GSP could you possibly explain what you run into in trying to keep heated a large qty of gold like this to pour. I feel you can get the point across better than I can. I don't think people understand the problems of trying to heat up quantities like this.
That's hard for me to answer because I never thought much about it. Of the many 100s of melts I've made, I only remember a few times that the melt froze before I poured it. Usually, that was because of something dumb I did. For example, I pulled the crucible out, only to realize that I hadn't prepared the mold yet or, maybe, I hadn't yet dumped the metal out of the mold from the previous pour. When this happens, I just put the crucible back in the furnace and re-melted the metal. You soon learn how hot the melt should be in order to not have any problems. Generally, you can do this by looking at the colors. When the material isn't well melted, it will be darker than the color of the inside of the furnace. Depending on what you are melting, the right color is often yellow or light yellow. When it's ready to pour, the furnace, crucible, and melt will all be very close to the same color.
You have to look into the furnace often and you can't do that without the right equipment - it's too hot. An old method is to make a square ping-pong like paddle about 12" x 12" square, or so, with a handle, out of about 3/8" plywood. In the center, cut a slot about 1/8" wide x 4" long, vertically. The kerf should be about the right width with a circular saw. To look in the furnace, you just hold the paddle to your face and look through the slot with one eye. I usually don't remove the lid to look at the melt. I look through the exhaust hole in the lid, which is usually about 4" wide on a #20 furnace.
If you have problems getting enough heat to get a good melt, it is usually because the furnace isn't adjusted properly or the gas pipe wasn't large enough to start with.
Here's how I used the natural gas furnace:
(1) I put a piece of burning paper in the furnace chamber and partially turned on the gas only - no air. Leave the lid off. Adjust the gas till it swirls to about the top of the chamber.
(2) With the lid off, let it heat the furnace for 5-10 minutes.
(3) Then I closed the lid and let it heat the lid for a few minutes. Adjust the gas so the flame is coming out of the exhaust hole in the lid about 6" - 8".
Warning. If you close the lid too early, before the chamber is heated up properly, the flame can go out. If this happens, shut the gas off and turn on the air to exhaust the fumes from the chamber. Turn the air off and start over
(4) Turn on the air slightly. The flame (the tail) coming out of the hole will be shorter. Adjust the tail until it is about 3" long. At this point, the roar of the furnace should be about maximum.
(5) Let the furnace heat until it's about yellow inside - maybe 45 minutes. You may have to adjust the tail every so often. Just remember - more gas, longer tail - more air, shorter tail.
(6) For certain things, you need the furnace to be hotter or cooler. To raise the temp., first increase the gas (the tail will get longer) and then adjust the tail down to about 3" with the air. To lower the temp., first decrease the air (the tail will get longer) and then decrease the gas to about a 3" tail.
Warning. The order of making adjustments is extremely important. If you do it in reverse, you will have not tail and you can blow the flame out. For safety sake, you should always have a tail.
