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Clay for building furnaces


Kyle Vance

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So in all of my prior attempts to build a bloomery or hearth, I have had the issue of the clay in the local soil disintegrating into a fine dust upon heating, even if the clay is separated from the other sediment, it still seems to be useless. I am not sure exactly what to do other than using firebrick for everything 

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Find a pottery supply store and buy a 50lb bag of EPK.  It's a brand of kaolin clay, costs around $12, and mixed 1:1:1 by volume with sand and the organic material of your choice (peat moss, chopped straw, horse dung, camel hair, etc) and enough water to make a play-doh like paste, you're good to go.  California is not known for its clay deposits. 

 

The organics are not even strictly required, they just make it less prone to crack if fired before dry, and slightly more insulating.

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There seems to be a lack of local suppliers as well, so I guess shipping is the only option. Do you think adding a bit of the local clay like sediment would affect it too much if you were to exceed the 1:1:1 ratio?

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I can't imagine you don't have a clay supplier in CA unless you are out in the wop-wops. Might check near by universities/colleges/secondary schools. Sometimes they have robust clay programs and you might score a bag of dried clay.

And I bet you can find some native earthenware clay somewhere near by. Wouldn't some of the earliest hearths have been built with earthenware cob or does that not hold up to smelting? Very interesting stuff. Do you have any pictures of your previous attempts? How did you clean your local clay?

 

Sorry for butting in, but I'm interested.

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I used a gravity separation to remove all the sand and silt, and then added a bit of sand back, and built a descent little furnace, which crumbled upon a bit of heat. it had turned to reddish powder through the entire thickness of the walls 

 

I could perhaps talk to the pottery guys at the college i attend, however, i am not well connected with them unfortunately. (and if they are anything like the chemistry department, you have to technically steal the waste if you want to keep anything lol)

 

I live in the southern part of the central valley if that helps any

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2 hours ago, Kyle Vance said:

I used a gravity separation to remove all the sand and silt, and then added a bit of sand back, and built a descent little furnace, which crumbled upon a bit of heat. it had turned to reddish powder through the entire thickness of the walls 

 

I could perhaps talk to the pottery guys at the college i attend, however, i am not well connected with them unfortunately. (and if they are anything like the chemistry department, you have to technically steal the waste if you want to keep anything lol)

 

I live in the southern part of the central valley if that helps any

Is there actually any clay in it? It may just all be silt. If it's clay, it should bake hard, not turn to powder. 

Jeroen Zuiderwijk

Facebook page: https://www.facebook.com/barbarianmetalworking

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2 hours ago, Kyle Vance said:

I used a gravity separation to remove all the sand and silt, and then added a bit of sand back, and built a descent little furnace, which crumbled upon a bit of heat. it had turned to reddish powder through the entire thickness of the walls 


I've had the opposite problem building glass making furnaces - if the clay specification isn't good enough it melts!

Have a go at searching on Google Shopping for the brand of kaolin clay that @Alan Longmire suggested and see if Google Shopping can find a supplier who'll deliver it to you - having a known quality of clay to hand with known composition and known impurities is an enormous help.

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9 hours ago, Taylor Hendrix said:

earthenware

 

That is the issue, you can make earthenware from most clays, but it doesn't stand high temperatures.  It's fine for a forge, and can be used for a hearth melt, but not a furnace.  The walls of a short-shaft smelting furnace have to be able to take 2400 F without melting, and ordinary red clay won't do that.  Lots of misguided attempts at archaeometallurgy have shown this.  The English archaeometallurgists in the 1990s were having very little success getting blooms from their reconstructions of furnaces because they were making the walls too thick and from the wrong clay.  The furnaces kept splitting open and melting into the charge, until Lee Sauder went over in 2002 and showed them how it really worked. Left them gobsmacked that this American non-archaeologist showed up, built a furnace from stoneware clay with walls that were thick at the base, but tapered into less than 3/4" at the top, effortlessly produced blooms five to ten times the size of what they were getting with their cob furnaces using the same ore and charcoal.  The English teams were producing lots of slag, but that's not the ideal outcome...

