Crucible crack - replace or risk it

[Mike Farmer]

I had to do an emergency shutdown on my 80 lb. electric melter just before Christmas. My wife needed surgery and we got a call from the surgeon she had selected asking if we could be in New Orleans the next day for a pre-surgery appointment. We live in Dallas so we had to leave that night. (The procedure she needed was new and only done in New Orleans or New York.)

My crucible was about half full of glass. I could have taken time to empty it, but we wanted to get on the road as soon as possible, so I just programmed the controller for my normal 2-day ramp down. I had a neighbor check on the furnace after 2-days. It seemed to be cool, but both the temperature controller and limit controller were showing errors. So I had her unplug the furnace to be safe. When I got home two weeks later, I discovered that both my thermocouples had opened up. I was using the heavy 8 gauge K-type thermocouples installed in alumina protection tubes. It appears the metal sort of crystallized over time and the thermocouples opened up. It is strange to me that they would both fail at the exact same time.

The crucible appeared to have survived. However, before firing the furnace back up, I decided to do a closer inspection using one of those extension mirrors to look at the sides closer to the bottom of the crucible. What I found was disappointing. There was a hairline crack. I pulled the crucible from the furnace and found the hairline crack appears to be just below the glass line and runs a little less than halfway around the crucible. It measures about 19 inches long. The crucible circumference is around 45 inches.

I have attached a couple of pictures to this post.

So finally to my questions -

Should I replace the crucible? Or should I try to slowly bring the furnace up and try to get a little more life out of this one, knowing there is some risk? Either way, I am planning on ordering a new one.

If I do decide to fire the furnace back up with this crucible, does anyone know whether or not you can see cracks in a crucible at working temperature? Would they show up as a dark line or brighter line?

If I can see a crack at temperature if it is there, it seems it might be worth the risk. I can bring the furnace up, use the remaining glass, inspect the empty crucible at temperature, and decide whether or not I can charge the furnace again.

Please let me know if anyone has thoughts on this or experience with a similar crack.

(The last attached photo shows part of the non-cracked region.)

[Scott Novota]

It will slowly work out though that crack if you fill it to the crack. If you fill it over the crack it will flow at a rate in linear lock step with the amount above the crack. The weight of the glass above the crack acts as pressure.

At which time it will run down the side of the pot and pool around the bottom of the pot and start to eat though the soft brick. The tensile strenght of the soft brick will start to decrease as the glass starts to impregnate the soft brick. Then the weight of the pot with glass in it will cause it to drop though the bottom of the kiln/furnace like the gold safe at the start of the movie in the movie "Itailan Job".

I know, because of first hand experience with this one. I did not have the advantage of knowing of the crack. I have pictures at home I will try to upload to sate your curiosity.

In short, I wish I had known the crack was there so I would not have used the pot.

[Pete VanderLaan]

The difficulty with this stuff is that you want to know if it's cracked, so you whack it and remove it to look for the crack. The worst thing you can do to a half full crucible in a forced shut down is to whack it at all. These things are vitreous. They don't take well to being whacked. It is an open argument as to whether the crack showed before or after the whack. It's frustrating. They look basically the same.

If it were me, I would replace it. Short cracks high up on the pot I will gamble with, not this however.