This post is from my series on building a backyard foundry.

You remember back when I said in part two of this series that I was temporarily using a flimsy stainless steel tub as a crucible until I managed to obtain a 3 1/2 inch (nominal) pipe nipple? Turns out that when you think “I can probably get one more melt out of this thing before it is destroyed”, that is the time to throw it away. The crucible failed. Fortunately, the crucible was still in the furnace.

As I also mentioned before, there was a design problem in that I re-routed the tuyere to come in the bottom of the furnace, which meant that if there was a crucible failure in the furnace, then the molten metal would head down the air pipe to the fan. Which happened. Fortunately the fan was blowing cool air and the melt froze in the air pipe. The remaining small amount of molten metal in the furnace could be easily scooped out with a long handled steel spoon, so no significant harm was done. I was going to be rebuilding the air pipe anyways.

I’ve done so. I’m still not very happy with the design, but it is better. Basically I have a T:

FURNACE BOTTOM
||
||
| =========== air input
||
||
||  <— aluminum foil plug that will melt
BUCKET

An immediate problem was that ash and coals are going to fall into the pipe and fill up the drain. I solved this problem by putting a pipe cap with twenty or so holes drilled in the sides on top of the air pipe. That certainly lets enough air in, and, bonus, directs it towards the charcoal surrounding the pipe. Whether it will let the metal out in the event of a crucible failure, I hope to not find out. I think it will.

I’ve been trying to come up with ideas for better designs for future furnaces; should there be multiple tuyeres to distribute the airflow around better? Should the floor of the furnace slope towards the drain? If I ever build another furnace I’ll experiment with these ideas.