 

Anyway: almost any stoneware clay can be used, EPK is just the cheapest and easiest to find commercially.  To find your own, get a soils survey map and look for deeply buried wet clays. The term "gley" is sometimes used to describe these clays that form well below the surface in water, in an oxygen-free environment.  They'll be white (kaolin), blue, or dark gray when first dug, but will oxidize to white or light tan over time.  The deposits are usually in floodplains, anywhere from five to ten feet below the surface.

 

https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx is the standard web-based utility in the US, but it is NOT intuitive to use!  Your county ag extension office is a better place to ask.

 

Before you try any kind of furnace construction, read everything on this page: https://www.leesauder.com/smelting-research

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Had a minute and looked for ceramics supply houses in the southern central valley, and you're right, pickings are slim.  There's a couple of pottery studios in Fresno and one in Visalia that offer classes and firings, they probably can get EPK if they don't have it on hand, though.    

 

Then I found these guys in Santa Rosa: https://www.creativeceramicsandglass.com/kaolin-epk

 

And now I see a lot of places are rationing the stuff. Apparently the mine closed down for a month in August and it has scared people.  No wonder it's going for $1/lb when it used to be $0.25/lb.  

 

Any kaolin will work fine.  

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Well, in the meantime, will the normal red bricks I have work well enough for a remelting hearth, or will they throw the melt off somehow?

 

I know how they respond to high heat as I used them for a improvised coal forge when I started and you had to replace them after 2 uses

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Might be worth it to price fireclays. Greenstripe is what I usually get but I've purchased GoldArt as well. Fireclay is cheaper than EP Kaolin from my supplier. And a note regarding Kaolins: prices vary greatly. EPK is by far the cheapest version. Others can be over twice as much. Bummer EPK is not available in bulk right now. We mostly use it in our glazes, so nobody wants to run out.

 

I suspected as much regarding the lowfire clays and the smelting process, but a good guess. Some fillers and tempers do wonders for a clay depending on the application.

 

Laguna Clay is the big southern CA clay supplier. If they also supply industry, they might have bulk pricing.

 

I'm enjoying the Sauder link, Alan. Thanks.

 

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15 hours ago, Kyle Vance said:

will the normal red bricks I have work well enough for a remelting hearth,

 

Nope, they'll melt or crumble.  For a remelting hearth you can go to a big box hardware store and buy a box of firebrick.  The half-thickness ones do fine for remelting hearths.  They eventually crack, but they don't melt.  

 

9 hours ago, Taylor Hendrix said:

fireclays.

 

These can work, but you're right, you have to add temper or they'll crumble.  Grog (previously fired clay crumbled up) works well.  In fact, most home smelter folks recycle furnaces this way.  Stretches out the fresh clay as well.  I've always had a bag of EPK on hand, so not much experience with fireclay.  Definitely worth looking into, though.

 

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Ball clay works as well, but you may have to play with the tempering.  The reason we use the kaolin by preference is that it doesn't melt, which means it doesn't tend to add to the slag, and it doesn't have anything in it that would preferentially reduce instead of the ore.  It doesn't scavenge carbon or oxygen, in other words.  Red clays do.  

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4 hours ago, Alan Longmire said:

 Red clays do.  

Alan, I'm a clay wonk so don't get me started.

 

2400 deg is in the region of our cone 10 aka stone ware temperatures. We often fire for 10 plus hours to get cone 10 to bend, so most pottery supplies will have some dry clays suitable for Kyle's needs. Kyle, make friends with the clay folks at your university. They are awesome folks.

 

I know you know what reduces in the red clays, Alan, don't you. Crazy, right? Fe!

 

Does the charcoal charge not take care of reduction, Alan? Can the clay walls actually parasitize the reduction process with all that organic carbon in the house? That's kind of cool. So, with this bit of new info, the best clays will for sure be the lighter ball clays, fireclays and kaolins. Less iron, more better. You can temper with all kinds of things. Try cat litter. Recycled furnaces are awesome sources for temper(grog). So is bits of IFB.

