# Fused silica crucible for induction furnace



## elfixx (Jul 9, 2011)

I received my induction furnace last week and I have been making some test since. I've tryed 3 type of crucible to melt gold and silver and encounter some problem with fused silica. Everything goes well using graphite crucible and ceramic shell/graphite crucible but I am unable to melt or even heat to redness anything in fused silica crucible. Is it simply impossible to melt metal in those with the use of a induction furnace or am I missing something? And if it is indeed impossible to do so, why is it?


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## samuel-a (Jul 9, 2011)

Waht is your working frequency?

I'm no expert, but as far as i could learn so far, clay/silica is used at high frequency (above 40-50Khz ?) where SiC/Graphite/alumina are used at low frequency.
I inclined to believe that frequency adjustment derived mostly from the melt charge characteristics (such as weight, shape, element, metling point and more) rather then crucible type availability.

When you tried with the silica, did the crucible itself heated to some extent? if so, what?


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## elfixx (Jul 9, 2011)

My furnace is a variable frequency type, it adjust itself from 1Khz to 100Khz depending on the requirement it sense. On both test I've done using silica crucible, the metal got hot but the crucible stayed cold. The reason I want to use these type of crucicble is that they provide a much cleaner melt when melting/casting bullion. Graphite decompose and leave graphite dust ontop of the bar and ruin the final look of the bar.


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## HAuCl4 (Jul 9, 2011)

Simply put, the fused silica crucible does not conduct current, whilst the graphite crucible does. Therefore one heats up and the other doesn't. Whatever you want to heat up has to either be a conductor of current or be heated by being in contact with a conductor of current.

If you put a silver or gold ingot inside the silica crucible, it will melt. Powders of metals inside the silica crucible will only melt if the frequency is high enough.


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## elfixx (Jul 9, 2011)

I'll have to forget about these then... Is there some other type of crucible that could provide such clean melt to cast bullion? How about SIC crucible?


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## HAuCl4 (Jul 9, 2011)

elfixx said:


> I'll have to forget about these then... Is there some other type of crucible that could provide such clean melt to cast bullion? How about SIC crucible?



Put the fused silica crucible inside a slightly bigger graphite crucible, and the gold dust inside the silica crucible. Keep it simple.

Haven't tried SiC with induction. It should be pretty easy to test. Do you have pics?. :shock:


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## samuel-a (Jul 9, 2011)

elfixx said:


> I'll have to forget about these then... Is there some other type of crucible that could provide such clean melt to cast bullion? How about SIC crucible?




As HAuCl4 suggested SiC, or Alumina crucible, but not pure alumina (above 99.5%) since it is quite vulnerable to thermal shock.
Both types will serve dozens if not hundrands of melts if treated right...


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## goldsilverpro (Jul 9, 2011)

Is the silica crucible conductive - I don't think so. Most commercial induction crucibles I've used consisted of a conductive graphite crucible in a clay shell. At least, that's what they looked like.


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## Noxx (Jul 10, 2011)

Do you have a spec sheet of your furnace ?


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## elfixx (Jul 10, 2011)

Medium/high frequency 3-5kg capacity furnace
1-100Khz
8KW
1phase 220v
water cooled

That's prety much it
I have picture comming soon


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## HAuCl4 (Jul 11, 2011)

samuel-a said:


> elfixx said:
> 
> 
> > I'll have to forget about these then... Is there some other type of crucible that could provide such clean melt to cast bullion? How about SIC crucible?
> ...



elfixx asked about SiC. I said I had not tried it. I did not suggest SiC. I doubt it will work. I said the easiest would be to put a fused silica crucible (that will not heat up on its own) inside the graphite or clay-graphite crucible that is standard with these induction furnaces.

I believe elfixx wants to try Miller processing, and therefore needs a suitable crucible setup. Clay-graphite crucibles alone are useless for Miller. Usually a clay crucible is placed inside another crucible (called guard pot) for induction Miller rigs. Probably 4metals has more to say on the matter. I did not use induction in my old Miller rig.

I believe that with his furnace, he can melt ingots placed inside fused silica crucibles (without the outer graphite crucible), but will have difficulty melting powders placed inside fused silica crucibles.


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## Harold_V (Jul 12, 2011)

HAuCl4 said:


> I believe that with his furnace, he can melt ingots placed inside fused silica crucibles (without the outer graphite crucible), but will have difficulty melting powders placed inside fused silica crucibles.


That becomes a function of the available frequency. As the charge diminishes in size, frequency must rise. Same thing applies to the nature of the charge. Small particles do not respond to low frequencies, whereas a large single piece may heat perfectly well. Again, a function of frequency. 

If an induction furnace is large enough, it can operate at line frequency (60 Hz here in the States), but you're talking a huge induction furnace. 

All materials can be heated by induction, including rubber. All that is required is a high enough frequency. Think microwave oven. That's all it is---an induction heater. 

Harold


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