Castable Refratory: what to use for mold's inner wall?

P.Brewster

P.Brewster

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Jul 25, 2007
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I'll be pouring a new forge out of castable soon. Obviously I'll use a steel shell for the outside wall. What is best for the inside wall? How do I remove it?

I mentioned to the salesman at my local refractory supply place that I planned to use a cardboard tube, and then burn it out. He said the pressure of the curing refractory might bust the cardboard.

thanks
 
Go ahead and use the tube but wrap it in a layer of cloth, or material that will ease the pressure.
 
Patrick

I used a piece of stove pipe. After the refactory cured, I pried the pipe away from the refactory and collapsed it in on itself. I put duct tape on the outsied of the pipe to keep the refactory from trying to stick to the pipe.

Brian
 
First one I poured, made the inner form from a 2x4 and apiece of 4"pvc. Cut a section from the pvc so it fit over the 2x4 width wise. That gave me a flat floor.
 
Thanks guys - good advise.

birddog, it took me a minute to picture what you meant.... now I get it; that is pretty slick. Was the 2x4 fastened to the PVC in any way?
 
I have successfully used both PVC tubing with a slit down the side and stove pipe. The most critical part however is the mixing of the refractory, the word "pour" concerns me since water amounts added will literally mean the difference between your shell lasting months or years. The proper mix for many refractories will look rather dry and will not "pour" in any liquid sense of the word but instead will crumble in like a graham cracker or Dutch apple pie crust and proper vibration will then liquefy it and flow it into position. What I did was made a stiff spring attachment to replace the blade in my saber saw crumbled the appropriately mixed refractory between the inner and outer molds and then put the saber saw against the outside metallic shell and flowed it all into position (it will go from moist gravel to a true liquid when vibrated). Shells that I have done with too much initial moisture cracked and began to flake away on the first two uses, while drier vibrated shells have lasted me for ten years.

The best curing I have accomplished was by baking the shell at 450F for many hours while I built a bon fire large enough to make a huge bed of coals. I then transferred the baked shell into it and piled coals on top and cooked until it was glowing gently throughout. This shell had the least cracking of any I ever did.

Also the best shells I have done were normal refractory on the top and sides but high alumina on the floor (high alumina will run around 3 times more $$). The high alumina is much denser however and necessitates the use of temporary dividers that are removed at the end of vibration and only vibrated enough to bind it to the other mix. Too much vibration will cause the high alumina to spread out in a circular fashion into the surrounding material and go for the bottom of your mold.
 
I used a small metal garbage for the outside form. An old basketball was used for the inside. I made the forge 1/2 at a time and used pieces of foam to allow for the necessary holes. Took both halves to a friend with a kiln. I wrapped the forge in K wool and been using it for more than 10 years(haven't made damascus in it).


Pete
 
Patriqq, it was attached with drywall screws. I also covered it with grease before pouring the refrac. After it set in the summer sun for a day, I gently tapped it out.
 
I've made two castactable refractory forges using non galvenized stovepipe, 8" for the outside and 5" for the tube inside. Once the castable had cured and in use, the inside pipe just oxidized and crumbled away. Used them for about 6 years now and still going strong.
 
good info -

Kevin,

thanks for explaining the consistency. I like the saber-saw trick.


Gene,

I wondered about that - a steel liner will burn away without problems... good to know.
 
You could also use a palm sander like an aggregate vibrator on the form. Probably not going to hit as hard as the saber saw thingy but just something to consider.
 
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