Safety, Aluminum Sources, Melting, and Pouring
We are now at the most exciting and most dangerous part of the aluminum casting process. You will be working with fire, an extremely hot fire, fed with forced air. But the biggest hazard lies in a possible spill of molten metal. Before I even start the fire I wear tall, heavy leather boots, this is no place for sandals, or plastic tennis shoes. I also wear a pair of welders suede leather leggings, and a welders suede leather apron. It is wise to wear long pants and long sleeve shirts. This is a pain in the summer, but even with good clean charcoal there will be cinders blowing out of your forge. Make sure there are no flammable’s within a 10 foot radius of your work area. The most dangerous would be a forgotten 5 gallon plastic gas can laying near the forge. If you do not already have a fire extinguisher, then buy one, the biggest baddest [dry chemical A-B-C] one available, and keep it close by. When it is dry I take my garden hose and water down the entire forge area. [JWR Adds: Never do this anywhere except on soft soil. Wet gravel and concrete will spall into “formidalbe projectiles”, in a steam explosion.] It is critical to protect your head, face, hands, and lungs. I wear a respirator, not a simple dust mask, but a respirator with a valve. Molten metal gives off toxic fumes that you don’t want in your lungs. I also wear a standard military issue boonie hat, this keeps cinders out of your hair. I wear a full [Lexan] face shield, the kind that you wear when brush hogging or grinding. Lastly, you need to wear thick leather gloves, since your hands will be closest to the heat. Lineman’s gloves or welder’s gloves are best. And if you have not guessed already, you need to do this outdoors. There is one last safety step you will perform when doing the actual pour and I will cover that later.
You are going to need some aluminum to melt. Look around, chances are you already have pieces of aluminum scrap. The trick is to find or cut down scrap pieces that will easily fit into the crucible. Yes, you can heat up an automotive transmission in a charcoal fire and bust it down into smaller pieces, but that takes an enormous amount of work. Stay away from aluminum soda cans, they provide very little metal when melted, and are coated in latex that gives off foul fumes, and can contaminate your melt. The absolute best aluminum scrap around are door and window frames. Almost every window made has aluminum bars that hold the glass in. Once the glass and rubber gaskets have been taken out, the window panes and door frames, can be pulled apart into nice long bars. If there are any steel rivets, screws, or hinges they need to be removed prior to melting. If they are painted, don’t worry about it, the paint will burn off quickly. You don’t even need to cut them down, just feed them vertically into the crucible, once melting temperature has been achieved. I have a scrap dealer/junk peddler in the neighboring town who sells me all the window panes and door frames I can carry for just a couple dollars. Unfortunately not all of us have a good scrap dealer, but do look around, if you can’t find one then you will have to get creative in finding aluminum to melt. Many scrap yards do not even sell scrap to the general public anymore, they only buy scrap, and this irritates me to no end. Be prepared to cut down many pieces of scrap into smaller pieces.
You will be astonished at how many chunks of solid aluminum scrap it takes to fill a crucible with molten metal. So before you begin have it all cut and ready to feed the melt. You also need to keep your charcoal fuel close at hand in a covered container. I had a cinder ignite my entire barrel of charcoal one night. You will have to re-fuel the forge quite often, it is also important to have your fire tools close at hand also. I rest mine on a metal table next to the forge, along with my melting scrap, and keep my fuel under the table. Attach your air supply to the air pipe on your forge, I place a small piece of plywood over the air supply to keep it from sucking in embers and ash. Make sure there is a flat level spot next to your forge, a cinder block or brick can be used also. This is to set the crucible on to re-fuel. With a pick or shovel cut some small 1″ x 4″ trenches in the dirt near the forge, make sure the trenches are away from your main working area. This is where you will dump any leftover aluminum after the pour. You next will fill your crucible with scrap, for these starter pieces, make sure none protrude out of the crucible, get the crucible as full as you can. Fuel the forge with charcoal, fill it at least halfway up, also put your forge lid into place.
