This area will be devoted to sharing tips and techniques in sandcasting and lostwax casting operations. Your contributions are welcome.
The methods and materials involved in any form of metalcasting operation are VERY hazardous. Educate yourself on the proper safety precautions before attempting any metalcasting operations.
Things to keep in mind ...
With molten metal, WATER IS AN EXPLOSIVE! If you took physics in high school you may know that materials expand when heated. They will expand even more when you change from one state to another ie. liquid to gas or solid to liquid. For the techquies water expands 1600 times its original volume as a liquid to a gas. For a normal person that means one cup of water with expand to 1600 cups of water when it goes from 211 F to 213 F in an instant. For the rest of us that means one thing, BOOM!
I remember the first time I watched a green sand pour in a commercial foundry. Little explosions everywhere and you either learn to relax or you went and found another line of work.
Winston's note
As a frame of reference for things I say, it will help to know the equipment and materials I use. For the last 15 years, I have used an oil-bonded sand system; before that, I used a water-bonded sand. For core sand, I use a sodium silicate binder, gassed with carbon dioxide. I use a Simpson wheel muller to recycle the casting sand. My main melting furnaces are propane-air fired. Crucibles for all metals are silicon carbide, the largest being a #10 (30 pounds brass maximum melt). Primary metal alloys cast are silicon bronze, gunmetal bronze, yellow brass (leaded), white brass and various aluminum alloys.
To get good castings on a consistent basis, you should use foundry ingots of known composition. This is not always possible for hobby casters, so don't expect high quality castings if you are using scrap as feedstock. This is really important with copper alloys; if some alloys are mixed in a melt, total junk is cast (voids, hard spots, slag inclusions, etc). With aluminum alloys, things are a bit more forgiving; rule-of-thumb, if it was cast, you can recast it. For beginning casters, I suggest starting with aluminum alloys, move up to a silicon bronze, then to tin-bronzes and yellow brass (yellow brass is NOT for new guys).
Extruded aluminum products do not make good castings. Aluminum casting alloys contain copper or silicon as alloying agents and there isn't enough of these in extrusions. You can make a decent casting alloy if you have a source for extruded aluminum ... melt the aluminum, then add 8% fine copper scrap to the melted aluminum; the molten Al will dissolve the copper very quickly and yield a good alloy which machines nicely. Do a two-step procedure when making your own alloy ... make the alloy on the first step, pig-out, remelt and use this melt for the casting pour; this gets rid of dross and yields a more uniform alloy.
When dealing with aluminum scrap, be careful that you don't get any pure magnesium mixed in. Magnesium added to molten aluminum in a gas fired furnace can result in one hell of a fire. About the only way to put out a magnesium fire is with LARGE amounts of dry sand. This happened to me once and I know a couple other casters it has happened to.
Aluminum alloys and silicon bronze alloys change very little in composition during the melt ... little or none of the alloying agents are lost; you can recycle these materials and get consistent castings. Alloys containing high levels of zinc (brasses) change considerably during a melt due to zinc lose; to recycle these alloys, you must add back the lost zinc. I do a lot of yellow brass work; my basic procedure is to use half new ingot and half recycle in a melt.
Always use clean metal in a melt. Remove all grease, oil, paint, etc before placing in the crucible. Many lubricants and paints will give off toxic substances when they burn. I sandblast all scrap/recycle before melting.
Use silicon carbide crucibles where possible, no matter what alloy you are working with; they have a longer life than clay-graphite pots and are worth the extra cost.
Use a different crucible for each alloy you are melting. NEVER use the same pot for different copper alloys since cross-contamination can ruin the whole melt.
If you are a single-man operation, I don't recomend using anything bigger than a #10 crucible. With a single-man pouring shank, this is about 40 pounds of weight of molten metal in a big spoon. You may be able to handle more weight than this but you can not control it and you want to have TOTAL CONTROL of molten metal at all times.
