So I'm getting ready to start building and documenting my large furnace build with a 55 gallon drum as the shell. I'm trying to figure out the refractory material. The goal is melting bronze and copper regularly and occasionally iron with solid fuel, lasting as long as possible. I have ceramic blanket which will fill most of the shell, probably around 3 inches around, and then I have some fire clay for the hot face. The fire clay is rated to 2300°F by itself, and I'm experimenting with additives to increase that temperature. Resources I have available are pure silica sand at about 150 mesh, silica powder at 500 mesh, perlite, and sodium silicate. Anyone have experience in this area? I have several samples of various ratios between these components and will fire them once they dry to see how they react. It seems perlite will add to insulative properties but reduce strength, heat tolerance and longevity, so I wonder why it's always recommended in refractory mixes. Please don't suggest ordering a refractory cement online.
I just discovered silicate-alumina refractory recipe. This stuff is insane, like a full 1000° hotter than clay refractory. Is there any reason people don't use this instead of clay?
I wouldn't mess around with home brew refractory recipes, unless you have no other choice, at least not for the hot face, especially if you have iron in your sights. Not sure what part of Texas your in but there are forum members and sources available. People have successfully used fire clay and sand for hot face backing. Most of the castable refractories are alumina-silicates (mostly alumina) and there are many castables with 3000F+ service ratings. The phos-bonded refractories tend to do better with iron. Melterskelter has had good luck with the BluRam phos-bonded plastic refractory. Might search and read up on the pros and cons of high and low mass furnaces. Best, Kelly
In response to your first quote, perlite-cement-fireclay-silica sand melts to a foamy glassy green substance when approaching iron temps. Sodium silicate recipes also foam up when hot and then soak up atmospheric water and dissolve again. There is a commercial concrete product if you want to roll your own refractory: "Ciment Fondu" which is calcium aluminate cement, good to 1800 deg C/ 3272 deg F depending on which of the three types you use and the aggregate mixed into it. Most commonly available commercial dense castable refractory has white ciment fondu which has the highest alumina content, I can only find the grey ciment fondu which is good to 1350 deg C/2462 deg F when mixed with brown fused alumina which can be bought as sand blasting media. Commercial castable refractory for me costs about $50-90 Australian per 20 kilo bag and eliminates one more variable from furnace construction. https://www.azom.com/article.aspx?ArticleID=1411
I can understand not wanting to pay for shipping on 50# bags of refractory. Rather than doing that I drove 5+ hours each way to buy bags of 3200F dense castable refractory and greensand in person at a big foundry supplier, and I have never regretted it even though that meant driving all the way through Toronto twice. I assume it cost me more in gas money than shipping would have been, but then I would have missed out on an amazing tour of their warehouse and a chance to visit with an old friend who lives quite near there.. I don't think I would use any diy materials or recipes to make a furnace lining again. I try to build my own foundry gear as much as I can and really enjoy doing so, but personally, refractory and crucibles are the 2 exceptions where I'd rather pay for the real thing and know what to expect in terms of performing very well. That said, even though I abandoned them forever years ago, welded steel pipe crucibles and a homemade sand and fireclay Gingery charcoal furnace lining were good enough to get me started and totally hooked. I seem to remember there is a long section on creating viable diy refractories in the foundry tutorial ebook which is linked here somewhere. I believe it is in a pinned thread. You may find some good ideas and recipes there, good luck! Jeff
Thanks for all the insight! I want to be clear that I have an ample supply of lab-grade material to work with (lucky), I'm just trying to figure out the right ratios of ingredients. With the exception of the perlite you can't buy any of this stuff at a hardware store. So far it seems that the addition of oxides to the mix is really what makes a strong refractory. Unfortunately, I only have lab sand, not oxides, so none of my tests are on that level. Alumina blast media seems rather coarse. One recipe I've found is literally just aluminum oxide powder and a binder, and it comes out looking like a sharpening stone and was impervious to the temperatures the creator was capable of. I'm tempted to go that route but having all this ultra-refined dirt seems like such a waste!
This is the tutorial jeff referenced above. Maybe you've already seen it: http://forums.thehomefoundry.org/index.php?threads/foundry-tutorial.1487/ Itll take some scrolling to get to the pertinent section, but theres quite a bit of the kind of information you might find helpful in terms of a formula. Pete
Do you know Mizzou is sold at armadillo clay in Austin? I did what jeff did and made an 8hr round trip there to pick up a box of kaowool and 2 bags from them. Best money I ever spent. I applaud anyone trying to go the home brew route, but ask yourself if youd rather be spending your time reinventing the wheel to save a few bucks or casting metal? Keep your eye on the end goal. I've seen people fail and give up on casting because of this very thing. (omm comes to mind)
I already have several furnaces, so working on this project doesn't create any downtime to my castings. Also I'm not "reinventing the wheel". It's not that hard to experiment with different amounts of components to find out which works the best, especially if you have most of them in abundance, for free. All of these refractories use the same ingredients. It's the ratios that they keep a trade secret. A few also have some other additives that help in retaining plasticity, extending working time etc. I refuse to pay 300% markup on recipe when I have all the ingredients at home. And 150lb of refractory cement is not a few bucks. Do you give up making your own brownies when you can just go buy a little debbie? ANd you have butter, sugar, flour and cocoa available...for FREE?
Perlite and portland become a flux at temp. I don't eat brownies. If you come up with a mix that is good for you then it was time well spent.
Here's a link to a good article on refractory aggregates (in different types of ciment fondue concrete) and their service temperatures which I found very useful as my furnace uses this type of dense refractory. https://www.azom.com/article.aspx?ArticleID=1411 A lot of the aggregate material can get expensive such as brown alumina and fused white tabular alumina, as for the cement, I've only found the grey ciment fondu available, not the specialist refractory grade white or brown calcium aluminate cement. I'm told it used to be made in Australia but with the demise of the steel making industry it now has to be imported. Back in the day it used to be fairly cheap as there was enough demand to get a cement plant to calcine the lime and alumina in their cement roasting rotary kiln as a dedicated batch. This 1989 video shows the digging of marl clay as a substitute for gannister clay for the iron furnace lining at the 11:10 minute mark, they later show it being used as the furnace lining and mention it has to be relined every time the furnace gets used, so while it can withstand the temperatures involved it's not that durable physically.
Great find! A wealth of information thanks. Sure is a lot of hand crushing with those hammers. Man you think they would've cast at least a rotary crusher or something by that point! Nice montage at the end shaking out all the castings. I bet it's like Christmas the day after pouring
One thing to keep in mind is that this type of furnace (drop bottom cupola) has the metal in contact with the lining so the calcium carbonate in the marl clay is beneficial to the iron quality by forming slag with the sulfides.
