I have been reading a ton on forge building and this is by far the best explanation / design / how to I have came across. Please make this a sticky.
Maybe it will save some people some time. Wish I found it sooner.
Frosty wrote this on another thread,
"Sodium silicate is a popular internet expert material and it WILL stick stuff together but it melts at a lower temperature than a decent forge's every day operating temperature. Using sodium silicate as the binder means using a 2,000 f. material to build a 2,600 f. piece of equipment. Colloidal silica rigidizers won't take much more than sodium silicate but it doesn't break down at high temps. it melts.
Rigidizers MUST be shielded from the flame face, they WILL NOT take forge temperatures no matter what someone on the web says. Nor will perlite.
Prepare your ceramic blanket refractory OUTER liner. I use the name Kaowool only because it's what I have available and I'm too lazy to remember any of the other brands. Some things are standard though, 1" 8 lb. is preferable and is commonly a 2,300 f. refractory blanket. And NO, Kaowool is NOT appropriate for the contact surface unless you're going to be running your forge at high orange and below. The ceramic blanket MUST be shielded from the flame. It will compress and covering it with a brittle substance like castable hard refractory endangers the refractory. If you bump the forge too hard and the blanket compresses under the hard liner the liner will break up. That is why we rigidize the blanket, ONLY the blanket. You don't thicken it you WANT it to flow into the blanket's fibers and dry. Firing it fuses the silica with the fibers making the blanket stiffer.
Sodium silicate falls under HAZ MAT. rules and requires PPE to use safely. Colloidal silica only requires a good dust mask bug eye goggles and a cool shower. Hot water opens your pores before it can rinse the almost molecule fine silica off your hide. This is the reason emergency showers are all cold water. The stuff is listed as an irritant and otherwise has a pretty clean MSDS.
It's inexpensive, carries no HAZ MAT restrictions and works a treat. Just mix it with clean water and spritz it on. Spritz the blanket with clean water first to butter it, this will allow the colloidal silica rigidizer to penetrate more deeply. This is a GOOD THING.
That's rigidizing, no magic, no secrets it's been standard practice since before ceramic blanket refractories were invented.
Use 2 layers of 1" 8 lb. Kaowool. Place them in the forge shell cut slightly oversize so it holds itself in position by compression. Rigidize it and let it dry. Fire cure the outside layer now, a Bernzomatic torch will do the job nicely, just bring it up to red heat, that's plenty.
Cut and lay the next layer of Kaowool. Butter and spritz the first layer's inside surface and the surface of layer 2 that will contact it with rigidizer. This isn't a must but it does provide a better bond, it's a "can't hurt, might help" thing. Once layer 2 is place butter and rigidize it. Let it dry and flame cure it.
Press the liner with your finger, it shouldn't compress without pressing hard. If it does compress butter and rigidize it again.
Once the insulating outer liner (Kaowool) is dry and flame cured, cut your burner port(s). I use a sacrificial hole saw, don't hog it and it won't tear the rigidized Kaowool. Drill it over sized so you can plaster it with the flame face refractory.
Once you have all the mechanical mods made it's ready to lay in the hard refractory flame face. Recently "Kast-O-Lite-30 hi." has become the popular consensus "best stuff." It's a castable, 3,000 f. high alumina, bubble refractory. It is concrete hard at it's top operating temp 3,000 f.
High alumina refractories are not subject to being dissolved by borax containing refractories like we use during forge welding.
Mix the refractory with only enough water to make it workable, butter the kaowool and plaster it approx 1/2" thick with the hard refractory. It isn't going to want to stay in place hanging from the roof. However Kast-O-Lite sticks to itself well, heck the stuff sticks to anything meaning there is NO problem plastering one half letting it set and cure then roll it over and do the other half.
However, if you wish to make a mold and cast it between the insulation and the flame face it's okay, just a PITA. The stuff may look like wet low slump concrete but the aggregate is crushed so it doesn't flow well. Commercially it's troweled or gunned. (sprayed) The wetter you mix it the weaker it is when cured. It's still darned strong stuff.
Once the inner liner is set and cured. Kast-O-Lite cures like concrete, it does NOT dry, it hydrates and cures. Once set the recommendation is to keep it at 100% humidity between 70 f. - 120 f. I just wrap my castings in a wet towel and give it over night to finish setting and do some curing.
The stuff will cut you a LOT of slack though you can get away without the "proper" curing time, just sitting over night will work for our purposes. I HOPE I don't have to tell you not to let it freeze!
Kast-O-Lite as it is doesn't need a kiln wash to protect it from abrasion and chemical erosion, it isn't susceptible to caustics like borax so there's not need to protect it. However an IR re-radiating kiln wash WILL increase efficiency. Butter it and paint on the Metrikote. Let it dry and flame cure it. Mount your burners bring it to red heat, let it cool.
It's ready to go to work.
I know I started off with a list of beginner's mistakes but left off one of the biggest. Trying to understand all this stuff will only make you crazy. We see guys spending months trying to find the "BEST" . . . whatever. Unless you know and have experience with the craft you won't know the "BEST" if it falls on your foot. Honest best is a very personal and subjective thing. What's best for me isn't worth doodly to many other guys but the way they do it is just silly.
You really need some time at the anvil before you can sift the good Youtubes from the idiot junk. Especially building smithing equipment, Youtube forge builds in particular are bad and often just plain dangerous. Not all of them but the vast majority.
And yeah, I've built a forge or 20.
Frosty The Lucky."