Saturday, 26 May 2012

Post Re-Refractory Firing

Well, today I fired up the flowerpot furnace to get it ready for some smelting. A short while ago I repaired the furnace by fixing a couple of cracks in the lining and the terracotta pot. I needed to remake the furnace in preparation for some planned melting.
After adding refractory, it’s important to let the refractory lining cure (same as cement) so that it can dry out as much as possible. Next I needed to do a slow firing of the furnace to remove any pockets of moisture. You do this slowly so that the moisture isn’t turned to steam and then cause an explosion in the lining. If done right, the slow firing will do this admirably.
So I added some dry straw, wood shavings, small timber offcuts and some greenish apple wood and fired it up. Lots of smoke (cough) but eventually.
We have flames too. As I said … this is a slow firing process, I don’t want to get the furnace too hot too quickly, so let the fire take it’s course.
Adding more apple cuttings to the fire helps things along.
When the fire is going along well, and it has risen in temperature slowly, I connected the air supply to help the burn get hotter.
The vacuum was set to it’s lowest setting and started to fuel the burn nicely.
But I didn’t want to abuse the furnace too much, so I left the level of air pressure nice and low.
After a while, I turned the pressure up so that the furnace got up to a nice heat level.
Then I left the fire going until all of the fuel had pretty much burned out.
A crack opened up in the side of the furnace where I had repaired it, but this is OK, the crack is about 2mm wide at it’s widest and doesn’t go all the way through (it’s only in the terracotta) and, more importantly, it didn’t blow up.
With this furnace, I’m going to have to take it easy on the first couple of burns until the cracks settle into place. It will still be good for smelting aluminium and probably copper, but I won’t be using this furnace for anything of any significant heat. That’ll be the job of Furnace Mark II.
I also plan to operate this furnace in the lean-to attached to my workshop, rather than out in the open, so it shouldn’t suffer too much from the weather or rapid temperature changes.

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