Have a "sit-out" metal bucket with an inner refractory material in the bottom for storing hot crucibles after a pour. It is easy to touch or knock over an empty pot after a pour since you are usually busy.
Always put a piece of heavy cardboard under the crucible in the furnace before a melt. At high temperatures, the glaze on the outside of the pot will soften and the crucible may stick to the bottom of the furnace or burner block. The burned cardboard will leave a deposit of carbon which will prevent this sticking.
Don't melt too much in crucible ... leave at least one inch of free space. On the other hand, melt a fairly full pot with each fire ... if you are using a flux or doing lots of high temperature bronze work, the inner wall will gradually become less with each melt; if you melt varying amounts with each fire, rings of different wall thickness will result giving stresses in the crucible.
Alway pre-fire a new crucible empty the first time. Take it to full red heat and allow to cool. Do a "ring test" on a crucible before each melt ... with the crucible empty, tap it gently with a metal object and it should give a clear "ring". A dead sound indicates a possible cracked crucible ... discard it. Test your new crucibles and you will develop a "feel" for how they should sound. I have gone through several cases of crucibles over the years and three have broken during a melt or pour ... never leave a furnace unattended during a melt and always wear heavy leather safety equipment during a pour. I had one pot develop a pin-hole leak about half way up the side during a bronze pour ... if I hadn't had on a heavy leather apron, the pissing stream of molten bronze would have done away with my pisser.
Pack the metal charge in the crucible loosely before a melt, allowing room for expansion. A tightly packed pot can crack as the metal expands on heating. When you have a molten metal heel, add subsequent additional metal gradually, in small portions.
Keep a logbook or notes on each pour you make. This information should contain the following info at a minimum: pattern description, alloy cast, pour temperature/time of superheat, setup notes, results of pour. This will help you learn quicker and act as a reference for future patterns of similar size and shape. I not only maintain a daily logbook but I also have a "cookbook" for each pattern I cast, containing all of the setup parameters and special notes regarding a pattern. I also take 35mm pictures or VHS videos of special setups. (It is my intent to edit some of these videos and make a movie for beginning casters; this video will be for sale or rent. It may be a while before I find time to put this together, so watch this space).
I have built all of my own melting furnaces through the years; all have been propane-air fired. They ranged from one gallon to 30 gallon steel drum shells in size. Their basic design is about the same as outlined by Bill Ammen and Dave Gingery in their various foundry books.
If you make your own furnace, one note here on the castable refractory. Commercial castable refractories come in various densities and temperature ranges. I prefer the lightweight refractories (40 to 55 lb/ft3); they make a quieter running furnace which is easier to move around; the high density refractories last longer in use but the furnaces are extremely loud in operation).
There are some real bargains in used school foundry furnaces and equipment. As more and more schools delete foundry classes, these items become available at bidder prices. In my local area, three of the hobby casters I am presently helping get started bought their furnaces and equipment at school auctions in the last year; none paid more than $200 for complete setups.
Few hobby foundry folks can afford or justify expensive pyrometers to measure the temperature of the molten metal before a pour. I have both electronic pyrometers and optical pyrometers. Several years ago, I got tired of buying the expensive thermocouples (they don't last long with molten bronze). I am as near-sighted as a fence post and I wear trifocal glasses, therefore reading an optical pyrometer became almost impossible. Since I operate the furnace at the same conditions everytime (air flow, length of flame feather coming out of the exhaust, sound, etc), I reasoned that the temperature should be fairly constant. I checked this with an electronic pyrometer at time intervals after all of the metal in the crucible had visually melted. The results of several checks showed an almost straight line plot of time after final melt vs temperature of the molten metal. For the last 12 years, I have been using this empirical method of determining the temperature of a heat. Depending on the size and shape of the pattern, I will superheat brasses and bronzes from 1 to 12 minutes after final melt; for aluminum alloys, I superheat from 30 seconds to 2 minutes. (This is based on using a #8 crucible and my furnace setup ... your mileage may vary but it gives you a place to start experimenting ... KEEP NOTES).