melts away leaving the vague shape of the original piece until well along in the heating process. Finally, the oxide crumbles and floats on top of the molten copper. It then has to be skimmed and the high heat of the copper tends to make my skimmer less than sturdy. It can be done, but melting aluminum is a better experience.

Gary Brady Austin, TX

Well first of all, read up on foundry practice obviously... ABYMC.com and backyardmetalcasting.com are good starts.

You *can* make your own crucibles, but the thing with pottery is it has to be fired slowly (over at least 5 hours, much longer if you need a more stable product), and any typical formulation is about as good a conductor as your average 3000 degree rated castable refractory, not to mention the expansion is greater. Try this: toss a coffee mug on a fire. Watch it snap into pieces. Thermal shock in action.

Even Vince Gingery in his crucible book goes only as far as grog and clay with a little feldspar, as I recall. Something like 77% 20-80 mesh grog,

20% fireclay and 3% spodumene (lithium feldspar, a flux). Now fireclay alone has a high silica content, which - when not tied up as something else - tends to form quartz or crystobalite. Both of these have dramatic changes in density at specific low temperatures; the "quartz inversion" occurs at 573C (1% change, IIRC) and cristobalite at 180C (0.5 to 1%, I forget exactly). Not only that, but the high grog content, coarse grog no less, means high porosity - holes that insulate, just what you don't need! The only thing it's really got going for it is the high melting point - up around cone 25, at least (2800-3000°F I'd say).

Well, yeah...

LA Graphite is the cheapest, IIRC. Clay-graphite takes more care than SiC or steel crucibles, but can also handle iron if you desire to try that. Don't use steel, it'll dissolve and not only will your alloy be askew, but the crucible might burn through as well.

Once you make a furnace and get a crucible, you need fuel... go with propane, it's just easier. You can use wood, coal or charcoal (stay away from briquettes), but melting something hot like copper isn't the easiest goal with it. Not to mention, the uneven and often all-or-nothing heat control is sure to crack any but the best crucible.

You asked what for mold, sand mold is fine. Read for yourself, I don't have to say another word on this!

As for melting and pouring copper... use a slight reducing atmosphere. The posters above who say pipe just kinda sorta collapses royally screwed something up. Copper is a noble metal and as such, its oxide is very easy to reduce to pure metal. Not to say there won't be any trouble: oxygen (gas, not oxide!) is soluble in copper, just like CO2 in water. When it freezes, it comes out and gives you trouble. The common fix is borax plus either soda (baking soda, washing soda, etc.) or boric acid, with crushed charcoal (any kind) on top to absorb the oxygen. If you don't mind alloying, you can add 1-5% zinc (pennies) and that'll combine with the oxygen, keeping it clean. There's still the matter of hydrogen, but oxygen is probably your #1 concern.

I personally have recast scrap wire as ingots for storage. Works fine, though it did come out all bubbly!

Beware of shrinkage defects in the casting BTW: copper - being pure - has a steep melting/solidifying point, so it'll want to suck in metal all at once. Use big gates and even bigger risers. Pour around 2200°F.

Tim

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