Using forms

[hpmer]

I am attempting to use forms to make a castable brick with odd cutouts and protusions and am having problems getting them out of the mold without them cracking or breaking up, with part of the clay sticking to the mold etc.

Should I let them dry in the mold longer? If so, how long, until they're completely dry?

Any tips for how to use these things?

[josephjcole]

A little more info would be good... What are you using for castable? If you are using manufactured refractory castable, then it shouldn't take more than a day or two to be able to remove them from a mold. It's key though that they dry slowly, if you are using a wooden mold make sure that the mold is covered in something to keep the wood from pulling moisture out of the castable.

You say however that the clay keeps sticking to the mold... clay is different than castable....

any more info on what you are using for castable, and what the molds are made of?
Joe

[hpmer]

I'm using a mix of .5:1:1 sand:clay soil:ash, and the forms are wood.

[grizbach]

You need a release on the mold , you could rub a candle on it . Or tape wax paper or cardboard to it . If these stick to your clay , they will just burn off.

[peterberg]

It's absolutely necessary to use a release agent. For example a wax of some sort. What I am using is simply motor oil. Use a small paint brush to apply, then wipe excess with a cloth. Works like a charm for refractory castable.
And oh yes, make the mold in such a way that it's possible to take two or three sides off. For example a mold made of plywood held together by screws.

[hpmer]

And how long would you suggest leaving it in the mold for the clay to set up before removing?

I know it depends on how wet it was to start out, but are we measuring in minutes, hours, or days?

[josephjcole]

I imagine it depends on how fragile the shape of your brick is... I've made brick with fairly wet clay/sand mixture "slopped" into a mold after which you can remove the mold immediately. In industry the clay is put into the mold dry and compressed with a huge press and then the mold is removed immediately.
So unless the shape of your brick is very fragile I would think you could remove it fairly quickly. If it's more fragile then wait until the clay has hardened a little bit, which depends entirely upon the humidity/temp of your room. For regular brick size (4.5"x2.5"x9") somewhere around a couple of days? The bigger it is the longer it will take. A little portland cement will help it set up quicker (it really doesn't take much).
Joe

[dakman]

I am considering using flexible duct hose, such as the kind shown in this picture, to form the lower elbow and heat riser for an RMH.
I would cast an insulating refractory around the ductwork. Once the refractory was cured, the ductwork liner could be pulled out prior to firing. (Provided a release agent was applied liberally prior to casting.) I have read much about the need for turbulence in these areas, and it seems that the ribbed shape of the duct work would lend itself nicely. Comments? Thanks,
Dakman

[canyon]

Sounds interesting, however, I question the idea of removing the flex duct after casting. I think you will find it very difficult to keep either the cast piece or the ducting or your temper in one piece. If you use the thin aluminum you could burn it out relatively easy.