Now for the last safety step, you need to do a practice run for your upcoming pour. Go and get your mold and set it down carefully near the forge. Get your hooks, one in each hand, and practice lifting it, practice attaching the second hook to the lower manipulation ring, and practice pulling the bottom up on your crucible for the pour. What we are doing is getting your orientation right, the orientation of your body to the forge, crucible, hooks, and mold. Figure how you are going to move, where you are going to stand, and where everything needs to be located when you move the molten metal from the forge to the mold, for the pour. Make adjustments until you are comfortable with the location of everything and your movements are smooth and fluid. You can leave the mold in place if you like, but put a piece of plywood over it. If it is going to get in the way, then put it up, but remember how and where it needs to be placed. Move the mold slowly and carefully!
Don’t turn on the air supply just yet, I squirt a little lighter fluid over the charcoal and then drop in a lit match. Let the charcoal catch for a bit before turning on the air supply. Take your crucible and set it in the forge on top of the charcoal. Things will begin to heat up, I like to do the actual melting at night, the darkness allows you to see what is going on in the forge and crucible. Have patience it takes some time for the concrete of the forge to heat up, you will not melt any metal until this concrete is hot and refracting heat.
Let the fire burn for a time and then grab your skimmer, I run the skimmer down into the charcoal bed to gauge how much has burned off. If you see a large quantity of ash, then its time to re-fuel. You must turn off the air supply before re-fueling. At first when I re-fueled I just dumped the charcoal into the forge, this was difficult because much of it would wind up in the crucible, too much. When I re-fuel now, I first turn off the air supply, then I grab one of my hooks and carefully lift the crucible out, and place it on the level spot you made earlier. A minute or two out of the fire will not make a big difference. Then I scoop out several handfuls of charcoal with my gloved hands, and drop it into the forge. I then take my hook and lift the crucible and set it back atop the new fuel and turn on the air supply. This also gives you further practice in manipulating a hot crucible. The metal is not molten yet and is still relatively light. It is later on that great care must be taken.
I need to cover why you are resting the crucible on an unsteady bed of charcoal. On one of my earlier melts I rested the crucible on the forge bottom and just kept fueling around it. The aluminum heated up and began to melt then it re-solidified, and no matter how much fuel I kept burning around it, it would not melt. After an hour of this, I took the crucible out and looked at it, the sides were orange hot, but the bottom was not glowing at all. It is crucial that the bottom of the crucible be brought up to a high temperature, if not, there will be no melt. I welded up a little table for the crucible to rest on while in the forge, at first it worked, but when ash built up I had the same problem as before. You could also scoop out the ash at intervals during the burn, but you will be losing heat and wasting fuel. A 3″ or 4″ diameter pipe that is 8″ tall will not fall over in its 8″ diameter forge. It may shift some and later it may spill some aluminum, but a small aluminum spill in the forge is neither dangerous or explosive, and the spilled aluminum is easily removed and recovered once the forge has fully cooled. My very first attempt at a crucible was a chopped down empty propane cylinder. I got the aluminum to melt, then the bottom of the propane cylinder burned out, and the entire charge of aluminum went into the forge. I did not even notice it, one second it was full of aluminum, the next second it was empty.
After your third or fourth re-fuel, take a good look at your crucible, gently push down on the aluminum with your skimmer. If it gives at all then it has begun to melt, re-fuel and keep the heat on it. You will see a layer of charcoal and ash beginning to form on the aluminum. This is not a concern and the ash layer actually protects the aluminum, allowing gasses to burn off while keeping other unwanted gasses from entering the melt. Before long your charge of aluminum will have melted, you will see just a small amount of silver molten metal in the crucible covered by a layer of ash. A filled to the top crucible now has less than an inch of molten metal. You have reached melting temperature and the pace is going to quicken, the walls of your forge are now very hot and refracting. This is the fun part, when this happens I find that time flies by, and all your concentration is on the melt. Re-fuel and begin feeding aluminum into the melt, when the crucible is taken out, the entire pipe is orange hot. It will stay orange hot while you are fueling. I only feed the melt when it is in the forge. If you are fortunate enough to have the long aluminum bars, simply put it in the crucible vertically and hang on to the cool end with your tongs. It will quickly melt into the crucible. If you are using broken bits of scrap, only feed them to the melt with the tongs. The temptation to put them in with gloved hands can get you burnt in a hurry. Mind your fire tools, the hook you just used to remove the crucible is still very hot when you set it down to grab your skimmer. I got branded one night for not minding my fire tools.
