Perlite and sodium silicate forge (Did that get your attention? Because I could use some advice...)

[jonathan_winters]

Hi all - I've searched/read a ton here and while I've thoroughly enjoyed your content, I haven't been able to find the info I'm looking for. I'm hoping it means my search skills are weak and not that this is just an incredibly stupid idea! Either way, I've read through enough posts to feel confident you'll let me know which it is. (And if it is stupid, well, then I'm glad I found that out before I started!)

 

For some context: I've built a few rocket stoves successfully using a mixture of perlite, sodium silicate, sand, and a bit castable high-heat cement -- this mixture has held up great in that context (wood fires) and it is super easy to work with. 

 

Fast forward to now - I'm just finishing my new workshop and getting back into blacksmithing, and decided it was a good time to switch to a propane forge, which I'd like to build myself (I enjoy the DIY process of making my tools, even if it isn't really any cheaper). My mind immediately went to using the same type of rocket stove mix for the forge because of how dead-simple it is to use. I even had that thought affirmed by the seemingly endless YouTube videos showing people making similar forges

But after checking into melting point of those components I realized it won't work for the forge. I'm guessing that this is not news to you...

(As an aside: I'd be really curious to hear how those forges are working out for the YouTubers who actually tried to use one of these...)

So, my questions are:

First, what are your thoughts on proceeding with my mix from the rocket stove as the *insulation layer* in my forge, with just a layer of some sort of castable refractory (kastolite or satanite)? The big question in my mind here is how deeply the forge's high temps will soak into the surface of the forge, and whether that castable refractory would be sufficient to contain the mix if it melts.

And if that is a workable option, how much of the castable refractory would I need? "How much": both in terms of how thick would you go, and how many pounds would it take to achieve that thickness. (I'm haven't been able to find how many cubic inches a pound of dry refractory will cover). 

If you think using the perlite/sodium silicate mix will lead to rapid and catastrophic failure (or even just if you think using it at all is a terrible idea), do have recommendations for similarly-usable and cost-effective castable materials that would work for the bulk of the insulation layer? (I know it is hugely popular here, but I really dislike ceramic wool, so that would only be an option for me if it was just *so* much better than everything else out there.)

 

While I'm at it, I've attached an image of what I'm thinking for my forge design (in the image it is sitting vertically on my bench, as if it was ready to be cast).

The opening down the middle (as pictured) is 16" long, 4" tall, and 6" across the bottom and 4" across the top. I'm thinking of dropping that down to 10" long, but I don't think I'd want to go any smaller than 6" wide to work with (just for maneuvering work pieces). The outside dimensions are 10.5" at the widest and just a hair over 10" tall, giving at least ~2" of insulation all the way around. 

The pipe is just a placeholder for the burner of similar dimension. Right now just a hole for one burner, but if I keep it 16" long I'll add a 2nd hold for an optional 2nd burner. 

The flat sitting out in front of it are for a cast "skirt", to rest my work piece on as it is heating up. 

I'm a very visual person so I threw the mold together just so I could have something to touch and turn around in front of my eyes -- this is very much a "rough draft", so I'm still VERY open to feedback on what could change to improve efficiency or usability! 

[Mikey98118]

"First, what are your thoughts on proceeding with my mix from the rocket stove as the *insulation layer* in my forge, with just a layer of some sort of castable refractory (kastolite or satanite)? The big question in my mind here is how deeply the forge's high temps will soak into the surface of the forge, and whether that castable refractory would be sufficient to contain the mix if it melts.

 Sodium silicate and Perlite both melt at less than 2000F, so even to use this mixture as secondary insulation is a poor idea. As tertiary insulation, they are just fine. This leaves you in need of a flame impingement layer, and a primary insulation layer. If you want to get by with thin coatings as your flame impingement surface, your primary insulation layer had better be good to 2600F. If you interpose a 1/2" semi-insulating hard refractory layer between a heat reflective flame coating and the primary insulation layer, than 2300F insulation will serve.

[Mikey98118]

Semi-insulting hard refractory equals Kast-O-lite 30. You can purchase a small amount of it from Glenn.

[jonathan_winters]

Thanks - that is along the lines of what I was guessing. 

I don't have a point of reference for what a "small amount" of Kast-o-lite would be, or how much is a "normal" amount. Do you know how many square inches it covers per pound? Or ballpark "for sure enough, but not way too much" to buy for running a 1/2" layer?

[jonathan_winters]

I finally found it!!

Quote

Kast O' Lite 30 requires 92 pounds per cubic foot. This mean 5 pounds will cover 0.054 ft³ or 93.3 cubic inches.

Based on this, it looks like I'll need about 10 pounds to cover my interior surface (assuming I stick with this design) - that's not too bad. 

[Mikey98118]

Most people find a two-inch thick layer around the hard refractory shell works well. A one-inch thick layer between a hard refractory liner and a sodium silicate/Perlte filling would also work.