Not a bad plan. My point is that being able to change the shape of the fire ball to better than fixing the fire pot. If you fill in all your hearth exept an oversized fire pot/bowl then just use loose cat litter and ash then you can dig out and shape the pot as desired.

Menchelment, have you tired charcoal in this forge?  Could anyone fill in a noob on whether charcoal would work well in it?

Should be fine. Just shape the fill to be more of a tall trench than a wide bowl.

Here is where we wound up when we ran out of cat litter. I don’t see how the cat litter provides much binding, even when wet.

 

I’m half tempted to finish all of this off (cap this) with the high temp cement mixed with perlite. Or, I’ll pull out half of what we just did, mix in cement, form, then mix cement into what I pulled out and put it back in, then final form. I guarantee that will bind.

Doesn't look to me like you soaked the cat litter in a bucket of water until it would stay in a lump when you crush it in your hand. Did the container say it was just clay?

Done...

 

Nevermind.

Cat litter was 100% clay. I pulled it all out and mixed the perlite/cat litter mix with some high temp cement as a binder. What you see on the top is the high temp with perlite in it. 

No wonder. Just clay and sand didn't register? Perlite has zero use in a solid fuel forge like this, what's to insulate? It's hard to get a better high temperature binder than bentonite. Bentonite mud is what's used to plug the tap hole in cupola iron melters. Toothpaste consistency bentonite clay is smooshed against molten iron where it dries in place till it's tapped with a sharp steel bar. At temps around 2,700 f. it isn't fired like pottery, just baked hard like an adobe brick.

3pts. sand to 1pt. bentonite clay and just enough moisture it can be squeezed into a hard lump in your fist. When Thomas says soak in a bucket of water he means add a LITTLE water, mix and let rest overnight in a sealed bucket so the moisture is evenly distributed. When it passes the squeezed lump test ram it in place with a wooden mallet, end of a board or B'ball bat, etc. till hard. scoop, scrape till you have the shape you like. Put it to work.

Making mud pies leads to shrink checking as it dries worse than any mud puddle and being bentonite it'll take weeks or months to dry from mud. 

Frosty The Lucky.

I tried sand before and it turned to glass. The bottom of the forge got very, very hot with with sand, hence the start of this thread. I did what was advised and it dried as pictured, loose. I could not get the cat litter clay to bind. I soaked the litter in a wheelbarrow with water in it. There’s no mud. 

In that case, I would say you have the wrong cat litter.

Okay. If the kitty litter didn't turn to mud soaking in water it wasn't cheap clay kitty litter. I've been fooled too and the stuff didn't work at all. We were making kiln wash with Zircopax and bentonite like high end crucibles, etc. I ended up buying a 50 lb. sack of bentonite drill mud though it's available online, at pottery supplies, feed and seed farm supplies, forget the makeup and food supplement versions they're CRAZY expensive. Cheapest clumping kitty litter you can find, make sure there's no sand in it. 

If it got too hot under the fire you don't have a thick enough layer of soil. 

Perlite has a lower vitrification temp than silica sand, it just doesn't make glass. One of the guys in our club is a professional bronze caster and has a special spot to collect sand that isn't silica. Silica sand in his molding sand fuses if the casting is more than pretty thin, say 3/8" thick. I think the sand he collects is mostly schist or shale ground down by the Knik river.

Frosty The Lucky.

So, he’s got a knack for the Knik?

3 hours ago, JHCC said:

So, he’s got a knack for the Knik?

There's a pottery place not far from us the other direction called "The Knik Knak Mud Shack." The old man who owns the place has about 3 two car garages worth of blacksithing tools and equipment, including at least 2 Little Giant power hammers he won't sell, he's convinced people will be excited about driving all the way out and paying to look at it. I asked about buying some maybe 30 years ago got the same story. He'd be happy to let me set my gear up as part of the display though. Wuite the oportunity you know. 

Knik is pronounced K Nik', with your teeth slightly clenched.  The area is named for the tribe's territory. 

Frosty The Lucky.

3 hours ago, Frosty said:

about 3 two car garages worth of blacksithing tools and equipment

That’s what happens when you give in to the Dark Side. 

Frosty, the cat litter claimed 100% clay. It’s the Special Kitty brand. Perlite was used more for weight reduction. The forge is currently portable. I modified the legs and installed equipment leveling casters with ratcheting levelers. When I had sand in it, the forge was very heavy. This is much lighter and I am fairly confident it won’t transfer heat nearly as much as sand turning to glass. 

 

I have have some projects that have high clayey soils which require removal and export prior to building. Some are moderate to highly expansive, some aren’t. Do you think this type of clayey soil would be a good hearth lining material? Perhaps as a binder with the silica free sand you mentioned? Is the silica free sand a fine sand or a bit more coarse?

 

I’m relatively pleased with how this thing is curing. I’ve kept a radiant heater under the forge to keep the hearth bottom warm. Very minimal shrinking and I’m going to put the final high temp mortar coating on the fire pot after more cure time - no perlite in it though. Probably another week of curing before doing this.

Yeah, I was fooled by "natural clay" kitty litter too, it had enough sand in it we couldn't make a kiln wash with it. That's when I bought a sack of drilling mud, bentonite. 

Clayey soil like you're talking about is perfect, just don't get it wet. Just moist enough to clump in your fist is perfect. Sand, crushed fire brick, or pottery grog (aggregate) in the mix allows movement between particles as it warms and cools preventing thermal checking. The spaces provided by the aggregate (grog) allows any moisture to escape when it gets hot so steam pressure won't make it spall.

The sand Pat collects is on a sand bar in a river, he sifts it for what he needs to make molds. I've never been to his river bar so I can't do more than guess at what it is. How it behaves in his bronze molds and the geology of the mountains is all I have to go by. It doesn't fuse like silica sand and the over all geology of that part of the Chugach range is meta sedimentary. I'm guessing it's shale or slate ground to sand. It's not the sort of thing you buy at the local sand and gravel pit.

I'm thinking a casting supply or maybe buy and crush Salvation Army dinner ware. Old broken porcelain crushed to sand would do it. 

The problem now is getting carried away trying to find something "perfect". I think close will have to do. 

Just use what you have till you find something you like better, this wears out or . . . It's made use it. Don't get too invested in what I say, I'm just some guy on the internet who doesn't use a coal forge that often. Seriously when I dig out the old rivet forge I ram in some clay/sand mix, use it then scrape it into a bucket for next time. This is about as far from a precision piece of equipment as digging a hole in the ground. It's just not near as hard on my back to use.

Frosty The Lucky.