Continue re-fueling and feeding the melt, when the crucible is half way filled with molten metal, you need to skim out the charcoal and ash layer, otherwise known as dross. First take your skimmer and stick it into the charcoal, get it red hot, this prevents the aluminum from sticking to the skimmer. Now work the skimmer washer across the dross layer and pull up as much dross as you can. Some aluminum will come out with the dross, don’t worry about it. Pull the skimmer out of the crucible and forge, and rap it against the metal side of the forge towards the bottom. The dross and captured aluminum will drop off onto the ground and can be collected and disposed of later when everything has cooled. Repeat until the molten aluminum is mostly cleaned off. You do NOT want to stir the molten metal, you just want to get the trash off the surface. If you stir the metal it can capture air and gasses you don’t want in the melt. With the melt cleaned off, have a look at it, you will see the quicksilver of the molten metal but aluminum also has an orange aura around it. Even when it is out of the crucible, but still molten, it will have this orange glow around it. Continue fueling the fire and feeding the mold until the melt is nearing the top of the crucible. It is up to you to judge the amount of metal you need to fill a mold. And you get better at this each time you do it. For the 6″ X 6″ sphere you will need the crucible to be pretty full. It is not just the pattern cavity that needs to be filled, but the sprue, riser, filter trench, and channels as well. It is far better to have too much metal than not enough.
With your crucible nearly full, I like to carefully take it out of the forge, you should be pretty good at this by now, for one last re-fuel. Put it back into the forge and turn the air supply back on. This last heat will bring the impurities to the surface. Be sure to orient the crucible so its lower manipulation ring can be accessed from the standing position you practiced earlier. Get your mold and tools ready it is almost time to pour. After letting the final heat work its magic for several minutes, turn the air supply off, and repeat the process of skimming the dross from the metal. Hooks in hand get into your pouring position, lift the crucible out, it will be heavy so exercise caution. Attach your second hook to the lower ring and move the crucible over the mold. Your target is the funneled out sprue hole. Slowly raise the bottom of the crucible and let the metal run into the sprue. Do not stop pouring for any reason, a brief interruption can cause the metal to solidify and wreck the casting. Slow and steady, slow and steady! While pouring, keep your eye on the riser, when you see the aluminum come up to the surface of the riser, you are finished. Stop the pour and move the crucible over to the trenches you cut in the dirt earlier. Pour out any remaining metal into the trenches. You can use an ingot mold instead of trenches for this as well. You need to get all the remaining metal and trash out of the crucible before it cools. Once I have poured off the remaining metal I will hold the crucible with both hooks, bottom up, and tap the mouth of the crucible on the ground several times. This does a good job of cleaning it out for the next melt.
| Look at your mold, if you see aluminum bubbling out of the sprue, your sand was too wet and not properly burned out. Chances are the casting is blown, but this is not always the case, so don’t lose all hope. Once in a while you get a poor fitting between the sand in the cope and the sand in the drag. When this happens aluminum will briefly come out from the seams. It will burn the wood and smell terrible but don’t touch it, just let it cool, and once again, this may not necessarily ruin the casting. If you see just a little steam and water coming out of the vent holes then chances are the casting is a good one. But there is only one way to find out for sure, open the mold and have a look. Keep in mind that it is still very hot inside the mold. I wait 20 minutes before breaking the mold open. Even after 20 minutes you can get a steam burn so wear gloves. While you are waiting clean up your mess, put your tools and air supply away. The forge will stay hot for 12 to 15 hours depending on the outside temperature, so don’t worry about cleaning it up until the next day. The crucible will stay hot for a while, just let it cool.
In the next installment, I will cover: breaking the mold, possible disappointment, and cleaning up the casting.