To Use Refractory or Kaowool in Gas Forges? That is the Question.

When we make a gas forge we need some thing to keep the heat in the
chamber otherwise we just have a large inefficient torch.
Obviously we need something that won't burn. A steel case would work
temporarily but quickly it would start to glow as it picked up heat.
This is a heat conductor. We use a metal shell (either pipe or box) to
hold the lining and the burners etc. The linings are fairly fragile so
the outer shell also protects them.

For a long time kilns relied on heavy hard firebrick as their lining.
Fire brick is essentially a refractory material. It is a clay
combination with previously high fired crushed ceramic. This creates a
stable block (brick) that can be heated fairly harshly and tends to
stand up to abraison well. Eventually the fire brick will crack and
crumble with repeated heating and cooling.

Castible refractory is the same high fired ceramic that is mixed into a
matrix of high temperature concrete (cement). This allows you to mold
the
material just as you would with concrete. This can be very handy when
you are making odd shapes such as pipes etc. Castible refractory is
heavy much like concrete and is quite dense.

This density causes it to be a heat shield and a heat sink. This means
that the refractory will soak up heat until it reaches
thermal equilibrium. That is if your forge is made from castible
refractory material it will have to soak for a long time to
come up to operating temperature. Until you reach this equilibrium
point the walls we be stealing heat from your bar.

The length of time it takes to come up to equilibrium depends on the
mass of the material. Heavy thick walls need to soak for along time
(think 1 to 2 hours).

The ceramics industry developed a better material with the
trade name Kaowool. Also Known as Durablanket, Fiberfax, Cerablanket,
ceramic
blanket. There are probably other trade names that I haven't come
across. This material is a high temperature spun fiber much like the
familiar fiberglass insulation material for houses. The difference is
that it will withstand up to 2300 degrees F (or 2600 for the premium
quality). It comes in various thicknesses. 1/2 inch, 1 inch, and
2 inch are common, and different densities. Most gas forges
and kilns use an 8 lb density.

This Kaowool is actually an insulator. The tiny fibers create air
pockets in the structure of the blanket. These air pockets keep the
heat in the chamber, and restrict its transfer to the outer wall of the
forge. I did some tests with digital pyrometer on a 1 inch thick piece
of Kaowool. The interior temperature was running about 2200 deg. F. and
the other side of the blanket material was at 400 deg. F. This was after
the forge had be running for about 1 hour.

You should also note that flux for forge welding eats ceramic blanket
very quickly. It will eat fire brick too but is a bit slower. Put a
drip pan in the bottom of the forge to catch excess flux if you are
forge welding.

What this means is that there was a loss of 400 deg. to the atmosphere
and the room from the wall of this forge. Although there was a dramatic
difference in temperature from inside to outside I was still losing
400 deg to the environment. 2 inches helps solve this
problem. Now the outer surface runs at a bit above room temperature.
Extra insulation is worth it.

Some thing worth mentioning is ITC100. This is a coating that is quite
thin that is applied to the inside surface of the forge. What this
coating does is it acts as a reflective space blanket. If you are
familiar with the mylar space blanket it has no insulating properties
but it is reflective and returns a significant portion of the infrared
heat back to the person. ITC100 works the same way. Many smiths swear
by it.

I talked to a thermal ceramics technician and he had varying respones
from his customers. Some thought ITC100 was a great product
and saw an
increase in efficiency of their kilns. Others saw no significant
benefit. It is fairly expensive so I have opted to use refractory mortar
as a coating to help keep the Kaowool intact. The mortar creates a hard
surface that protects the soft Kaowool from getting abused by pokes
from the steel. The mortar like the ITC100 has to be replaced
ocaisionaly. But this is not a big problem.

I hope this helps explain some of the mysteries with high temperature
insulators and refractories.
If you are building a gas forge and require more information then
please see my href="/links/?u=http://www.artistblacksmith.com/items_for_sale.htm">Gas
Forge Construction Plans.