Pre-fire Prep for the Mokume Mini Kiln

Begin by carefully stacking the cleaned metal, paying close attention to the proper order. Make sure the surface of the sheets remains clean. When they are stacked and the sides are squared up, firmly squeeze the stack tight with one hand (it's okay

to touch the outside pieces now) and with the other hand, apply a band of Scotch tape completely around the edge of the billet stack, as shown in the photo at right. Fold any excess tape over onto the top and bottom sheet. This not only temporarily holds the billet together, but also protects the edges from becoming dirty during the clamping process. Now, put it aside in a safe place while you prepare the clamp plates.

From your selection of clamp plates, select the pair that best suits the size of your billet stack. Check it for flatness and paint the faces that will hold the billet with a barrier coating of yellow ocher, or Scalex. This will act as a separator and keep the metal in your billet from fusing to the clamp plates. Then set them aside to dry. You may also use typewriter correction fluid or sheet mica for this purpose. I prefer a combination of a paint-on separator and mica. The mica allows you to paint on the flux with no risk of contaminating the metal stack with the yellow ocher or scalex. Later on in the firing, if the mica begins to burn away, the scalex or ocher is still there. For high temperature firings like platinum billets, I use all three! At any rate, do whatever it takes to make sure the billet does not fuse to the clamp plates. Then, take the kiln C-clamp and loosen the ball-joint swivel on the adjusting screw and the screw itself. These will be stiff from the previous firing. Apply a drop of oil to both points and work until they turn freely.

On a fireproof pad, place bricks (any kind will do) on which you will set the kiln when loaded. Make sure these are in position so that the kiln will be oriented properly for your torch.

The excess tape that has been folded over the top and bottom of the billet now needs to be carefully trimmed away with a sharp knife. Make sure you leave the tape that is still covering the edges, as this will be removed later. Next, take the billet stack and place it between, and in the center of, the two clamp plates. Pinch the whole clamp plate billet stack assembly firmly with one hand and with the other, take the Vise-grip C-clamp and very tightly clamp it in the exact center of the assembly.

Note: Alternately, if you have access to a hydraulic press, you may use it to smash the billet layers together prior to clamping. This will help achieve greater contact between the layers, but is not absolutely necessary.

Adjust the two other regular Vise-grips and simultaneously clamp these to either end of the assembly with firm but even pressure. Make sure the pressure is even on both ends or this can deform the metal in the billet stack. Release the Vise-grip C-clamp in the center.

Now it is time to clamp the whole billet assembly into the kiln C-clamp. First, check the level of the top adjustable clamp bolt (if you have added one to your clamp) to make sure the billet will be properly centered in the chamber and in good view. Place the billet assembly in a vise to hold it steady. Then tighten the C-clamp onto the center of the billet assembly. I tighten as hard as I possibly can by hand and then use pliers for another half turn. (It is important to tighten the clamp enough to create flexing in it, so that as the billet fires, and softens, it will retain positive pressure on the metal.) Now, if everything looks good, release the two vise-grips simultaneously and remove this whole contraption from the bench vise. Inspect to make sure that the billet stack has not changed position (sometimes it twists), and check that you can see no gaps between the layers. Then, carefully remove the rest of the tape on the edges of the billet.

It is at this point that I would carefully apply a generous coating of borax paste flux to the edge of the billet stack. This, I have found, protects the edges of the billet from contamination during the firing process and reduces the amount of metal that must be trimmed away later on. For the first couple of firings, you may wish to use no flux. It is very easy to mistake the shine of molten flux on the side of the billet for telltale signs of sweating. Try a couple of firings without the flux until you get a sense of the quality of the glow the metal has at fusing temperature. Then move on to using flux.

The clamp assembly may now be loaded into the kiln. It is important that the billet assembly be fired in level position. Do not fire the kiln with the billet layers perpendicular to the ground. If the metals reach the completely liquid eutectic point, this thin molten layer of metal can act as a lubricant and the sheets can literally slide out of the clamp assembly.

Both halves of the kiln are now aligned and bound together with a couple of light, steel wires. Set the kiln in place and position your main burner torch so that the tip of it is about 1/4 inch from the outside opening of the torch port. You can make a special torch holder or use vise-grips to build a temporary one.

Firing requires your full attention, so make sure you will be undisturbed for at least 30 minutes and can concentrate exclusively on the firing. Once you begin the firing, do not turn off the torch until the bonding is complete. To do so would allow oxygen into the kiln and contaminate the metal. So, unplug the phone, turn off your beeper, chase out the kids, and tell your significant other to keep a significant distance away!

Light your hand held-torch first and use it to light the main burner torch. Keep this flame directed at the main torch until you have both the gas and oxygen turned on and adjusted to a reducing flame. (A reducing flame is one which has a yellow tongue coming off the inner light blue tip, at the center of the flame.) The yellow should be clearly visible but the flame should not be sooty. Keep close tabs on the flame during the entire firing. Some gas and oxygen regulators will change pressure slightly while in use and this change can be enough to shift the flame from slightly reducing to slightly oxidizing, which could be disastrous. Billets containing platinum should be fired with a neutral or slightly oxidizing flame.

Caution: Do not allow the chamber of the kiln to fill with gas before lighting, as this can cause an explosion. When firing, always wear glasses to protect your eyes from UV rays.
 
 

First Stage

The initial stage of firing is for steadily and evenly raising the internal temperature of the kiln. If you are using two torches (which I highly recommend), adjust them to work together to keep the chamber heated as evenly as possible. Hand-hold the second torch so you can easily change its position to heat up the colder areas of the chamber. If you are using just one torch, keep an eye on the hot spot opposite the torch port where the flame bounces off the wall of the chamber. Try not to let that side of the billet stack assembly get too far ahead of the colder side. This will probably require cutting back on the flame size as you reach visible red heat.