High Temp Crucibles

[Chris Marriner]

First post here - so I hope that this is in the right place... or at least an acceptable place...

I have a 14" High temp aluimina crucible.
I have it inside a custom - but similar design to an AIM wire melter.
The main lid is 3" brick with an access port door of 2" brick.
The main lid sits on top of the crucible - the thought was to keep the heat in - when opening the port to gather...

I’m guessing that the crucible has some insulating properties… and that when heating – the temp in the firing chamber will likely be different than inside the pot…

For example – I stuck my pyrometer into the air chamber above the pot – and it is about 200-220 degrees (750 degrees) less than the firing chamber (970 degrees)… Presently at 1440 firing - 1250 inside pot...

I assume that at some point this will even out… or should we have left approximately 1/2" for air flow??

Is this temo difference stressing out my pot and risking possible shock??

Is there a point where crucibles basically become just a vessel and transfer heat evenly because it’s lost it’s insulating property??

As it's heating up... I've got a lot of time to wonder about all these questions...!!!

THANKS!!

[Pete VanderLaan]

It sounds like there is no controller on the furnace which makes it really dicey. The pot should be brought up at 45F per hr until you hit 1000F and then hold for four hours. Then go to 1100F over four hours and then hold at 1100F for four hours. Then go back to 45F per hour until it starts to glow. Then just turn it up.

It sounds like this particular horse has already left the barn.

Electric furnaces have thermal planes, there's no way around the fact. It's part of why I like it turned up slowly. The pot has zero insulating capacity. You are just seeing reflective heat versus latent heat. You don't say how close the pot is to the elements. Too close is bad. I like about 1.5 inches clearance.

In my opinion, Trying to run a furnace with a pyrometer is a recipe for trouble.

[Chris Marriner]

Pete -
The horse is on it's way - but fear not!!! I forgot to mention the controller... 3 button Orton - programmable up and down... I have it programmed up 75 degrees per hour - so hopefully the 30degrees isn't a problem... and I missed the soak at 1100... (Fingers crossed - I will build those in for future firings) (See the ramp I'm using at end)

The pyrometer was only to see how well the inside of the crucible was heating...
The thermocouple for the controller is in the "firing chamber" with the elements - so I'm guessing that it won't read the same temp as inside the crucible... until it reaches that stabilization point... Glad to hear it has no insulating properties - and the difference between reflective and latent heat... sounds normal.

Elements are recessed into a t slot in the brick to hold them in place without pins or insulators. The crucible is approx 1.5" from the elements... a little more in the corners of the octagon...

Should there be a gap to allow the heated air from the elements directly into the air space where the crucible is?? or is minimal gap better to reduce the work of the elements??

Melting Spectrum System 96... I know... but it's what I had...

Another thing I'm worried about is heat loss when opening the port to gather... but I guess once the crucible and the glass reach the magic temp - that it will probably retain heat pretty well and its only a few seconds per gather...


75/hr to 2200
hold for 4 hours
down 30/hour - 2000 for squeezing
hold 2.5 hours
up 50/hour to 2125 - working temp...
Indefinite hold... Then I can switch programs to cool down...

Then a separate program for cooling at 50/hour after scraping the pot. Do I need to slow around 1100 on the way down??

I just popped the lid to take a peek and the glass is getting soft!! (and everything still looks intact!!!)

SORRY FOR ALL THE QUESTIONS!! and thanks for the SPEEDY answers.

Originally Posted by Pete VanderLaan

It sounds like there is no controller on the furnace which makes it really dicey. The pot should be brought up at 45F per hr until you hit 1000F and then hold for four hours. Then go to 1100F over four hours and then hold at 1100F for four hours. Then go back to 45F per hour until it starts to glow. Then just turn it up.

It sounds like this particular horse has already left the barn.

Electric furnaces have thermal planes, there's no way around the fact. It's part of why I like it turned up slowly. The pot has zero insulating capacity. You are just seeing reflective heat versus latent heat. You don't say how close the pot is to the elements. Too close is bad. I like about 1.5 inches clearance.

In my opinion, Trying to run a furnace with a pyrometer is a recipe for trouble.

[Pete VanderLaan]

A lot of people actually isolate the elements from the open chamber above glass line. With caustic glasses, like the one you are using, their has been some substantial evidence that it prolongs element life

[Dennis Hetland]

How much is your 100 lb 16" crucible? How many do you have?

[Pete VanderLaan]

Originally Posted by Dennis Hetland

How much is your 100 lb 16" crucible? How many do you have?

*******
$358.00. Holds 108 lbs Normally in stock. It's rare for us to have less than two in stock. I can't do an inventory check on the weekend.

[David Russell]

I am thinking abut changing out my hi temp 22" this summer, it has been hot for four years. Nuff said.

[Pete VanderLaan]

Originally Posted by David Russell

I am thinking abut changing out my hi temp 22" this summer, it has been hot for four years. Nuff said.

********
Crucible life is based on frequency of charges at fixed temperatures dependent on the material melted at what temperature also considering how frequently it is turned on and off. In my world, I change out a pot at 70 cycles and never more than 90. I don't have leaks and always have excellent glass. Turning pots on and off reduces this by up to 35%. That's my world.

Anything different than this approach, based on 40 plus years of observation is dancing with the devil regardless of who your crucible manufacturer is. Some are sworn by, some are sworn at. None of us do well being made into something we really are not. At High Temp, we have tried to keep pots available on the shelf at very fair prices. That was not the norm ten years ago. People had to wait a minimum of four weeks and up to four months to get a replacement pot. We strive to keep inventory.

[David Russell]

I do adhere pretty much to those values, even still i think the longevity speaks well for the product, just my 2 cents.

[Pete VanderLaan]

I have seen strange outliers to the norm. I stick to the norms and don't try to create unreasonable expectations.

[Dennis Hetland]

How much for your 300 lb pot?

[Pete VanderLaan]

you can look at all the pots, specs and prices at www.crucibleconnection.com