Building a charcoal furnace to melt aluminium --------------------------------------------- 14/Jan/2003 I've just bought the first two Gingery books (The Charcoal Furnace and The Metal Lathe) and spend far too long looking at webpages written by people who've done this already. My aim is to be able to cast metal parts, since I'll then be able to make some of my own bike spares. When you drop a bike it's always things like the clutch lever and mirror mounts which break, and they're way overpriced to replace. My second aim is to build a Gingery-style metal lathe. A new lathe is around 600UKP, and I don't want one that much! I'm more interested in the process of making one. I figure I should write an account of my activities in case it helps someone else. Plus, it'll let me keep track of how much money and time I spend doing this. I'll also try to answer my own questions with a bit of research. I started looking for materials in B&Q and Homebase. Homebase is near my flat, but isn't very big. There's a huge B&Q warehouse in Edinburgh, but it's miles away and I have a motorcycle rather than a car, so carrying heavy or bulky stuff back is difficult. However, down at the industrial end of the city (Leith) there is a lots of building yards, metal yards and scrapyards. A quick bike trip around the area revealed plenty of hopeful looking places. The first task was to find scrap aluminium, ideally in small pieces so that I could carry it back on the bike. Having never been to a scrapyard before, I decided to try the one on Constitution St. It was very straightforward. They had sorted out different kinds of metals into different skips (I had wondered if I'd need to hunt around the piles of wrecked cars to pick out the bits I wanted!). It was just a case of picking the bits you wanted and then getting one of the guys to weight it. I filled a bag with some two inch strips which had been coiled up, and ended up with 6kg of metal for 4UKP. Aluminium is 2.5 times more dense than water, so 6kg will be just over 2 litres. I'll need more than that to build the metal lathe since both the headstock and the bed need about a litre each, but it'll do for now. The Gingery books measure everything in quarts, and a quart is just under a litre. I went to B&Q in the evening. They sell Perlite in 10 litre bags for 4UKP. I got a couple of these, since I planned to do a perlite and fire cement furnace lining. Consequently, I also got a couple of 1kg tubs of fire cement, which were about 2UKP each. Now that I've found a fire clay supplier, I'll probably stick with the Gingery recipe. The fire cement can get used for patching any cracks in the lining. It comes in plastic tubs, so it's not going to dry out. B&Q also have a big range of sand, but I didn't fancy carrying 25kg of sand on the motorbike since it was blowing a gale. It ranged from 1UKP up to 4UKP for a 25kg bag, depending on what flavour you wanted. I'm not clear about the differences between the kinds of sand. I figure that fine sand is best for making moulds. And it's important to get silica sand rather than stuff made from ground-up limestone, since that explodes when you heat it. I'll try to find out more. They sell cement in B&Q too. Some people use that in their refractory mix, but the Gingery book just uses sand and fire clay. I've not decided whether or not to buy cement. Cement is just any powder which forms a solid cohesive mass when you mix it with water. When you add the right amount water you start chemical reactions (add the wrong amount and it won't set so well). The cement grains absorb water, and grow needle-like crystals outwards, like a sea urchin, which locks the grains together. As the crystals grow, the cement sets. After a few hours, the grains are locked together and the cement takes on an initial set. They carry on growing for up to a month, and the cement gets harder and harder. 15/Jan/2003 Today, my task was to find fire clay. I went on another bike ride to find places which looked like they might sell it. I found a company whose sign said that they sold mortar, so I went in there. They didn't have any but they pointed me across the road to Christies. It's just along a bit from the scrapyard, and they're a fireplace and chimneys repair supplier. They sell 25kg bags of fire clay for just over 4UKP. My tactic of riding round until I find a vaguely relevant supplier, then going in and asking them where I can get stuff has worked quite well. This evening, I went to Homebase and bought a galvanised steel bucket which was 6UKP. It's only about 12 litres (3 gallons), but it looks like there'll be plenty of space for a reasonably thick refractory lining and enough space left over for charcoal and a one litre crucible. I got a plastic 3 gallon bucket for mixing too, and that's only 2UKP. They had little galvanised steel "paint kettles" which looked like they might hold one or two litres, but they're quite thin so I'm still looking for something to use as a cruicible. Right now, I need to find a friend with a car so that I can buy fire clay, sand and maybe cement. I'll take advantage of this rare car oppurtunity and buy lots! I'll be using sand for the furnance lining, make moulds and I'll stick some on the ground to catch spills. I need to figure out amounts. Gingery says 5 gallons of sand and 2.5 gallons of fireclay should be more than enough to line a 5 gallon bucket furnace. My bucket is only 3 gallons, so that'll be 3 gallons sand and 1.5 gallons of fireclay. In metric, that's about 12 litres of sand and 6 litres of fireclay (which is more than my mixing bucket will