Furnacey fun

I made the rest of my casting patterns the other day, and after painting them with glossy paint, set to finishing the furnace.

I lined it with the clay mix from before after grinding it down to a very fine powder. Because I knew I was short on volume I added all the sawdust I could find, most of a bag of perlite, some more sand, and the last of my bag of bentonite. I also filled the bottom of the body with dirt to fill up some of the extra space.

When lining furnaces it's important that your clay not be too wet. That was my problem the first time, and the loss of moisture caused a lot of shrinkage, and thus, cracks. This time I made the clay much drier. I spread the dry clay out as thinly as I could on a plastic ground sheet and sprayed a very fine mist of water over it, then mixed thoroughly with a rake. I repeated this until the clay was uniformly damp. It looked a bit like breadcrumbs and clumped together when squeezed, but did not feel wet. After that, I covered it for a while so it could percolate a bit, and to give me time to get the body ready to be lined.

The inner form was made by wrapping the metal from an old canola oil tin around a pair of wooden discs. Before doing this I rolled it up as tightly as possible and tied it like that for a few days. This meant that the metal was springy and would hold the discs, but also that it would pop in on itself when the discs were removed. I packed about an inch of clay down straight on top of the dirt, making sure there was a former for a drain hole. That done, I set the inner form on the clay bottom and began to ram clay in around it. When I got to the level of the burner I put the pipe in and continued to ram. You should build the lining up in very small layers, working around the furnace as you go.

To line the lid I just placed it on a piece of plastic sheet on the ground, inserted a piece of PVC in the middle as a former, and packed the clay in. I rand wires all through the lid to add stability. After using the concrete here it occurred to me that I should have used concrete in the bottom instead of dirt too...

Unfortunately, I was STILL short of clay, so the lid was only 2 thirds full, and the body lining stopped short about an inch from the top of the steel wall. To compensate for this I ground the steel on the body back to the level of the clay, I also filled the top of the lid in with quick set concrete. This is not a hotface, so concrete was fine. It was just to add stability.

I also began work on a lifting mechanism for the lid, but haven't finished it yet.

I cast my first four lathe parts the other day, and they came out reasonably well. Some had little pockets caused by loose sand, but the pockets are in non-critical places. It took all day, but I got the carriage, cross slide, compound base and compound slide castings done, and turned all my scrap into ingots. I'll upload some photos soon, but at the moment I'm busy machining the castings on a friend's milling machine.

Disasters and discoveries in the alloy aisle, plus a new furnace

So, given my somewhat less-than-satisfactory results with the aluminium bronze, I started looking into other options. I looked at buying pre-alloyed aluminium bronze, but shipping will bite me. My next thought was Za-12, a zinc/aluminium alloy with excellent castability and hardness (google it).

While at a scrap yard looking for the piece of steel that makes up the front leg of my new furnace (read on for more), I discovered that the man who owns the scrapyard also sells ingots of zinc, 5kg each. AU50c/kg! At that rate, I'll be going with za-12, I hear it's easy to make at home given the low melting point of zinc.

Now, on to the furnace...

While 'making' the aluminium bronze, I overheated my furnace, driving the Portland cement reaction backwards, and making my refractory more like 'refractury'. A short while later, I found an abandoned water heater, one of the little ones. I also found a stock trolley in a skip. After putting my thinking cap on, I decided to build a completely new furnace, rather than relining the existing one.

I cut the water heater down, cut a ring out of it for a lid, and welded it onto the trolley, about 20cm above ground level. Then I welded a piece of 40mm box section steel -- the one that the nice guy at the scrapyard gave me -- onto the front, to provide support for the weight of the furnace.

The recipe for the refractory that I picked cannot be found here, because something has gone wrong with my link. Instead, I will just list the ingredients.

40% bentonite
25% 200-mesh silica sand
20% fire clay
10% alumina
5% borax


It has no cement in it, so is essentially a ceramic reaction, which I won't be able to drive backwards, no matter how hard I try. Hopefully.

I haven't done the lid yet, because I ran of of clay... But I won't be able to cast again for a while anyway, so that's fine.

