Alright so here's the scoop. I'm nearly finished building a 55 gallon drum furnace. I put in the refractory hot face and cast the lid a few days ago but I'm running into a couple problems. The first - and cause of all further problems - is that the refractory I got is not drying! My tests which were all very small worked perfectly (about the size of a cup). They dried brittle and then I fired them resulting in a great refractory. I have a dehumidifier running with plenty of airflow over the clay but there is just too much moisture. It seems that when you start getting into a 50lb 20 inch disc of clay that it refuses to dry without a kiln the size of a shed. I'm ready (I think) to fire the main body of the furnace to finish the drying but the lid is so soft that when I try to pick it up, the stabilizing rods cut through the clay and it slides right out of the barrel shell. I'd like to be able to start lining up the lid for attachment but right now I have no idea how to dry the clay all the way through, short of waiting a month for it to air-dry. Any ideas? I'd like to salvage this lid somehow. I've already emptied around 9 gallons of water from the dehumidifier and the clay is still wet. About 1cm in from the outside is firm plastic, the inside is still slip-like. I'm using a hawthorne bond based fireclay. Waiting to see what y'all have to say. Possible solutions I think is to cut 2 more kaowool circles to enclose the clay..or re-cast the lid with the "top" of the barrel. Once it's been fired and the top is solid I'll remove the steel barrel lid so the clay can fit more flush with the top of the furnace
As for cracks from drying - I'm not concerned, any major cracks will be filled with refractory mortar after firing. I just need to be able to move that lid around
You could probably help speed the drying at the risk of cracking with some kitty litter silica gel in a small bucket sat inside the furnace and frequently swap it for freshly baked dry silica gel. There's a temperature limit when drying the stuff for reuse where you can overheat it and destroy it's dessicant properties. Some people here have used the heat from an incandescent bulb to gently heat the lining too. Weighing silica gel before and after would give an idea of how much water it gathers, having such a thick lining is going to risk it blowing off in chunks from the steam generated when it gets fired for the first time.
The furnace itself is fine, I have the blower going and it's already dry and ready for firing. The lid is the problem. It's 2" of clay, 1" of ceramic blanket, then .5" of clay. I can't get to the deeper inch of the thick side. Even once it dries, it seems to want to slide right out from the barrel shell. I have 4 quarter inch threaded rods going through the lid. You think they will hold it all together?
Hard to say what will happen, internal rods can expand faster than the refractory due to expansion co-efficients and actually cause cracking. Some people have exploited geometry to retain their lid material: either a dome or conical shape so that when the inevitable cracks occur in the refractory it will try to slump but comes up against the sheet steel retaining ring (with some features for the refractory to key against and grip) and gets wedged tight.
Agreed, Mark. There is a reason domes are dome-shaped. Asking a ceramic material to defy gravity in the shape of a flat disc is asking a lot. It can be done. A good number of folks here have flat lids. But domes are inherently more stable. I made a couple furnaces with flat lids and they worked for a while. But my dome-shaped lid has lasted much longer and has taken far more "abuse" in stride as I have used it regularly for a year and it looks like it is good for quite a while to come. Denis
How do you guys suggest making a dome-shaped lid? The size lid I’m working with doesn’t exactly hold its shape in any way. I’d have to make some kind of form 22 inches big and short of hammering away on some sheet metal I can’t imagine how I’d make that.
22" is your OD,....right? What's the furnace bore? Cheap salad bowl oughta due for an inside form. Use wax or petroleum jelly for release agent. I presume the fireclay is only intended to be backing. If so, mixing with silica sand reduces clay and moisture content, but think you may be beyond that. Best, Kelly
And it does not have to be a round dome, though I think Kelly's suggestion is probably very practical and simple. But you could also just cut 8 or ten 1X6 or so trapezoids wider on one end than the other so that stacking them up they form multi-faceted dome with a central void for the vent. They could be simply duct taped to temporarily hold them in shape. Or a person could use damp soil to pack a dome-like form and lay plastic over it to form the inside of the lid. Use a template to keep the shape roughly uniform all the way around the form. Pack the refractory over it. Place a steel hoop around it to from the band at the base of the dome. Must be a lot of other ways as well. Denis
Apologies roundbug as I can't recall your original post on the subject but most builds that I'm familiar with that use fireclay use a mixtures of it and silica sand as a backing but use a castable refractory for a hot face which is cast first, then the clay sand rammed behind and using the dense refractory as the inner mold which facilitates post firing. The problem with doing it the other way around is the thickness is hard to cure without a very long and gradual firing.......much like the pottery folks do with thicker sculptures. For 2" thickness of clay I'd think you'd be looking at a number of days with mild heat if not several weeks. The reason it drops out of your lid frame is shrinkage. Anything with high water content will likely have high shrinkage. Castable refractories go through a chemical reaction for curing and don't depend on evaporation. Best, Kelly
