[TheForge] chip shapes and materials

[Demon Buddha]

Jerry Frost wrote:
> 
> From: "Demon Buddha" <osan at netlabs.net>
> 
> 
>>
>>
>> Bruce Freeman wrote:
>>
>>> Frosty,
>>> Your idea on thermally conductive chips is excellent. What we need is 
>>> small (maybe 3/4" dia) balls of a
>>> highly refractory and thermally conductive material.
>>
>>
>> Bruce, simultaneous hig refractivity and conductivity are mutually 
>> exclusive properties by definition.  It's like wanting a surface with 
>> both high transmissivity and reflectivity at once.
>>
> 
> That would be my misuse of terminology. How about this, High heat 
> resistance and high conductivity? It's what I meant.

	By "high heat resistance", do you mean high MP?  if so, then you are 
right on.

> Agreed, almost as much a PITA as slip forming a lattice.

	I must disagree with you on this point.  The casting isn't difficult, 
just time consuming... mainly because you must wait for the moisture to 
  be drawn from the slip.

 > So how about we
> compromise and slip form hollow spheres?

	I have a better idea than that.  how about a polystyrene core that sits 
inside a hollow cavity into which you case refractory cement.  When it 
sets, it is at a far higher strength than clay so that it is less likely 
to fail during burnout and firing.  Refractory cement also tends to be 
more conductive than the high temperature clays, a good thing.

 > That'd be really simple with a
> two part gang mold. Pour in the slip, swirl it around while the plaster 
> sucks a little moisture from the clay, pour out the excess, let it set a 
> bit and break the gang mold open. Getting even wall thickness on the 
> spheres would be a simple matter of weighing the whole sheebang after 
> you pour the slip out. If is't short, pour more in and swirl some more. 
> If it's too over weight . . . Oopsie, redo time.

	It'll work but I think it is a lot of hassle.

> Yes, you want it to store heat but you want it to shed and absorb heat 
> quickly too. Given a choice I'll take chips that are highly conductive 
> over a high specific heat. Bottom of my desirable list is holding heat. 
> Insulating chips are not on my list at all.

	But if this is the case, then there is no point in using the chips. 
This is why coal and charcoal are superior: they hold heat in the blast 
area but radiate comparatively litte outward into the atmosphere, if the 
fire is well structured.
> 
>>> I'm also intrigued by the idea of using carbon for the
>>> chip.  Sure it would burn, but probably not very much.
>>
>>
>> I don't think it would be at all strong enough.  Go poking 1" stock 
>> around in a mass of hollow graphite obkects and I think you'll have 
>> much breakage.
>>
>>>  Graphite crucibles are practical, and they take much
>>> more stress than these chips would have to.
>>
>>
>> Thermal stress perhaps, but they don't usually get bounced around that 
>> much against hard objects.
>>
> 
> 
> Good points but clay graphite crucibles are known for being reasonably 
> tough. Then again I may be thinking of something else, I've never messed 
> with one.

	They are pretty tough, but don't forget that metal is going in and out 
and this will be hard on the refractory.  I cannot say that I'm thrilled 
about the idea.  I came up with this many years ago and wondered how 
well it would work.  Apparently it is used commercially, if I recall 
correctly from an earlier post.  If so, then clearly my intuition is flawed.