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Melting aluminum (ericlippert.com)
170 points by eurg on Sept 13, 2014 | hide | past | web | favorite | 35 comments



Great article, I love his detailed explanations of why he does things the way he does them (the boots without laces, the natural fibers) when it comes to safety.

Don't be fooled by the low-tech appearance, this guy is top notch, he's been doing a ton of research and has quite a bit of experience. You don't set this up the first time out of the gate like this and if you plan on doing any aluminum casting yourself this would be a great starting point.

I also like how he evaluates what he's done, notices what could be improved and makes notes to do those things better next time. That's how you get to where he's gotten in the first place. There are other aluminum melting tutorials out there that make me cringe.

If anybody is wondering why concrete can explode when heated rapidly, concrete will always contain a percentage of water and when you heat that up faster than the steam can escape from the material it will cause a steam explosion.


Thanks for the kind words, I appreciate it. Indeed, I did a lot of research on home foundry setups and tool building before I started this project, and I am deliberately going slowly and carefully.


This was a wonderful post, thank you for taking the time to share it!

One question—I thought aluminum was really reactive with oxygen, and normally ever piece of it we see is coated with a thin film of the oxide. Does this present any special issues when heating it up to melt it?


I'm guessing the oxide is not a problem, it'll just float and you scoop it up. What I'd like to know is if more oxidation is taking place while the aluminum is being heated and when it is in liquid state. Heat should accelerate the process, right? And we need a shielding gas to keep the oxygen away while welding aluminum.

  > Twenty minutes later I have melted enough sprues and ingots that the
  > crucible is almost completely full. I take the lid off, skim the dross — the
  > bits of aluminum oxide and impurities that float — off the top and put it
  > in the ingot tray to cool; this will be trash. (Trying to recover aluminum
  > from aluminum oxide is not really worth it when aluminum is so plentiful.)


Correct; the oxide is not a problem. There's not much of it, it melts at a much higher temperature than the pure metal, and it floats, so you just scoop it off before you pour.

Whether oxidation or reduction is taking place during the melting process is determined by the amount of oxygen arriving via the air blast; too much oxygen and you get a hot oxidizing atmosphere, which oxidizes both my iron crucible and the top of the melt. Too little oxygen and you can actually get a reducing atmosphere, the furnace runs a bit colder, less heat escapes through the flue. So it's a tricky balance to strike and I'm still fine-tuning it.

I add potassium chloride and sodium carbonate to flux the melt and remove dissolved hydrogen; the KCl helps keep more aluminum oxide from forming apparently, though some home foundry sites report that they don't see a difference if they skip that step. Both chemicals are cheap, so I'm not too concerned.


It's only oxidizing at the air/metal interface. It's a very small amount and the oxide already in the basin is lighter than plain Aluminum so it floats to the top.


ever piece of it we see is coated with a thin film of the oxide.

The chemical composition of the oxide [0] is the same as the lenses on iPhone cameras, the so-called 'sapphire.'

[0] http://en.wikipedia.org/wiki/Corundum


It's not just so-called sapphire, it is sapphire (artificial, and a very thin layer of it but it really is the real deal).


This is an excellent article, and shows why you need to do all the fussy finicky things that you need to.

It touches on two stories of personal interest to me. First, my maternal grandfather spent a fair amount of his time in a silver mine in the blacksmith shop. This was a small enough mine that mining was done with picks, shovels and the like. They would wear out, and need to go to the blacksmith shop to be refurbished. We had an old open coal forge that had been used by the previous owner sitting unused in one of our sheds. Grandpa came by one day to fix up one of his pickaxes. The forge had a blower powered by a hand crank. He first scooped the coal out from the hole in the middle of the bottom of the body of the forge. Then took a coffee can, placed it over the hole, and packed coal around the can, and sprayed water on it. He had me turn the crank as he lit the coal, gently at first, then stronger.

Once the heat was up, he began heating the tip of the axe. As he heated, he told stories of how you heat, then cool the tool. Too much heat could burn the tool. And when to take it out to shape the tool on the anvil. And how there was different types of quenching--water, oil. Depending on what the tool was to be used for.

The other story is that my brother (now retired) was a professional welder and instrument maker. He made instruments for telescopes. As a gift, he once gave me a pair of dice made of aluminum. Not particularly fancy, but he noted that the trick with aluminum is that it will suddenly melt on you without turning red.


without turning red.

Metal light emission depends almost entirely on blackbody radiation [0] which depends only on temperature. Aluminum melts at lower temperatures than iron and that's why iron glows before it melts and aluminum doesn't.

Blackbody radiation is also the principal source of sunlight (and all starlight). It was one of the first physical phenomena that required the quantum theory of energy to explain.

[0] http://en.wikipedia.org/wiki/Black-body_radiation


Indeed, though the iron crucible I use gets hot enough to be red hot. Whether the molten aluminum has a faint red glow about it or not I cannot say; I don't usually melt it at night.


