The following is excerpted from Steve's new book, Mokume Gane A Comprehensive Study

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Making and Firing the Mokume Mini Kiln

First I want to cover the construction of the simple mini kiln, which I designed for laminating mokume billets used in smaller scale work. It is easy to make, and up until this past year, I used this type of kiln exclusively for the production of my line of mokume wedding bands. Although the chamber is small, it is capable of firing gold billets weighing up to 10 ounces, and is plenty big enough to make sheet for modest hollowware projects or knives.

One of the most critical issues that we have to deal with in producing quality mokume gane has to do with the ability of many hot metals to absorb oxygen and form oxides on and below their surfaces. This firescale severely disrupts proper bonding, and unless the causes of this are eliminated, it is impossible to make a billet that will withstand the stresses of rolling and patterning.

The early Japanese swordsmiths solved this oxidation problem by heating their billets in a blacksmith’s forge, which, by its very nature, provides a reducing, or oxygen starved atmosphere. This worked well for them because they already had the forge and it was an easy way of heating the large billets of copper-based alloys that they usually made. However, most of us do not have access to blacksmith’s forges, nor do we need to create such large quantities of precious metal mokume for our purposes. Considering the above, this very naturally points us towards the use of a small kiln.

Gas-fired kilns provide an easy means of excluding oxygen from the air around the firing billet, but, again, I found commercial kilns far too clumsy to operate when creating mokume gane for my individual needs. For this reason, I developed a very inexpensive and serviceable kiln using two standard-sized, soft fire bricks. These bricks, known as K-23 insulating fire bricks, are available from ceramic suppliers for a few dollars each.

The kiln itself has four main parts: The firing chamber.  Burner port.  Spy hole and secondary burner port.  Recess for the clamp.

The largest billet I make in this kiln is approximately 1 1/2" x 2 1/2", I find a spherical 3 inch chamber located in the center of the kiln (each brick contains half of the chamber) to be quite adequate for this. You can make the chamber larger or smaller, depending on the size of the billets you plan to fire, and the cavity may be somewhat elongated by carving the inside to allow adequate clearance for the billet assembly.

The burner (torch) port is drilled through one brick toward the bottom of the chamber and situated in such a way that it and the spy hole, together with the spherical chamber, allow an even circulation of the flame from the torch. The spy hole itself is drilled in the front section of the kiln (the brick without the burner port), a little above the center-line of the chamber, and beveled inside and out so as to make viewing easier. The recessed area that accommodates the C-clamp should not be carved until you have bought the clamp. Do not skimp on this item, since failure of a C-clamp during firing can ruin hundreds or even thousands of dollars worth of metal. I now use a 4 inch heavy-duty drop-forged C-clamp available through industrial tool and welding suppliers.

To carve the insulating fire brick, I prefer a sculptor’s clay loupe tool, but just about anything will work on this soft material. Take care not to put too much pressure on corners and thin places, as the brick breaks easily, and keep it flat on a smooth, clean, surface to support it as you work.

Using a pencil lay out the location of the chamber and clamp on both bricks and begin carving. Once the chamber is complete (each brick contains half of the spherical chamber), carefully carve the fire brick so that the clamp will lie down into a matching recess carved into one of the bricks. To carve this, I use a linoleum knife and coarse files. Position the C-clamp over the brick and redraw your lines, if necessary. Keep in mind that the handle of the clamp used in tightening should extend through the top of the bricks, while the body of the clamp will come out the side, between the two bricks. This is desirable because the cooler outside air will keep the clamp temperature down during firing, and thereby maintain its strength and pressure on the billet. The fit of the clamp into the recessed area should be as snug as possible, but not so tight as to inhibit removal of the clamp from the kiln.

Don't worry about small gaps between the clamp and the brick. During firing, the pressure of the burning gases inside the kiln will push out of these gaps and keep any oxygen from outside the kiln away from the metal. The object, of course, is to locate the clamp mouth, which will hold the mokume billet, in the very center of the chamber formed by the two bricks. When one brick is finished, mark out the position of the other side of the clamp on the other brick and carve it in the same fashion. The easiest way to do this is by placing the clamp into the finished side, then place the other brick on top of the clamp and wiggle it back and forth a bit. The clamp will make marks where it contacts the brick and show you where you should carve. By going slowly and repeating this procedure throughout the carving process, you can achieve a very snug fit.

When all of your brick work is finished, it is best to bind the bricks in some way to help hold them together. This can be done, in a pinch, with two or three strands of steel wire or a couple of long hose clamps wrapped around the circumference of each brick. For a more permanent and more protective binding, your local sheet metal shop can fabricate a frame of galvanized steel for very little money. Make the frame about 2 1/8” deep so that the brick extends 3/8 inch above the top edge. This will allow the clamp to be fully recessed into the brick without touching the frame. Any gap between the brick and metal frame should be filled with a thin mixture of casting investment so the brick will be fully supported by the metal. Filling this small gap will be much easier if you moisten the brick before pouring the investment. I prefer using a kiln with this kind of frame, which is much sturdier and will greatly extend the kiln’s life.

As for the clamp, you need to make slight modifications. The side of the clamp mouth connected to the body of the "C" clamp has a greater mass and will absorb more heat than the adjustment screw side of the clamp. Also, without some extension to this side, the billet will not be positioned in the center of the chamber. It is necessary, therefore, to add an extension piece to this side. This can be accomplished by welding a piece of steel rod or a bolt to the inside of the upper clamp mouth, or even just inserting a loose 1 inch long by 1/2 inch bolt with nut between the clamp mouth and the billet assembly. What I prefer, is to tap that side of the clamp and use a 2 1/2 inch long by 1/2 inch machine bolt as another adjustable element of the clamp. In that way, you can always make sure your billet is optimally positioned in the chamber.

You will also need clamp plates between which to sandwich the billet. You can make several pairs of these from mild steel or, for plates that are stronger at high temperatures, use high carbon steel or moly-bearing stainless steel. They should measure slightly larger than the intended billets, from 3/8" thick for very small billets to 1/2" for larger ones. Check them for flatness every time they are used, as they will deform under the heat and pressure of the firing process; regrind or replace them as necessary. The only other tools you’ll need for my mini kiln method are two pair of regular Vise-grips pliers and one Vise-grip locking C-clamp.