The Coated Tyle Furnace

The old version of these instructions has moved here

Here’s a way to smelt your own iron, in a bloomery furnace built of cheap and readily available materials.

Furnace Materials:

7 or so 10” cinder blocks

2’ section of 12” square flue tile

            about 50 lbs. dried, ground clay (any clay will work here, but the stickier the

better. If you are purchasing dry clay from a ceramic supplier, we have               found ball clay works well)

            8 lbs. Cellulose building insulation

            5’ of 2’wide chicken wire

            fan - high pressure/high volume, e.g. shop vac

            pipe fittings and hose

            ¼” thick copper for tuyere

Smelting Materials: (for @ 20 lbs of bloom)

            @70 lbs roasted ore

            @ 200 lbs wood charcoal (not briquettes!)

            scrap wood for preheat

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Preparing the ore and charcoal:

Finding your ore is, of course, a major project in itself, which we don’t have time and space to go into here. But when you find it, you first need to roast it. Heat the ore to a bright red color, in a wood or gas fire, and hold it at that temperature long enough to make sure it’s heated through. Quench the ore in water while it’s still hot. Your ore should now easily break up into small pieces. In this furnace you want the ore bits to be about the size of a sunflower seed. Breaking it up this small will also produce a lot of dust and fines, which is ok- we’ll smelt it all. Pick out any obvious impurities.

            Procuring your charcoal is also a major project. There are many reasonable methods to produce your own, which you’ll probably want to do, as real wood charcoal seems to be selling for about 50 cents a pound these days. Break up your charcoal until the pieces are about  1”, and sift out the fines through a ¼” mesh. Save the fines for the furnace base.

Forge a Tuyere:

Using ¼” or thicker copper plate, roll up a tube of  ¾” or 1” interior diameter, about a foot long. If you flare one end it will make it easier to attach to your air supply. Although we will be operating at temperatures way above the melting point of copper, it will conduct away the heat to the outside of the furnace rapidly enough to keep it from melting.

Building the furnace:

First, you need a dry, elevated base for your furnace. Stack the cinder blocks into a rough square, leaving a space open in front, like so (plan view)

Fill and pack the center of the hearth you’ve created with charcoal fines, level to the top of the cinder blocks.

Next, you need to build the furnace shaft itself. On one side of your flue, drill a hole large enough for your tuyere 6” or 7” from the bottom, centered right to left. Make sure it’s large enough to allow the tuyere to protrude inside the tile 2 ½”, and angling downwards 22-25 degrees.

On an adjacent side of the tile cut a slag tapping arch out of the bottom edge, about 3” tall and 6” wide.

The best way to cut these tiles seems to be to drill a series of small holes with a hammer drill, then chip out the piece you want to remove. If the tile cracks a bit, it’s no big deal, as it’s going to crack as soon as you light a fire in it anyway. Set the tile on top of your charcoal base.

Now, to sheath the furnace with clay. This will hold the whole thing together. Weigh out your cellulose, and thoroughly shred the clumps. Mix in the clay, coating all the cellulose with clay. Next add water, and knead it to the consistency of cold gooey oatmeal. For 50 lbs of dry clay, this may require almost 50 lbs of water. Plaster the outside of the flue tile with the clay mixture ¾” or 1” thick.

Wrap the furnace with the chicken wire, and twist the loops of the wire to tighten the whole thing up. Use the leftover clay to buttress out the bottom foot of the shaft where it rests on the base, and use some clay to lute the tuyere into place. Save a little clay for patching and emergency repairs.

The following drawing gives an idea of the whole assembly.

Now hook up your air supply to your tuyere. The following schematic gives you the basic idea. You may put a valve in the hose before the T; we prefer a dump valve on the T opposite the hose. Or you may use a variable speed fan. What’s important here is that the sight glass is removable for clearing the tuyere during smelting, and that all connections are reasonably stable and airtight.

The tuyere protrudes approximately 2.5” into the furnace and dips down at an angle of 22 degrees.

Now to Smelt!

Light a gentle wood fire, with natural draft through the tap arch, to dry and preheat the clay. On the first run of the furnace, this will take well over an hour. Poke any bubbles forming under the clay to let the steam escape.

            When the clay is mostlydry, fill the furnace with charcoal, block the slag arch with a brick, and turn on the air. From now on, you will keep the furnace full to the brim at all times. Set your air rate so you burn 4 lbs of charcoal every 10 minutes. This 4 lb charcoal charge will be your basic unit of measurement throughout the smelt. When you see a really nice bright white heat through the tuyere, and maintain that temperature for maybe half an hour, you may begin to charge ore.

            Sprinkle the ore evenly on top of the charcoal bed. At first, add only a half pound of ore to every 4 lbs. of charcoal burned, gradually increasing it until you are adding 3 or 4 lbs of ore for every 4 lbs of charcoal burned. It should take you 3 or 4 hours to work up to this charging rate. If you add it too fast, the furnace will start to cool, indicating you should back off on your ore charges. Your burn rate can increase to 7 or 8 minutes for 4 lbs charcoal.

            Monitor your fire through the sight glass. Use a rod to clear slag from the tuyere if necessary.

            After you’ve charged 60 or 70 lbs of ore, remove the block from the arch. Poke in there gently and see what’s going on. Your goal is to keep the lower part of the furnace full of slag. The slag is not just impurities to be removed- it is a vital part of the reduction process.  If the slag rises high enough to start blocking the tuyere, poke in and let some run out. Cool it in water, break it up, and return it to the top of the furnace. Any slag that is black and freely running, indicating a high iron content, can be recharged onto the furnace. Only discard slag if it is thick and gooey, indicating a low iron content.

            Tapping and recycling slag ensures a constant flow of iron rich material through and around the bloom, this is what makes it nice and solid. When the slag begins to get thick and gooey, you’ll know it’s about time to finish.

            7 or 8 hrs after your first charge, you’ll be ready to remove your bloom. Scoop charcoal fines from under the base of the furnace. Using a rod from above, knock the bloom out the bottom of the furnace.

            If you have enough energy left to start forging it now, you’ll save a lot of reheating time. Work the bloom gently at a low welding heat until it starts to consolidate, then take full welding heats until you have a solid billet. Now sing the smelting song:

Eeeeeeeeee blacksmiths are numerous,

Aaaaaaah but those who can melt iron from stone have grown rare.

Beekillers are many.

Lionhunters are few.

-West African Song