 

I have actually experimented with approx. 1/3 local soil, 1/3 sand, and 1/3 #1 potters plaster for the chinking around the tuyere in my charcoal forge. I have no idea what that would do to a smelt. Might be risky, but experimenting can be quite enlightening. I'd bet my diy pottery badge that charcoal fines/dust makes a fantastic temper as well.

 

I asked my buddy if he wanted to melt some ore with me. "Nope," he said. :(

 

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Exactly.  And cat litter is bentonite, yet another (very absorbent!) clay.  

 

Short-shaft bloomery smelting is such an inherently inefficient process that you want all the CO you can get to stay in the charge as long as possible.  If you use a clay that is fusible below 2400 degrees it will enter the slag stream.  If said clay has iron in it, it will rob CO from the charge, leading to more of the ore's iron getting entrained in the slag. Thus the 1990s English guys getting an efficiency of around 5% while Lee and Co. were getting 30-40%.  That's percent of iron available in ore that is extracted as metallic iron bloom as opposed to olivine slag.  

 

All this about clay chemistry affecting the smelt really helps explain why it's so hard to make a homemade crucible for Wootz, doesn't it?  

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Before last night I would not have understood the importance of the slag pool/stream, Alan. I read several of Lee's papers. Very interesting stuff. I appreciate how Sauder really made things simpler by not attempting any historical smelting. Castable is the way to go if you want to just make stuff from your iron.

 

No way am I running down the Wootz rabbit hole. I've got too much work to do!

 

Texas Blacksmithing Conference is going to be January 27th, 2024 just up the road in La Grange. They will have some smelters there. I'm looking forward to chatting with them this year, knowing a bit more about the process.

 

Good luck Kyle. I'll look forward to seeing some pictures if have a chance to take any.

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Yeah, the chemistry involved is pretty daunting, no wonder it took a couple thousand years from the invention of pottery before we figured out copper alloys.  Then another 2000 years before we got the clay figured out enough to do iron.  And even though aluminum is in all clay, and is the most abundant metal in the earth's crust, it was first isolated only in the early 1800s and was more valuable than platinum due to the difficulty of refining it prior to the discovery of the electrolytic method used now in 1888. That's why the tip of the Washington Monument is a five-pound aluminum pyramid, that was the most precious metal available at the time.

Kinda makes you want to time-travel to the 1820s with a load of storm doors...:lol:

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Not sure how much help this will be in this situation but "light insulating refractory brick" made from fairly pure alumina can be good as a secondary refractory (no use for a hot face refractory though - especially not with the basic slags involved in steel making arround).

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On 11/17/2023 at 3:23 PM, Alan Longmire said:

All this about clay chemistry affecting the smelt really helps explain why it's so hard to make a homemade crucible for Wootz, doesn't it?  


How helpful do you reckon the concept of acidic vs. basic refractories is?

My very approximate rule of thumb was that an acidic refractory in contact with the basic slags generated in steel making was a recipe for disaster - the two would react with each other. Steel making required a refractory that could withstand the basic slags so materials like tar and dolomite (magnesia), magnesia-graphite all the way to the specialized materials involving iron chromite.

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I honestly haven't gone that deep down the rabbit hole, just enough to ensure decent results when smelting in a short stack furnace. We did have a member several years ago who got really into smelting via thermite reaction, and for that he needed to research the high-temp magnesia and zirconia linings for crucibles.  Not many materials are happy at 6500 degrees F! 

 

You are correct that the soft bricks, while great insulators, can't survive hot face duty or slag.   Neither can ceramic wool, but we used that on the portable short stack Jesus Hernandez built for our experiments with Pigeon Forge ore in 2013-2014.  He made a pair of sheet metal shells lined with two inches of kaowool topped with about half an inch of castable refractory.  All we had to do on-site was make the base out of pre-made clay balls and install the tuyere.  From there on up the shaft was those two lightweight sections.  It worked well, but two runs was all it could do without needing patching on the lower section.  The slag did attack the castable, eroding grooves down the shaft.  

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