Castable?

[VaughnT]

I'm getting ready to begin on my gas forge but....

The pipe I have is 8" ID. I have plenty of length to cut from, but wasn't sure if I'm reading things right.

If I understand correctly, the castable refractory is just a type of cement that does the same thing that koawool/itc 100 does? Is that right? Is one measurably better than the other? Easier to work with?

I was considering sliding a piece of 4" pvc pipe down the middle of my pipe and simply pouring the castable around it. Seems simple, right? But what am I missing? I don't see a lot of folks doing similar, so I'm curious to know what I've got wrong.

Also, while we're at it, is there a formula to determine how many burners you need in a given size of chamber. I'm thinking something about 15" long and 4" in diameter after insulating the pipe. With a blown forge, could I get an even enough heat for basic knife heat-treating if I only have one .750" burner? Or would two burners be better (smaller diameter or same diameter?)

I have to say that I'm really excited to get back into working iron and appreciate all the information you folks have shared.

[pkrankow]

Castable is typically more dense and takes much longer to heat than wool, it also provides less insulating value than wool.

If you are planning to run your forge for several hours at a time (a full work day) then castable will perform excellent and last a long time. If you are using your forge for an hour or two at a go, then castable will be frustrating in how long it takes to warm. Wool is not a durable as castable. A hybrid approach, castable over wool may remedy this and give the best of both worlds. short heat time and high durability.

Phil

[CurlyGeorge]

I just built my first gasser. I used 1" wool and covered that with furnace cement, 2700 F. I obviously did not get the cement thick enough, because it is really cracking badly. I have been using the forge quite a bit, and it heats very fast and seems to hold the heat well. But as soon as I get a break, I am going to recoat the inside. My forge started out with a shell of 8" dia. by 12" long. :D

[maddog]

I'm getting ready to begin on my gas forge but....

The pipe I have is 8" ID. I have plenty of length to cut from, but wasn't sure if I'm reading things right.

If I understand correctly, the castable refractory is just a type of cement that does the same thing that koawool/itc 100 does? Is that right? Is one measurably better than the other? Easier to work with?

I was considering sliding a piece of 4" pvc pipe down the middle of my pipe and simply pouring the castable around it. Seems simple, right? But what am I missing? I don't see a lot of folks doing similar, so I'm curious to know what I've got wrong.

Also, while we're at it, is there a formula to determine how many burners you need in a given size of chamber. I'm thinking something about 15" long and 4" in diameter after insulating the pipe. With a blown forge, could I get an even enough heat for basic knife heat-treating if I only have one .750" burner? Or would two burners be better (smaller diameter or same diameter?)

I have to say that I'm really excited to get back into working iron and appreciate all the information you folks have shared.

Tube forges lined with solid castable or plastic/rammable refractory are often made just like your plan, with some kind of cylinder to form the inside. Sonotube is popular. I usualy roll a piece of galvanised flashing into a tube with round wooden end plugs to keep it's shape. I wrap the form in several layers of wax paper so it can slide out easily when the castable sets up.

Kaowool by itself doesnt need an inner form to hold its shape, especially if the liner is slightly oversized for the width of the shell and the wool is slightly compressed.

In making a hybrid, kaowool with a thin refractory liner, people often just trowel it on. I apply it to the kaowool sheet while its flat, roll it up and stick it in the housing. But most refractories tend to flake and crack when applied in thin layers, especially after the first few firings when there is a lot of movement in the material. Patching is just part of regular maintenance. The only material that I have found that works well like this is Plistix 900 which is fabulous stuff. If you go for a thicker inner liner of castable , say 1/2" or 3/4" then you will need a form to support the inside till it sets.

Kaowool is a great insulator. Most castable refractorys are poor to lousy insulators. IMO even if you are going for a heavy solid wall of refractory, you should wrap it in one layer of wool on the outside for insulation.

The rule of thumb for burner to forge matching is one 3/4" atmospheric burner to 350cu in of forge chamber. A blower driven burner of the same size can put out a lot more heat. Your design is 4"x15" comes out to about 200 cu ins. So you have plenty of burner but, with a long narrow chamber like that, you wont get an even heat. I don't make knives but I think this is important to bladesmiths. I suggest you consider splitting your inlet port into two 1/2" ports.