From: John De Armond
Subject: Re: casting question?
Date: Thu, 23 Sep 1999 05:36:49 EDT
Newsgroups: rec.crafts.glass

Terry Ow-Wing wrote:
> 
> I have NO experience in casting but what I heard from someone who does is
> that when you burn out the wax in your kiln the petroleum byproducts eat
> your elements!

Now the rest of the story ... (with apologies to Paul Harvey)

The issue is that high temperature elements (kanthal, etc) rely on a
protective oxide coating for their integrity at high temperature. 
This means that the environment in the kiln must be OXIDIZING, e.g.,
an excess of oxygen.  On the other hand, if the atmosphere is
REDUCING (containing reducing agents that desire a union with
oxygen), the oxide coating on the element will be reduced to metal
and the element will quickly fail.  Vaporizing/decomposing wax is a
reducing agent.  The solution is two-fold.  One, keep the kiln well
ventilated during burnout.  prop the lid open a bit, leave the view
ports unplugged, etc.  Two, use a lower temperature.  If the
elements are not run at their full power, they will not get hot
enough for the reduction reaction to proceed.  Since burnout
requires perhaps 1200 deg or less, this is no big deal.  Either set
the kiln controller to a low temperature or if the kiln (example:
paragon) has heat range switch(es), set it to "low".

To answer the original question, most any refractory material will
work. Since the mold is broken to release the  glass casting,
durability isn't terribly important. Plaster of Paris will work for
small castings. HydroCal is stronger under heat and is better for
larger castings.  Any mold needs to be carefully dried before
casting, lest steam pressure shatter the mold.  This is covered in
detail in most any beginning hot glass book.

John

From: John De Armond
Subject: Re: casting question?
Date: Thu, 23 Sep 1999 18:27:13 EDT
Newsgroups: rec.crafts.glass

glass wrote:
> 
> ==>>. You have already been told it will ruin your kiln
> ==>>> if you burn out in it.
> 
> I interpreted the information given here to say  was that you COULD burn it
> out IF you did so at low
> temp (1200) and kept the kiln vented.
> 
> NO?
> 

A little terminology check.  I would not attempt to melt (or burn)
out the bulk of the wax in a kiln.  It could be done but it would be
messy and generate a huge plume of smoke in addition to risking the
elements.  Melting out the bulk of the wax is best done with steam
as others have noted.  If you don't want to use steam, you can melt
the bulk out in your oven by suspending the mold over a pan to catch
the wax.  I guess a kiln could be used for this on low heat but an
oven is so much handier.

Once the bulk of the wax is melted out, there still remains some wax
that soaked into the casting material.  This needs to be removed or
else it will vaporize or burn in contact with the glass and make
bubbles or even explode the mold.  This is the process that I refer
to as "burnout".  It can safely be done in a kiln on low heat and
with the lid cracked - been doing it for years.  It involves heating
the mold until the wax is vaporized and/or oxidized away.  Typically
900 degrees or more.  

I should note that some (maybe most) jewelers who make investment
cast findings burn out the bulk of the wax investment in a muffle
furnace.  They can get away with this because the investment is
small and thus contains little wax and because the muffle furnace is
designed for that.  Assuming you're going to be casting objects at
least as large as a paperweight, the amount of wax would overwhelm
the kiln and would probably catch on fire in the kiln.

John

From: John De Armond
Subject: Re: Broken wire in fusing furnace
Date: Tue, 21 Mar 2000 03:23:13 EST
Newsgroups: rec.crafts.glass

Mark Porter wrote:
> 
> Tine,
> 
> I've done some repairs to kilns and heavy dryer units and it is possible to
> get a little extra life out of that element, first unplug the unit, then
> very carefully bring the coils together and have them overlap about 2 turns,
> that is all that is needed.  The first time that you power it up it will arc
> a little and this will kinda tack weld the ends back together.  The wire
> that these elements is made of is very brittle especially with age.
> 
> Hope that this helps,

Works quite well if you put a little Borax on the joint as flux
before applying the power.

John