What is the composition of the lid? You mentioned that its based on Hawthorne fireclay, but is there anything else mixed with it? That may or may not matter I guess, just wondering what mix you've decided on. But to address your original question, the only fireclay I have experience with is from AP Green. Mixing it with water does not initiate any chemical reaction that I've seen. Thin sections will dry seemingly hard and crusty, but thick sections will not. Water only acts as a vehicle to change it from powder to plastic, and the stuff is sticky. I have about 10 lbs of it that I mixed with water prematurely (didn't need it for the original purpose I'd intended) and have kept it in a kitty litter bucket for 8 years. It's still plastic! The thin unfired section you have for a hotface may be little more than a precarious pile of dirt until you apply heat. Unlike commercial refractory cement which is chemically activated by water, even the dry thin sections of my fireclay would have almost no green strength. My fireclay will only cure with heat. I'm not talking sintering (firing). That's another matter. I'm only talking about initial dryout. Again, hawthorne may behave differently, but this is how my stuff is. Heat activated. I hope this helps. Maybe there's pertinent information available from the manufacturer. Edit: having read Kelly's post, maybe I'm wrong about the curing. Instead of it setting up as a result of heating it, all I'd really done was drive the water out. Pete
The furnace body is 100% hawthorne bond, a little more than an inch thick followed by chicken wire and 4 inches of ceramic blanket. The lid is a combination of hawthorne bond and various amounts of perlite to make it lighter, and also to save my dwindling fireclay supply. I noticed firing the lid vent cap to 1000°C did not produce the same effect as firing to 1200°C like I did a while ago. At 1200°C it was like stone. 1000°C it's still rather fragile. I fired the body tonight. The lower third got sufficient temperature to harden properly. I didn't put enough charcoal in to vitrify the upper area..and without a lid I'm not sure I would want to fill more than a few lumps . Hardiebacker is better than nothing but hardly a "lid". Definitely need to fill the cracks with some refractory mortar now. A few are almost an inch wide. The outside shell did not change temperature from ambient save for about a 1cm around the tuyere. Perfect Once it has a proper lid I will fill it completely with charcoal and give it a go. So far I'm impressed what a few lumps of charcoal can produce. I think I'll end up making a new lid with koawool and a thin coating of clay, as I don't have much left after making the heavy disc-lid, assuming that one ends up not working. But we'll see Also yes I'm aware most of the debris in the chamber is grog from the walls, I was expecting that to happen from such a rapid drying process. That's okay, once the walls are vitrified that won't happen any more. Me poking at them to see which sections got fired properly didn't help either lol
Was really just talking about green state. Not my field, but the composition of fireclays looks a lot like castable refractory except they are low alumina and higher silicate content with the ratios being flipped. Clays like Kaolin rely on CaO/calcining at higher firing temperatures so you basically have dry mud until fired. It's very common choice for Adobe ovens and even naturally exists so has been used for millennia. Sounds like roundbug knows what he has.......just underestimated the challenge to get from green state to fired strength, which may never happen in the outer areas of the furnace structure. Best, Kelly
Yeah definitely forgot that many outer parts may never reach the temp required for vitrification. Not really necessary, the only part I want rock hard is the hot face. I guess if I want the whole thing vitrified I’ll just have to build a kiln big enough to house this one! ...but then how would I fire that one... If anyone is wondering about my choice of refractory..150lb of fireclay cost me less than $50. A single 50lb bag of Mizzou is like $70 + shipping. I used 50lb just for the floor of the furnace.
One thing to check regarding Mizzou and other foundry supplies is that a supplier somewhere a hundred miles from you may have customers somewhere in your area and may be running shipments via their own truck. I have one 85 miles away who does that. I get a few bags/boxes of stuff added on to the truck and meet it when it comes to town----free. Saves shipping and time. Denis
That's the same for me: the refractory supplier's nearest warehouse is 1800 kilometres away but they regularly ship to the local sugar mills for relining their steam plant furnaces, I just have to time it right.
So an update. I re-made the lid using the "top" of the barrel. Instead of clay I just filled the thing with kaowool and put a thin layer of clay mixed with sodium silicate. Nice and light. Great! I welded a bullet hinge on the sides and a handle to slide it open. First firing I tested aluminum. About 15 minutes to pour. Tiny amount of fuel needed, very little air. I used a few lumps of charcoal with a few briquettes to help get the fire started. There was a solid 2 inches of unused space all around the entire pile. Awesome. This furnace will be able to pour a LOT of aluminum. Let's move on to bronze...ugh. The efficiency of this build is not very good when it's 50%+ loaded. Mostly because I had a hard time getting the cut of the barrel to be even with my angle grinder, so there is quite a gap between the lid and the body. Heat escapes through there when I cover the vent hole and with fresh charcoal sprays embers all around the furnace. Despite the heat loss, I was able to melt bronze right before the bullet hinge seized up and the welds holding it to the lid snapped. The 1/8" steel bracket I made bent 90° when I tried to open the lid. I'm going to have to re-think how the lid covers the body. Seems like getting a solid seal is paramount. Might be best to just have a basic lift-off lid instead of fancy hinges. The vent hole is 4" in diameter, with a 2" tuyere.
Well done Roundy! I am about to start making a new furnace as well. What sort of burner are you using? I plan to use a drip burner, Cheers Charlie
I just have a centrifugal blower forcing air into the chamber. I got lucky with the angle and get a really nice vortex. I haven't read much about drip burners, is that something you drip into the air-stream? Sounds like a nice way to increase the heat quite a bit. edit: I should mention I have a ball valve that I keep mostly closed. The only time I need full airflow is with a fresh, fully charged chamber and need the air to push through all the wood.