[EDIT] I made the clay too wet, so it cracked and buckled all over the place. As a result, I had to let it dry and powder it right back down. Given how little extra volume I actually need for the lid, I've decided to add stuff to the mix willy-nilly to make up the volume. I have some perlite, which will go in, and I'll put in some sawdust as well, which will burn out, leaving a stronger, honeycomb-like structure. I might add some more clay and sand too, depending on how I'm feeling. More news later.

Experiments with aluminium bronze

Today I made some aluminium bronze, and it was a partial success I think.

I learnt a few things about the material, but I'm sure there's more to learn.

The first thing that I learnt is that it takes a surprisingly long time to melt copper; I was still trying after 20 minutes at full blast. Its melting point is 1084.62 degrees celsius, but either I couldn't get that hot, or there's some trick to melting copper. I got it to a very bright red-heat, but it wouldn't go any further.

So here's the lesson in that: according to The Complete Handbook of Sand Casting by C.W. Ammen, you should melt the copper first (p189), then add the other metals. I got fed up with w

aiting for the copper to melt, so I added my aluminium, which began to melt almost instantly, forming a puddle in the bottom of the crucible. Once this puddle had formed, the copper began to melt really quickly. If anyone can explain this, please do. Anyway, in future, I'll put copper in the bottom, aluminium on top, then more copper, then fire it up.

The second thing I learnt is that either aluminiun bronze is not nearly as golden as the internet would have me believe, or that it's a lot harder to get the ratios right than I thought. I aimed for 10% aluminium by weight, so I put 9 parts copper and 1 part aluminium in (2kg copper, 222g aluminium). One thing that leads me to believe that the ratio is hard to get right is that there was an awful lot of dross (or something that looked like dross) stuck in the bottom of the crucible after the pour. Thus, much of either or both metal oxidized. At what ratio, I can't say, but surely it is likely to skew my intended ratio.

In all honesty, I don't care about the colour, and the colour came out being pretty nice anyway. It looks almost like steel, only shinier. What I care about is the mechanical properties.

Here's an idea of the colour: the watch is stainless steel, the light has made everything a little yellower, though there are some patches about the place on other ingots that actually are that yellow. Also, notice how smooth the side is, and the two big flat bits on the top. The bumpy bits were loose bits of sand. This implies that I can get a pretty good finish with this alloy even with reasonably coarse sand.

The third thing I learnt is that aluminium bronze needs to cool slowly, otherwise it becomes very brittle. I quenched most of the ingots that I cast, with the result that I could break them by hand, or by dropping them from a height of about a metre. This had me worried, until I found an ingot I had forgotten to quench. I couldn't break it at all, I didn't try with a hammer though. In future, NO QUENCHING.

Here is a n ingot that I quenched, it was so brittle that I could snap it in my hand. It leaves a very sharp edge, I discovered the hard way. Notice how shiny the inside is? Also, you can see how it's quite crystalline. This has to be the result of the quenching, as the alloy ratio is almost certainly similar to the range of 'real' aluminium bronze, and I have one ingot, which wasn't quenched, which isn't brittle.

My theoretical pet metal

People who work with metal tend to have a favourite; under normal circumstances, I would say my favourites are steel and copper. When talking about casting, however, my (theoretical) pet is aluminium bronze. I say 'theoretical' because I haven't yet cast with it. It is, therefore, my favourite only by virtue of what I have seen and read about it.

Aluminium bronze is an alloy of copper (~90%) and aluminium (~10%). It takes on a vaguely brassy look, and I believe that Australian gold coins are made from it. Here are the reasons I like the idea of aluminium bronze so much:

great material properties - it conducts heat well, is highly resistant to corrosion, and is very strong and hard. It's used to make bearings in aeroplane landing gear, boat propellers, engine parts, and many other things.

Great aesthetic properties - it looks a bit like brass, but not as 'bright'. It's also antimicrobial , apparently, as a result of the copper content.

And finally, great castability - aluminium bronze is a 'short freeze' alloy, meaning that it turns from liquid into solid almost instantly. The solidification happens from the outside face and progresses inwards. This means, according to this paper, that, assuming you have enough risers and feeders etc., you can obtain almost maximum density when casting aluminium bronze. Virtually pore free!

These reasons have all combined together to make me want to cast my lathe parts out of aluminium bronze instead of plain old aluminium. I may cast them in aluminium first, to make sure everything goes all right, and then re-cast them in aluminium bronze. But we'll see...