Joined: December 15 2006 Location: United States Posts: 19
Posted: December 22 2011 at 12:27pm | IP Logged
Just a note about using various metals for casting - BE SURE they are DRY before adding them to a hot furnace/crucible.
Aluminum melts somewhere above 1300 degrees. Technically pure aluminum (no alloy mixed in) melts at 1218 degrees F. Brass and bronze are up above 2000 degrees, copper is above that.
Adding anything to a melt that is not completely dry can cause a steam explosion. The moisture (typically water) turns instantly to steam and expands approximately 1600 -1700 times. I have seen it happen, thought in my case it was fortunate that no one was injured. It is kind of like a molten metal hand grenade.
Here is a normal, safe procedure for seeing that the metal (aluminum, zinc, brass, copper and bronze) is dry for casting.
1) IF the furnace is cold, a crucible can be loosely filled with metal that is assumed to be dry or mostly dry. Firing the furnace from cold will heat the metal, crucible and furnace slowly and burn off any moisture. Be careful here that there aren't ANY parts (EVER ! ! ) that have trapped inside spaces. They might explode due to moisture or even due to expanding air.
2) If the furnace is hot and there is already some hot or molten metal in the crucible - the new metal to be added MUST BE PREHEATED - typically on the closed lid of the furnace. Don't over load the lid. A few pieces are enough. 5-10 minutes is usually enough time for a preheat. Then, with some tongs carefully pick up the hot pieces and lower them in to the crucible. Sometimes this can be done with the lid closed, sometimes you must open the lid, put the pieces in and then close the lid.
3) Zinc can be cast and melts relatively low (800 degrees). It is TOXIC so DO NOT breath any of the fumes. While not often found in firearms, it could be good for practice and testing molds. Virtually all the fancy old complex chrome work on cars was cast zinc and then it was chromed. Some interior plumbing fixtures are zinc. Zinc is also the metal that is mixed with copper to make brass. This should tell you to never breath the fumes from brass either.
4) Finally, recycled metals are often dirty. Some of this will burn off, the rest will become "sl*g" or "dross" on the top of the melt. This needs skimmed off before pouring. A skimmer does this, just before pouring. It looks like a big 3' spoon that has holes in it. Preheat the skimmer before using it. Do the skimming out of the furnace so the sl*g does not get spilled in the furnace. It can go into a metal sl*g bucket, to eventually get recycled at the scrapyard.
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