hold!). I think dry sand is about 1.5 time more dense than water. So, 12 litres required for the lining will be about 18kg in weight. So, I'll get one 25kg bag of sand for the lining, and another two for moulds and putting on the ground. I'll guess-timate that fire clay is roughly as dense as sand, so I'll get two 25kg bags of that - one would probably do, but it's not easy for me to transport this stuff. [ Added later: Building the furnace used less than a third of a bag of fireclay and about half a bag of sand ] I need some sheet metal to act as the former for the lid, a bit of metal pipe to attach the blower to the bucket, and some wire to reenforce it all. I'll keep my eye out for suitable bits of junk. Saturday 15th March ------------------- After much delay, partly caused by the need to build a shed, I've got going again. I picked up 25kg bag of fire clay two days ago from Christies on Salamander St for 4UKP, which got put into a rucksack and carries up on my bike. Yay, so it's possible to carry 25kg on a bike without any great difficulty. Yesterday, I went to the scrapyard and picked up a couple of steel tubes, suitable for the air pipe, some steel bar which might be useful for making tongs, and some random bits of wire for re-enforcing the lining of the furnace. This time I was scouring around piles of wrecked cars - obviously the valuable metals are seperated out and random lumps of iron are left out to rust. Unfortunately, there was no pipes suitable for making into cruicibles. I went to Homebase to get handles for the bucket, but ended up buying some chain instead since that'll allow me to be further from hot things when I'm moving stuff. I still need a second steel bucket if I'm going to do the "one inverted on top of the other" design, but Homebase have sold out. So, I drilled holes for two bolts at the appropriate end of the bucket to attach the chain. Then, I drew around the end of the pipe onto a sheet of paper, taped it to the bucket and drilled holes around the circumference. Then I use wiresnips to join the dots and make the hole for the air pipe. I use a medium-sized flower pot (6" diameter, 6" deep) to measure out 2 parts sand (one pot full), 1 part fireclay (half-pot) and 1 part perlite (half pot). That all got mixed together in the plastic bucket and then I added enough water to make it look like cement (I've never mixed cement before, so I'm using some imagination). That was enough to do the base of the bucket to a depth of maybe 2 inches. Then I put it straight into the oven at 70c for a few hours, then turned it up a bit. It was cracking a bit, but I'm not worried since it's just the base. I'll take more time when I'm doing the sides. It's now sitting out in the garden with some charcoal smouldering away in the bucket. Oh, and I bought a small fire extinguisher "just in case". ;-) Sunday 16th March ----------------- The base seems dry enough to let me get on with the sides. I forgot to watch at work for when the coffee can got empty, so I'm going to use an old paint can which we were about to throw out as a spacer. Currently, I'm pouring the paint from that into another paint can, and then I'll wash out all of the old paint. Wrapping some cardboard around it hopefully should work. That'll let me get the sides done and drying while I go to B&Q and get wood for a workbench. I ended up wrapping cardboard round the paint tin and holding it together with duct tape. I wrapped some paper around the end of the air pipe so it doesn't stick to the lining. Also, I wound random bits of wire around some more bolts which I put through the bucket sides. I'm not convinced that this was a good idea since it makes it a lot harder to ram the lining down. Anyway, the lining required four flower-pots of sand, 2 of clay, 2 of perlite. It's hard to judge how wet/dry the lining should be. The first batch of lining was made drier than yesterday's base, but the stuff I mixed for the top of the sides was a lot wetter. That made it easier to get the top flat, but I'm a bit concerned that the mix was wrong somehow. The air pipe makes it too big to fit in the oven. I need to find something which is the same diameter as the airpipe but shorter so I can quickly whip the air pipe out and replace it (should be fun) before putting the whole lot in the oven to dry out. Should've thought of that before starting putting the lining in. Got wood for the workbench, a big vice and an anglegrinder for good measure. I'll build the workbench tomorrow and that'll make working on this stuff a lot more pleasant. Thursday 20th March ------------------- I've been gradually drying out the furnace by putting it in the oven at about 70c for a couple of hours at a time. I don't want to leave it unattended, so it's been in for an hour before work some days and a couple of hours in the evening other days. There's still a few damp patches on the surface, but it's drying out quite well. The mixture looks like it's been fine, and it's been packed down well enough. When I removed the airpipe (to make it fit in the oven), the lining around the pipe retained its shape like a sandcastle does. I thought it might slump, but it was fine. So, the bucket went into the oven on its own, sans pipe. I took out the paint pot (still had paint residue in it) before heating, and the cardboard had enough strength to keep the lining in place. After a couple of hours of drying I took the cardboard out, and carried on with the heating. I'll do another stint in the oven to get the rest of the damp patches dry, and then I'll light a charcoal fire in it to harden the clay. Still