Second Stage

The second stage of the firing sequence takes the billet stack assembly from visible red heat, to bonding temperature. Watch the color of the metal to determine hot spots and keep the temperature of the billet stack as even as possible. Remove the flame of your hand held-torch occasionally, so its light does not distract and hide the first sign of glowing metal in the kiln. REMEMBER (especially those of you firing with one torch only), THE HOTTEST PART OF THE KILN IS THE CORNER FARTHEST FROM YOU, WHERE THE PRIMARY TORCH FLAME BOUNCES OFF THE CHAMBER WALL.

You may wish to use the pyrometer from your burnout kiln to keep track of the internal temperature. However, the thermocouple will heat much more quickly than the mass of the billet, so you need to watch the color of the thermocouple in relation to the color of the billet. You do not have to use a pyrometer. You do need to know the signs that indicate when bonding is taking place.

Listed below are some internal kiln colors and corresponding temperatures to use as a general guide for visually estimating when bonding temperature has been reached. Estimating temperature by color is a subjective judgment at best, and, depending on the exact alloys in the billet, actual bonding temperature will vary. If an alloy contains more copper or zinc than another listed here, the bonding temperature will be lower. If it contains more palladium or nickel, the associated bonding temperature will be somewhat higher. Use these as a guide for Liquid Phase Diffusion Bonded (eutectic) mokume only, and watch for the other signs I give you. Page 81 lists temperatures for electric kiln-fired mokume.

Make every effort to even out the heat in the kiln. Go slowly for the first few firings until you get a feel for the firing process. As the billet begins to glow, you should be frequently scraping the edges with a sharp steel probe. Scratch parallel to the layers, and also across them. This will actually drag small amounts of one metal across the surface of the other(s) and it will be there that signs of sweating can first be observed. When you believe you are at bonding temperature, there are several signs to look for. They are given below in order, from the most obscure, to the most obvious.

1. The shimmer of visible sweating where the metals meet, when the edge of the billet is scraped with a sharp steel probe. 
2. The shimmer of visible sweating where the metals meet, when a corner or section of the billet is heated locally by your second (hand-held) torch. 
3. The shimmer of sweating on the edges of the billet. 

The signs listed below indicate the temperature has gone too high. Turn off oxygen only and cut back gas. Reduce soak time according to how much you judge the piece has been over-fired, but do not turn off the gas until the metal completely solidifies.

• Partial melting of the whole edge of the billet, or of a single sheet in the laminate. 
• Liquid metal flooding over the entire side of the billet. 
• Collapse of the billet stack. 
• Hey! Where is my billet and what’s that burning smell? 

You would be surprised at how little temperature (and time) difference there sometimes is between #1 and #8. But let us assume that you are somewhere between #1 and #4. What should you do now?
 
 

Third Stage

This is the third or "soak" stage of the firing. When you are sure that bonding temperature has been reached, cut back your torch(es) so as to maintain the temperature slightly below (maybe 20º) what it was when you first observed the sweating. Maintain this temperature for five to ten minutes, depending on the size of your billet, and your observations of any continued sweating. This soaking is to make sure that the entire billet has the opportunity to reach diffusion temperature, and to promote the growth of the bond. This is quite a delicate stage and one that, unfortunately, only experience will fully illuminate.

After you have soaked the billet, turn the torch oxygen off, leaving the gas on. Let the billet cool in this manner for half a minute or so, allowing it to slowly and completely solidify.
 
 

Fourth Stage

The final stage of the process must be accomplished quickly and with great care while the billet is still red hot. Wear insulating gloves! Turn the kiln down flat on its back, snip the binding wire and remove the front (now top) half of the kiln. With pliers or other grasping tool, remove the clamp assembly and secure the back of the clamp firmly in a heavy bench vise located near your anvil. Carefully loosen the clamp and remove the billet and clamp plates. They will probably hang together at this point but they may separate and fall to the floor, be careful! Place them on the anvil and tap with a hammer to loosen the clamp plates if they are still attached, then quickly forge the billet in a circular pattern spiraling from the center of the billet outward. You may also use a hydraulic press or even a bench vise for smaller billets to achieve this forging. This process enhances the bond as well as the grain structure of the metal. After forging, quench in cold pickle at the appropriate temperature for the metals you are using. Preparing the billet for patterning begins in Chapter IX, on page 89.
 
 

For more information on making your own mokume gane and Steve's book, Mokume Gane A Comprehensive Study and video, "Mokume Gane in the Small Shop", click here.