# Graphite crucibles lifespan with soda ash



## autumnwillow (Jul 15, 2017)

So I'm building this furnace capable of fitting in an A50 crucible for pyrolysis and incineration then an A20 for the smelt.

Judging from the prices of these crucibles which are very expensive, how long do they last? Specially when you have to use soda ash as an oxidizing and a thinning flux, or is there an alternative to soda ash?

I've seen Chinese graphite crucibles break apart after just about 50 uses and this is without the use of soda ash.

The brand that I will be buying is a Morex not Salamander. Since Salamander is not locally available.
Its a graphite-SiC crucible if that info would help at all.


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## 4metals (Jul 15, 2017)

50 melts from a crucible is actually quite impressive. 

Since you are building a furnace, I assume it is a gas furnace. Make sure you design the bottom so any spills can flow under the crucible rest and collect under the furnace. A dusting of bone ash prevents any spills from sticking as well. This will allow for easy collection of the charge in the event a crucible does rupture.

Hopefully you will never need this but it pays to plan for the worst case and hope you never see it.


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## autumnwillow (Jul 16, 2017)

Well it turns out there are graphite crucibles which requires some tempering and break-in and these SiC crucibles which no longer requires such and is much stronger than plain graphite. I'm still wondering about its lifespan though as the A50 costs about $400.

With regards to the furnace build, I'm still studying which would be more economic to operate, a propane fired one or an electric. I can work with PID's relays and an element for the electric setup. Propane should be easy.

As for the structural I'm still a bit confused which is better:
a) a kiln built from firebricks with lined with refractory cement to seal then kaowool lining at the outer side
b) just a bunch of bricks with stainless housing - i see this often in commercial electric kilns, I wonder why they don't line the bricks with cement to seal like how other larger kilns (and I mean large like what they use at powerplants). They also don't use ceramic fiber.
c) a kiln just made from castable refractory cement. with outer kaowool lining - i see this often here in the forums and some in youtube


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## Lightspeed (Aug 23, 2017)

My findings on Furnace build:
The yellow type firebricks are fairly dense and may not be an economical refractory, as is castables, they both suck up huge amounts of energy before the furnace refractory will start behaving as such and not a heat sink, that being said once hot, they do retain heat. Myself I used ceramic wool at 2400f, 2 layers of 1 inch high density. Then applied a refractory zirconium wash coat, when the inside layer has had its day I can easily remove and replace. I used propane but soon found out there are burners and then there are burners, most just use bits of pipe, I did not and used an engineered burner. Check out hybrid burners on the net, If you really want bricks try to get white bubbled alumina firebrick, they are very light but more expensive, shapeable and cuttable to suit your needs, my furnace floor utilises these. With this setup I am ready to pour 5 mins after initial warm up, but I am not using 5kg crucibles, that is huge!!!

I have stuck to Salamander clay graphite which do not like flux of any type, it eats the inner surface in no time, I also use ceramic fused silica crucibles and melt dishes, while they do erode, I don't feel it's as bad as the clay graphite, I can also soak them if I need to remove flux from the inside, dry out and re use, I have not tried this with the salamander. Soda ash is aggressive as is Borax, soda ash is water soluble, where borax requires acidic wash to dissolve. I also use Potassium Nitrate for a heavy oxidiser when needed. You have probably finished your furnace build, how did you go?


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