need a lid. I'm a bit loathed to line another bucket, although that's the easiest way to do it. If I could get a strip of sheet metal from somewhere I could do a Gingery style lid. Saturday 29th March ----------------- I decided not to use another bucket for the lid, to minimize work! Someone I work with pointed out that there's a metalwork shop on Anderson Place, near where I work. They had big 1m square sheet metal which was too big to carry on the bike. Instead, I got a strip of steel - 2m long, about 5cm wide and 2mm or so thick, for about 2.50UKP. So, you could bend it by standing on it but it's pretty far away from sheet metal. Turns out, that was a big hassle when it came to drilling holes because it was so thick. The lid needs to have a circumference of about 1m, so out with the angle grinder. Lots of fun and sparks, but I really should've bought ear protectors first. I bent it round into a circle, and drilled a couple of holes through and bolted the ends together. I've not been doing much on the furnace recently. I've built and varnished a workbench, which now lives in the shed with a vise on it. Makes life a lots easier. Plus, it's been really good weather so I've been doing other stuff! Drilling holes around the lid was a big hassle. Originally, I had a cordless drill, and having never drilled metal before I broke lots of drill bits. Coincidentally, the charger for the drill died (the primary windings appear to be open), so I ended up getting a Black and Decker 550w drill instead. Ah, it's so much better. Drilling through the steel used to take about 5 minutes for one hole. Now it takes less than a minute. And I haven't broken any bits with the new drill. After I'd drilled about 6 holes around the lid, I got three metal coathangers and straightened them out to make supporting wire. They got twisted round lots to make them stay put. I also added in a few section of what is probably old fence mesh which I picked up from the scrapheap. At this point, I luckily remembered that I still needed handles, so I did a couple more holes and bolted on a length of chain, being careful to get it balanced so that the lid doesn't tip when I pick it up. Using chain is good because your hands are further from the hot bits. Then I used the refractory mix which was still in the bucket from a few weeks ago (I'd poured more water on it when it looked dry) and another batch (one flower pot of sand) to fill the lid. As a base, I sellotaped a couple of bits of wood togther. A jam jar wrapped in newspaper sufficed for the vent. That's all now in the oven at 70c until it dries out. While all this was happening, I finally got around to buying more charcoal to fire the furnace base. This time I bought lump wood charcoal and it's so much better than the briquettes I had last time - they light faster, and generally act more like wood. I lit a single layer of charcoal and a couple of firelighters. Then once the flames had died down and the charcoal was glowing, I added another layer. I added a third layer later, but the bucket was never more than 2/3s full. It was probably burning for about 4 hours. Afterwards, the bottom half of the lining had changed color to an orangih tinge, and it's noticably harder. I might put a layer of fire cement over it later, because the surface is quite rough and might chip quite easily. If the weather is good tomorrow, I'll take the (hopefully mostly dry) lid and put it on the furnace and give it another afternoon of burning charcoal. Charcoal is great stuff - once it's got up to temperature it doesn't give off any smoke at all, which is important since I've got lots of neighbours. I probably only need a single firelighter next time to light the furnace - it seems to suck plenty of air in through the air-pipe-hole. Sunday 30th March ----------------- The lid dried out quite well, after maybe about 4 hours in the oven at 70c, with the last half hour or so going up to 140c. There's a few small cracks but nothing to worry about, especially consider the amount of wire which is holding it together. I've lit another charcoal fire in the furnace, and the lid is sitting on top baking away. Once it was lit, I gradually filled it right up with charcoal. I've been adding a few more bits through the hole in the lid as the other stuff burns away. Unfortunately, the hairdryer which I was going to use as a blower has met an early demise. I removed the heating coil from the hairdryer, but forgot that it was also acting to limit the current to the motor. So, when I plugged the heater-less motor in to the mains, it made a bang and now runs no more. Hmm, I'll see if I can salvage the propeller bit and use a battey-powered 12v motor to run it. Wednesday 2nd April ------------------- Realised that a PC fan would make a good 12v/5v fan and there's a ready supply from other computer folk. Yay, it's going to be a hightech furnace. Friday 4th April ---------------- My digital camera arrived today so I can take photos of the First Melt - it does video capture too. I've now got a 12v PC case fan from a workmate which I'm just about to try. I've also bought a steel cake tray for making ingots (not sure what the nonstick coating will do with molten aluminium) and a pan and a steel sugar container thing as crucibles. This all came to a total of 8UKP, which is one of the more expensive purchases so far - but I want to try melting stuff over the weekend so I don't have time to scavange for them. I hope that I can lift the crucible out of the furnace with visegrips and welding gloves. Otherwise I'm going to have to make some long tongs