experiment4.com

The late, great Dave Gingery wrote a wonderful of books that describe how to build some pretty decent metalworking tools from scrap. If you are interested in metalworking, but don’t want to spend a lot of money, get these books! They are available from Lindsay Books.

The first book describes the construction of a small charcoal fired blast furnace, which is then subsequently used to make the castings to build the machines.

The best thing about this furnace is that it is incredibly cheap to build. And pretty easy too.

I build a “by the book” Gingery furnace (charcoal fired, with the standard Gingery lining) and I’m really happy with it.

The hardest part was finding the components – the books were written a while ago, and some of the stuff is a bit hard to find. Here is
where I found my stuff:

• The 5-gallon pail: these days it’s pretty hard to find a metallic 5-gallon pail – about the only thing I found that comes in these is roof tar. All the credit here goes to
  Bill, who was the first to discover (and use) a beautiful 5-gallon stainless steel stock pot from WalMart.
• Fire clay. I had a little trouble finding that. Pottery places I called or visited seemed to think that “raw materials” mean unpainted pots, and ceramic / clay places wanted me to get real specific about what cone I was going to fire the clay at, etc. Finally I went to the local construction materials company. And sure enough, they had generic fire clay, powder form, no questions asked. Perfect.
• Sand. Silica sand is the only thing that was easy to find. Home Base sells Silica Sand in 100-lbs bags.

11/18/2000 – Construction

I prepared the refractory mix a couple of days early, as the book recommends. I can vouch that following the instructions to the letter yields a pretty good lining: 5 gallons of sand, 2.5 gallons of clay, 3 quarts of water. I ended up with about 1.5 gallon of leftover, because the WalMart stock pot is a bit more squat than a standard 5-gallon pail. But that’s OK, because I will later make green sand with it (by adding more sand) – same components, different proportions.

I was a bit surprised to notice that dry sand and clay don’t mix that well after all. The clay tends to pool in the corners of the tub. I ended up putting on gloves and mixing the thing by hand, after adding water too. This worked very well.

Bill recommended that I “shape & bake” a sample, to see how it behaved. That was a really good idea, because it made me realize that the thing must be packed really hard to have any structural integrity.

The Gingery lining is basically a kind of fire clay. It’s important to pack it well, and to cure it well, otherwise it is fairly crumbly and won’t hold at all.

I wanted a pretty good fit for the air pipe, so I cut the hole in the pot this way: after cutting the pipe at the correct angle, I traced the outline of the hole and drilled through the center of the outline with a 1/2″ bit. I then used a jigsaw with a metal blade to enlarge the hole, but still staying within the outline. And then I finished with a file, until the air pipe fit in there.

Here is the furnace with the form in, ready to pack.

The lid

	My wife found a pretty nice steel pot to use as a crucible.
	Of course I had to remove the handles, and I realized I could use them for my lid.
	It was a simple matter to bend them to the right angle, and attach them to the strip with sheet metal screws.
	To make the lid, I didn’t use a form like the book says, but simply wrapped the sheet metal strip around the bottom of the pot, then built the wire frame.
	After that I removed the assembly from the bottom of the pot, and it kept its shape pretty well.

Packing the lining

	It’s very important to pack the lining pretty hard. As mixed, it has quite a bit of air in it. I pounded it a bit too hard in the pot however, and the inside form started to give way.
	Now the inside section of the lining is vaguely octogonal. It doesn’t seem to have affected anything though, except that I really had a hard time pulling out the bottom disk for the form.

Another difference from the book is that I laid down the bottom layer of lining first, then inserted the form and did the sides. It’s a bit easier, I would think. Most people seem to do it that way.

The Gingery lining must be kept wet until fired, which makes removing the form itself and the air pipe pretty easy. Apparently it’s a lot harder when you use a refractory cement mix.

My last action of the day was to put the lid in the oven, covered with a wet cloth, ready to fire in the morning.

11/19/2000 – Firing

Curing the lining is a whole day thing – I started by turning on the oven to 250 at 6am, then moving up to 550 at 8:30am.

Here is the furnace, all set up and ready to fire. Picturesque, isn’t it? What can I say, this is California.The blower is a small vacuum cleaner, the hose is taped (with paper tape, so any reflux will burn through and out instead of into to plastic hose) to the air injection pipe. For the initial burn I plugged in the vacuum with an X-10 dimmer module and controller - the vacuum by itself would pump way too much air.

At 2:30pm, I turned off the oven (but left the lid in there) and started the charcoal fire in the pot. Lighting the charcoal was pretty hard, there is no draft at all in the pot when the air is off. I finally turned the air on very low. That did the trick.

	When the charcoal was lit, I took the still-hot lid out of the oven and put it on the pot (thank God for welding gloves!) and gradually increased the air.
	I was pretty concerned about getting it really cured, so I kept adding charcoal. The burn ended up lasting about 90 minutes.

 

At the very end (you can see it was pretty late) I put on the last bunch of charcoal and turned the air on full blast. I got this very nice flame shooting up. Some spots of the lining started glowing red, so I figured it was probably pretty dry at this point.When it seemed that most of the charcoal had been burned, I turned everything off, covered the holes with brick, and called it a day. As it turns out, it seems that the rest of the unburned charcoal slowly smolders away after that. 14 hours later, after a whole night outside, the pot was still hot.

 

 

	The lining got really hard and reddish where it was hottest (i.e. the inside of the pot, and the bottom of the lid) with some traces of vitrification (darkish, semi-translucent areas). The parts that did not get heated up very well (like the air inlet, and the top/outside portion of the pot) are quite a bit more brittle.
	The lid finished curing during the firing proper – the temperatures reached in the oven were probably never enough to really cure it, but the firing did the trick.A few cracks appeared on the lid during the firing, but they are very fine and fairly shallow. They don’t seem to have affected the structural integrity of the lid.
	This is the bottom of the lid. Notice the change of coloration from the firing here too. The reddish portion is much, much harder.

 

---