Monday, February 20, 2012

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.