Of course, having a bucket of sand handy to put fires out does raise the question as to what would make the sand burn (at least if you are in the habit of reading Things I Won't Work With):

http://pipeline.corante.com/archives/2008/02/26/sand_wont_sa...


Anyone interested in the topic should check out the Gingery series on building your own workshop from scrap, which is all based on aluminium casting


He mentions Gingery as an inspiration:

"The project that I am gradually preparing for is to build the Gingery Lathe. The late Dave Gingery wrote a seven-part series of books in which he describes how to build a foundry, lathe, shaper, drill press and other metalworking tools from scrap at a tiny fraction of the cost of buying each. For me it’s not so much the cost savings (or the post-apocalypse skills!) as it is the opportunity to learn for myself how tools work at a fundamental level."

From the comments.


I have watched many chapters of the Gingery lathe casting series, because I have always loved process and tooling and youtube is now FULL of amazing documentary footage. I found it strangely enjoyable to watch.


One of the big drawbacks for me is the scraping, it's tedious and thankless. I guess that's why older people do it more, they learned patience.


What is scraping?


Many parts of machine tools like lathes need to be as flat as possible, because if the surface of the tool is not flat then that error is transferred to the object you are tooling. To get, say, a lathe bed flat, you buy or make a reference flat surface, paint the lathe bed blue, and then rub the surfaces together. The parts where the blue rubs off are the high parts. You take a scraper -- basically a modified chisel that takes off a thin layer of metal -- and scrape down the high parts. Wipe off the paint, repeat until there are no more high spots. It is very tedious.


Lionel Oliver has put together a great resource for this kind of thing for others who are interested [1]. When I was in high school I built a foundry (basically a stack of bricks with a draft hole and a place to put a small cast iron pot). I used it to melt aluminum and make castings in sand, and it was tons of fun. Looking back, I had no concept of safety at all but amazingly I didn't burn down our chicken coop, which is where it was built. The fuel I used to melt the aluminum was leftover lumber from our house remodel that I cut into cubes, and I had an old double squirrel cage blower set up to provide the draft.

1. backyardmetalcasting.com


That reminds me of the legendary Nils Provos and his blacksmithing ambitions: http://www.wired.com/2013/02/provos/

His videos: https://www.youtube.com/user/mintwart


If people find this interesting they might also like the myfordboy channel on youtube: https://www.youtube.com/user/myfordboy

There's tons of material in there on machining and especially metal casting at home.


I'm a huge fan of Keith Fenner https://www.youtube.com/watch?v=kANvdzoVUfw . He takes you all the way through a project.


Also tubalcain (google it).


Indeed, I watched all those youtube channels for quite a long time before deciding to build a foundry; there is excellent stuff out there! The Gingery books are really very good for books, but for a lot of these tasks it is so much more clear to watch someone do the work.


Have you looked into ironworking at all? I think building a bloomery furnace might be something you'd find interesting.

Some sites:

http://iron.wlu.edu/

http://www.wealdeniron.org.uk/

http://findsandfeatures.wordpress.com/2010/03/08/iron-ore-re...


I'm going to try to get good at working aluminum before I move on to iron; that's a whole other level. Fortunately I have some friends who are master ironworkers so I will go to them for advice should I eventually get there!


Nice article. As an unsafe counter-point, a few years ago I came home to my (then) 16 year-old son in the backyard tending to a pit he'd dug, full of a blazing stack of sticks that he was stoking with my leaf blower... sparks flying everywhere. He used this to first melt pennies, then my wife's aluminum cake pans. When I arrived he didn't have a shirt on, because he didn't want to burn his clothes. Ah, youth!


I used to collect old discarded car batteries, pull out the lead cells, and melt them down over the kitchen stove, and then pour them into glasses of water to make 'jewelry'.


I did the same, but made lead sinkers for hand-line fishing.


Just read the entire series (seems he was inspired in the summer of 2012, and recently returned to this backyard project). I found it really engrossing, and as he wrote in his first post, the spirit of hacking with tools outside the metaphorical software application.


Suggestion for next time - long sleeves. If a hot spatter lands on your arm, you could flinch and drop the crucible. And now you have a bigger problem.


I used to play knights & merchant as a kid so I was able to understand it :)

Why aluminium? Low temp, high availability?


Exactly. It is extremely inexpensive -- the majority of the aluminum I melt I found abandoned on the side of the road -- it melts at only 1200 degrees F which is an easy charcoal fire. It is easy to machine, doesn't corrode, is hard enough to build interesting things, and lightweight.


Have you considered or tried melting steel though?


As I mentioned in another comment, ironworking requires taking things to the next level in a pretty serious way if you want to cast it. Iron melts at 2800 degrees, you need ceramic crucibles, it is heavy, and so on. I'm not going to even consider iron until I have a lot more experience working with aluminum.

That said, I do have some friends who are experts at working iron, so if I do decide to take it up, I'll have a lot of good advisors.




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