Clay for casting and cob

[Bobwieser]

New to RMH, Thanks to all who have shared so much here.

I have a source for granite muck or dust from wet cutting of the large granite blocks into slabs. I'm thinking of using it as fire clay with perlite and heatstop 50# to cast a core and riser. Has anyon used it? Is there a good way to test the material?

[matt walker]

Glad to have you here Bob, welcome. I have no experience with granite dust, but I'd be inclined to say that it probably won't be a good substitute for clay. For sand, maybe, but clay is something else entirely. You could test by making a small brick and putting it in an outdoor fire and see what happens.

[Bobwieser]

Thanks for the reply. I'll give it a try
and report the results. I have a an unlimited supply of it. The ground is frozen and the muck is not.

[4seasons]

While I have no actual experience with granite I tend to agree with Matt that it would not work as a substitute for clay. It would be closer to a substitute for sand or even perlite. Granite dust has been suggested to be used as a substitute for wood ash as a garden fertilizer so I would think you may be able to use a fireclay and granite dust mix with some straw or fiberglass to make a castable mix. No guarantee that it will work, just thinking it may be possible. I think you can substitute pottery clay or dig clay out of your own yard to substitute for fireclay. I am actually building a prototype right now using clay from a pond cleanout with wood ash, sand and straw. I will report back as soon as I get it up and running on how it works.

[mannytheseacow]

Welcome Bob,

There are a number of different kinds of granite, so I think it might work depending on what you are working with. Matt and 4seasons are right, it won't be a replacement for fireclay but if you're mixing it with a high ratio of heat stop it will probably make a really good mix, assuming you are using the right granite dust. Specifically look at the feldspar content. Depending on if it's K-feldspar, or P-feldspar it might be a close mineral composition to fireclay, though it won't "bind". You'll need the refractory mortar for that. Or mix a good amount of ash in.

[Bobwieser]

I know the blocks come from Montreal I'll do research on it thanks. I have 8to cu ft of masonry perlite for the mix. I built an 8"RMH with free thick dense fire nrick. It's working good outside.

[Bobwieser]

Thanks for the replys. I learned that they cut perdominetly Stanstead Grey and some Caladonia. Stanstead comes from Quebec Canada not Montreal as I thaught. A quick search reveals that all granite must be at least 50 percent feldspar then Standstead Grey has silica and mica.
From your relplies are you saying it is an insulator? If so I think it's not good for the mass. It sure would be nice since it's so available for me.
I got a 5 gal bucket to play with. I'm thinking of mixing it with wood ash from my Garrison wood stove and add some furnice cement I have and see what comes of it. Maybe cast a micro rocket or rocket stove and fire it off.
I have perlite but I don't want to waste it but I might use some.
Manny I don't know if it's K or P feldspar. How much wood ash do you sugest for a sample cast?

[mannytheseacow]

Hi Bobwieser, I'm not a professional geologist, so take this post with a grain of sand (or quartz, har har.), but I know a bit about it.

Just thinking through your process here.... so, you're trying to emulate fireclay.

In "fireclay" you've got "fire" and you have "clay".

The "fire" part is referring to the mineral composition and the "clay" part is referring to particle size. Forgive my simplistic explanation, I'm not trying to be an ass, just stick with me!

The reason I asked about the feldspar is because you'll basically find two types. Orthoclase feldspar (K-feldspar) and plagioclase feldspar (P-feldspar). P-feldspar is often pink in color, the more pink, the more pure. If you want P-feldspar and you've got really pink granite, you're in luck. In this case you want K-feldspar because you're looking for the kaolinite minerals and aluminum oxides that fireclay have. Problem is, white or gray granites might be really pure K-feldspar or rather impure P-feldspar. I would lean toward thinking what you have there is probably K-feldspar or at least close enough to work with.

On to step two, mineral size. Clays are extremely fine, <2 nanometers. Granite comes in many forms depending on how deep inside the earth or quickly it cooled. Regardless of this, by nature the crystal size in granites are way bigger than clays, not to mention they have about 0 organic matter.

So if you want to use it I would be inclined to have a couple of thoughts. It might work, so far it looks like the color might be right. Determining particle size is a bit of a process. The ASTM has a standard for it and it only requires about $15 worth of equipment and some time. Even easier, you can mix some of your material in a jar of water and see how quickly it settles out. I'm not sure how fine this dust might be cut from the saws that your supplier is using, but I would go out on a limb and think it's not fine enough but it still might work by incorporating a binder like fire cement or some ash.

Fireclay isn't so much for insulation as it is for being able to withstand the heat. The vermiculite or perlite is for insulation, and you can get a big ass bag of it at any big-box hardware store for about $10, so I wouldn't worry about using up your perlite.

Now for the frustrating part. I sometimes get irritated going on these forums asking questions and most responses are "I don't know, try it and see". Fact of the matter is that most of this stuff is experimental and I have to tell you: I don't know, try it and see.

I might start with something simple like 45% perlite, 45% granite dust, 10 % firecement (by volume) and go from there. Or maybe try 40% perlite, 40% granite, and 20% ash. I bet it will work, however what the "right" ratio is....... I dunno? I wouldn't even be afraid to try adding some Portland cement. Honestly, you can make this things out of mud and they will last indefinitely.

If it's something you want to last for a long time, I would consider investing in some firecement for the critical areas like the feed.

Good luck!

[mannytheseacow]

PA Friendly Guy posted this a while back

They make rockets in buckets with cement. Somewhere in there it talks about adding a handful of ash to your water before mixing the water with the rest of your ingredients.

I've seen a lot of concrete stove ideas like this and on youtube, but I've built two concrete stove contraptions outside in the past and both failed miserably after a day of burning. I don't even think I was hitting the temps you would get with a J rocket.

[Bobwieser]

Thanks for the explenation. And for keeping it simple.
I did some research on Stanstead Grey. It's 65%to k-feldspar 25% quartz and 9%the biotite. The dust is pretty fine, I'll put some in water and watch for suspention. How long should it take? Please forgive me if I seem short. I'm using my phone and it's frustrating me Thanks Bob.