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.