first, which will be a hassle. I intend to make a work area by putting down plastic sheet on the grass and covering it with a good layer of sand. The metal shouldn't get through the sand, and if it did it'll be little worse than if it was straight onto the ground. The plastic makes it easier to clear up afterwards. I could make a wooden box, but plastic will make it easier to funnel the sand back into the bag. Saturday 5th April ------------------ Everything set up ready for a test melt tomorrow. The PC fan doesn't shift nearly as much air as a hairdryer, but it's a lot quieter. I'll run it for a while and see how it goes. I've wrapped some carboard and duct tape together to fit the fan to the steel pipe, and checked that it still fits into the furnace. I've got a biscuit tin filled with sand ready with some depressions to take the molten metal. I've rolled up a strip of aluminium into the crucible (a stainless steel pan). It looks like visegrips will be fine for picking up the pan, although whether or not welding gloves will be enough protection isn't clear (yet!). Cutting the handle off the pan was another angle grinding excuse. I used earplugs this time - good idea. At first, I tried to cut through the metal tags but then realised it was much easier to grind off the rivets. The rivet are much softer than the steel pan, and once you've taken off the heads the handle just fell off. Although I know that some metals are softer than others, that was a pretty good practical demonstration of the fact. Sunday 6th April ---------------- Yay! A succesful first melt. I've created a photo gallery capturing the day's events, so I'll just make some "notes for next time". - Don't use a plastic-handled screwdriver to clear the slag. - Keep a bucket of water around to dunk hot things into .. like a melting screwdriver. - Visegrips and welding gloves were fine. In fact, I had my hand inside the lit furnace picking out bits of the melted pan with visegrips and it was fine for a few seconds (started to char the gloves though). - PC case fan is fine for air. You don't need huge big blasts, and you don't need jets of flame shooting out of the furnace. Plus, it's really quiet. I powered it from a "wall wart" transformer and I just used duct tape to connect the wires rather than soldering a plug onto the fan! - The stainless steel cruicible turned a bit blue, but was basically fine. They're 2UKP new, so replacing it won't break the bank. - Masonary sand contains too much water - enough to cause a bit of steam, which bubbles through the metal and leaves it looking weaker. - It took about 5 minutes to melt the aluminium. Once the furnace is going, you should make the most of it and melt lots and pour lots! Or, get some marshmallows and toast them. - Always make sure you have some place to dump hot metal - a tray of sand is ideal. Monday 8th April ---------------- I'm using offcuts of wood (from when I built a bed) to make a 8x10" flask. Later this week I'll try to mix some green sand so that next weekend I can try casting something proper. Not sure what I'll do at first. A hairclasp for my other half sounds like an easy thing to start with. Just out of interest, I've added up the cost so far: 6kg scrap aluminium: 4UKP 2 bags of Perlite: 8UKP (only needed one though) 2 tubs of Fire Cement: 4UKP (not really needed) 25kg Fire Clay: 4UKP Steel bucket: 6UKP Plastic bucket: 2UKP Steel strip for lid: 2UKP Crucibles, ingot trays: 8UKP That's a total of 38UKP. I didn't need the fire cement (yet) or much Perlite so I could've saved 6UKP there. Plus, the cruicibles and ingot trays could've been scavanged to bring the total cost down to 24UKP for a fully operational furnace! Saturday 12th April ------------------- Today I melted down some of the aluminium strips which I got from the scrapyard ages ago. At first, they wouldn't melt. It turns out that the charcoal near the air inlet had burned away and so much of the air blast was being wasted. I added more charcoal so that the inlet had lots near it, and put lots around the crucible itself. That made a huge difference - the crucible immediately went red hot and the aluminium started melting. I got a video of the melt too. I melted one of the coiled strips, and poured it into a steel baking tray to make ingots. This caused the non-stick lining (couldn't get one without nonstick lining) to briefly burst into flames, but that only lasted a few seconds (long enough for me to think "shall I get the fire extinguisher now?"). So, today's net result is a couple of ingots. They both have a sunken appearance in the center. I think the metal in the middle stays liquid longest, and so it gets displaced. I'm not sure /why/ it gets displaced. If anything, I'd have thought that if the outside of the ingot cooled first then it'd shrink causing the center to rise, rather than fall. Hmm, the outside "donut" of metal must shrink, sucking molten metal from the middle, rather than whole ingot shrinking towards the middle. That'd explain it. I'm always surprised by how little metal I make. I start with lots of scrap and by the time it's melted down there never seems to be as much as I'd expect. Once I start doing proper castings, I should estimate the volume of metal required, and work out how much scrap metal I need /by weight/ since I'm obviously lousy at judging by volume. My wooden flask is ready to go, and I've mixed up clay/sand to a reasonable consistency but I'm not going to do any more today. I got lots of cool feedback from last weekend's pictures. Several people want to come round and see the furnace in action now.