Aluminium Casting and Why You Need to Be Careful

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Introduction: Aluminium Casting and Why You Need to Be Careful

With interesting projects comes the possibility for interesting problems or as a recent Instructables competition puts it, Epic fails.

Maybe I should be concerned that when I checked the emails this morning and saw the competition for epic fails that I had at least 2 options ;)

I have been melting metal for a few years now, in this instructable I will detail a few of my experiences along the way and a few things that I do not recommend for your own safety.

My first metal melting experience was with a coal fired foundry. It was created with a 20 litre stainless steel bucket on the outside and a 4 litre bucket on the inside, a pipe came through the side of the bucket and was connected to a vacuum cleaner on blow. The space between the 2 buckets was filled with normal concrete and while it did melt metal it also spewed small hot coals once it burnt down a little.

My first crucible was a section of 100mm galvanised pipe with some steel plate welded to the bottom. 5 or 6 years ago when I made this earliest foundry I did not have the skills to make the crucible myself so one of the fitters from work made it for me. While I no longer use I do still have it.

The photo above is pretty much my second foundry. It was LPG fired and was reasonably effective.

Like the first foundry it was made with ordinary concrete and through some miracle it did not explode or spit at me.

In ordinary concrete the cement adsorbs the water in the mix thus at high temperatures it can release the water.

Water is many times the volume in steam form (1600 x I heard once) and water turning to steam inside a closed volume can result in extremely dangerous steam explosions where concrete can go flying.

In the photos above I am melting brass from old tap ware in a commercial clay graphite crucible bought off Ebay.

In the second photo I have poured the brass into my so called 'metal muffins'.

As you can see here muffin tins make great ingot molds

Step 1: My Current Furnaces

Around a year ago I decided that more serious metal casting was on the agenda.

Like any mad scientist I went to Google, Instructables and YouTube and read and read.

Not wanting to spend a great deal I settled on a home made refractory mix of Perlite and Calcium Aluminate cement. I don't know the exact chemistry but CA cement performs MUCH better under heat than ordinary cement. Perlite is a processed natural substance, sort of like pumice, super light and fluffy, with excellent insulating properties.

In the photo you can see the furnace just after me making it.

On the outside is a section of 200L oil drum, then the home made refractory then the 'hot face' was a commercial 'muffle ring' made of Clay Graphite mixture. Muffle Rings are used in large furnaces to 'extend' the sides of the crucible so the solid metal can be piled above the crucible and not spill out. The metal will obviously melt down to a smaller volume and be contained by the crucible itself.

The muffle ring was approx. 400mm diameter.

As per proper commercial refractory one should always proceed slowly with warming up their new furnace to slowly drive off any remaining water in a controlled fashion, failure to do so can result in problems as described in the introduction.

Step 2: Current Generation of Furnace Running

Here is shown my first new generation of furnace running.

I dont know exactly what temperature I was achieving but I was able to (just) melt brass, my best guess would be in the vicinity of 1100 degrees C.

This burner was dual fuel starting on LPG to warm it up before applying waste oil then turning off the gas.

The burner used a spa pool blower and a dimmer style speed control to vary the airflow.

I was too stingy to spend any serious money on a 'proper' furnace nozzle so my oil delivery system consisted of old mineral oil in the blue tank then pressurizing the tank with my compressor, the oil was then delivered by a hose to a nozzle with a 0.5mm (or thereabouts) hole and squirting into the hot furnace and burning.

So long as you apply air in the correct proportions to the oil you can achieve a relatively clean and smoke free burn.

You can see in both photos the furnace glowing super hot, both of these are radiant heat only, the intense glow is not combustion. See in the second photo the perlite 'composite' lining of the lid glowing red hot also.

Note in the second photo my safety gear, this would be the very minimum; Polycarbonate face shield (performs incredible in the heat, still on my first one), Leather welding jacket, Cotton overalls, Leather work boots.

Note here I am wearing ordinary (but quality) welding gloves, soon after this photo I bought some proper aluminized gloves of eBay after some serious burns to my hands. I found that when exposed to furnace exhaust air that the gloves can just heat up too fast and by the time you realised that they were hot that you could not get them off fast enough.

Also around this point I discovered steel 7lb fire extinguishers as the best crucibles that no money can by. I contacted a guy who supplies and certifies fire extinguishers and he has given me a number of them during the last year and they make the ideal crucible and last for quite a number of melts before the steel flakes off too much.

Step 3: When Aluminium Catches Fire, the Thing That I Never Saw on Any Casting Website

Struggling to melt Brass I spoke to a rather helpful commercial refractory supplier who recommended that I use Kaowool to insulate the furnace. The ideal would have been the outside of the hot face but I didn't want to make another at this point so I loosely lined the inside of the furnace with Kaowool and I was quickly able to reach brass melting temperatures.

Kaowool is a ceramic blanket (mine was 50mm thick and rated to approx. 1300 degrees C)

One fateful night about a year ago I was melting aluminium scrap into ingots. I had asked my wife to grab the camera for us to take a video to show an interested family member when I saw the flames change from orange to a more yellow color.

The flames and furnace seemed to almost twinkle for a moment(best explanation I have sorry) before a bright white/yellow flame shoot up from the furnace to a height of a metre or so.

The flame was not explosive and was directed straight up out of the furnace (We would liken it to a fountain firework only much bigger and more intense). While we were outside we removed anything that might catch alight from nearby and turned off the air and pulled out the burner.

I would approximate that there was between 2 and 3 litres of molten aluminium in the crucible, this burnt for say between 2 to 3 minutes (was still burning after we had scurried around moving nearby 'stuff') and in this time it consumed all of the aluminium in the crucible and destructing the crucible itself.

In the photo to the right the spongy mess is all that remained of the crucible after the experience. I believe that the Aluminium burn melted or burnt the Kaowool on the sides in the bottom of the furnace.

The refractory base while damaged was not 'burnt through'.

My research since leads me to believe that the temperatures in the furnace after aluminium ignition probably reach towards 2000 degrees C but I have no way of knowing.

Not long after this I discarded this foundry and built another.

To answer the question before it arises the metal I was melting was extruded aluminium conductor.

Step 4: Summary

Still a year or so on I still have not seen anybody mention of Aluminium catching alight during casting.

Undoubtedly the melt was way too hot but still...

Just as I close I might add the 2 simplest things learnt so far from my casting experiences.

Safety Equipment is your Friend.

Volatile liquids that turn to gas then fireballs (Including Oil, LPG, Methylated Spirts) are your Enemy and should be treated with extreme caution.

Stay tuned, I hope to post another epic fail in the next few days.

Hmmm a home made Vacuum chamber, what could go wrong.....

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89 Comments

Great failures can lead to great success !

4 replies

Yup, look at Nobel, invented TNT, the last thing he invented in his career was a really good explosive...a bit of a fail though since it blew up and took him with it.

You're thinking of his brother. From Wikipedia's entry on Alfred Nobel - "Life & career" section:
"In 1888, the death of his brother Ludvig caused several newspapers to publish obituaries of Alfred in error. A French obituary stated "Le marchand de la mort est mort" ("The merchant of death is dead")"
{Britannica.com. 19 March 2012. Retrieved 26 January 2014}

he died of a heart attack at 63.

Certainly this and other experiences has taught me a great deal and the quality of my projects is starting to reflect it.

Stay tuned for more interesting projects and instructables in 2016

I was recently reflecting on how foolish I was to melt aluminum with NO SHOES, NO GLOVES, NO FACEMASK and NO WELDER'S JACKET. I was a total Idiot.

I remember a story about how America experimented with aluminum tanks until they discovered that phosphorus rounds burnt hot enough to ignite it. Seemed like a good idea until it wasn't.

1 reply

They made some warships out of aluminium too. When hit with a torpedo they would burn for days

This is why I don't use unknown scrap. Buying ingots or aluminum blocks from a reputable vendor maybe more expensive, but it is certainly cheaper than a stay in the local hospital's burn unit. The best take-away from this story is recycle the scrap, only melt known metal. It is certainly safer, and cheaper in the long run.

"Water is many times the volume in steam form (1600 x I heard once) and water turning to steam inside a closed volume can result in extremely dangerous steam explosions"

Krakatoa springs to mind, the result was catastrophic.

Was there by chance any iron oxide in the mix? If so, a thermite reaction is a possibility.

1 reply

Sure it is possible. I guess we will never know for sure. Best and most likely explanations are either a thermite style reaction (wouldnt have been a great deal of Iron Oxide) or Magnesium amongst the scrap which caught alight.

A recent experience with an alloy ingot that is likely contaminated with Zinc is once again showing the importance of identifying your scrap before a melt)

I too have had the experience of having the scrap that I took for aluminum catch fire when I tried to melt it. It was a Lou Reed moment - White Light White Heat. I shut off the gas, put two lids on the furnace and buried the base with dry sand. It went out. Water would have caused an explosion. So what I took for aluminum was likely magnesium or an aluminum/magnesium alloy. Scrap aluminum is generally an alloy of some sort. Because I do public events where participants carve into scratch mold and I pour them full of molten aluminum, I need to know I have predictable metal. So I make ingots first with my scrap by melting it and pouring into angle iron as others have described here. I also buy crucibles from Legend Mining Supply to avoid the steel scaling problem.

6 replies

You are correct. I work at a large aluminum foundry and water is not allowed near any tools except for the casting pits to cool the jacket. The melted and holders and even the aluminum ingots need to be dried in large ovens.

Yes, thats the thing with scrap 'aluminium' mostly an alloy, you might not know what you are melting. Certainly one thing I have learnt here from the comments is the importance of identifying your scrap.

Yes, I can get ingots from a foundry supply company, and I have done that. They are a known composition and they melt beautifully with little waste. They are also big and I don't really have the right equipment to saw them up. I am going to check out scrap from a sign maker I know. If that is suitable, I will have a consistent source.

Sounds like a nice luxury. Are they expensive?

My furnace is smaller than yours. I'm using #8 budget graphite crucibles which hold about 8 lbs. of aluminum, and currently cost $27.42 ea., and size "K" fused silica crucibles with about the same capacity which run $129.29 for a case of 6.

That seems pretty reasonable. I do have an A1 and A2 crucible which I normally reserve for brass. Have recently purchased a 16kg capacity crucible for when I hopefully achieve a cast iron melt. Not so easy to get here in New Zealand, have been recently talking with a metal casting supply company and they are able to supply crucibles though. I expect they will be much more expensive here.

With aluminum you need to make sure that everything that touched the molten metal is completely dry because of it is not it will violently explode. Also most aluminum is an alloy in which may contain magnesium. You better have a extinguisher on hand that can handle burning metals which is possibly what had occurred to you before it burned off.