9. On Investment Casting, Part 2



When we left off, we had sprued the positive form into place and assembled the flask in preparation for the investment - the surrounding with mold material - that gives the method its name. Mix the plaster according to recipes that can be found elsewhere, making sure to be precise in weighing out the plaster mix and the water both in the correct ratios and as a function of the flask volume, mix it thoroughly, and then use a vacuum chamber to degas it as well as possible. Fill the flask thoroughly, avoiding reintroducing air into the wet plaster, leaving voids, or damaging the positive or its attachment to the sprues. Gently but firmly bang the table to ensure that as many trapped air bubbles as possible separate from the positive and the sprues and float to the top of the flask, and fill with leftover plaster if necessary.

At this point you will need to wait 2~4 days for the plaster to cure preliminarily and be firm but damp to the touch, and can wrap the plaster to prevent further moisture loss. As a best practice, use a sharp marking tool to lightly carve things like your name, the intended metal, and the nature of the positive into the plaster. Whether or not you choose to do this, you can remove the rubber bottom in preparation for the burnout; the plaster will be hard enough to retain the wax and cause the top of the sprue to break as necessary.

Unfortunately, I don’t know much about the burnout, the step where you put the flask containing the cured mold material surrounding the wax, but I know it to require a fairly powerful kiln of the type suitable for making ceramic pottery, and that it takes around 6~8 hours to bring the kiln to temperature and burn out the positive. From a quick search online it looks like the target temperature is 1350 *F which must be reached slowly over about 3 hours so as to avoid cracking the plaster. When loading the kiln, though, it’s important to position the flasks with the

About an hour before you intend to pour, fill a small graphite crucible with an appropriate amount of metal for your cast and insert it into an induction furnace; the desktop furnace I used was roughly cylindrical and used a compatible cylindrical crucible of standard size. You will likely want a graphite stir rod as well. Set the furnace to somewhat above the liquidus/melting point of the metal or alloy, and not the solidus point: remember, we want for this metal to flow as freely as possible, and it will begin to cool and congeal the instant you take it away from the heat. In addition to the casting metal, add a 1983~2025 penny for its zinc content; this will help by preferentially absorbing oxygen from the air, rather than have your casting metal absorb it. I have heard the guideline that one should use at most half old casting scrap and the rest new casting grain, but I am doubtful of how true this is, especially for silver or gold, which are less reactive. My results have varied trying it both ways. Wait until the metal has come to temperature and is totally liquid to inspection with the stir rod.

When I did investment casting, Pumping Station One also had a vacuum casting table for improved draw-through of molten metal, so before the cast, one needed to begin by laying a silicone mat over the vacuum interface and activating the vacuum. In any case, the next step is the pour itself, the most hazardous part of the cast. This is for several reasons. First, it’s the part where molten metal leaves the crucible, and thus where spillage is possible. Additionally the faster you complete the pour, the better your outcomes are likely to be: after all, the metal will start cooling the instant it touches the comparatively cold plaster. As such, your use of tongs should be as deft and quick as possible while still being safe. Finally, if the plaster fails, molten metal will spill out onto the countertop. For all these reasons it’s all but obligatory to wear appropriate protective gear including gloves, an apron, and a face shield, and to refrain from wearing synthetic fabrics, which might melt on you. Using the tongs, first manipulate the flask inside the kiln to put the flat side down and the funneled side up. You may want to have someone assist you for this next part (and usually AA or DLS helped with this part, or I helped with it while they did flask manipulation): using a different pair of (purpose-shaped) tongs, secure the crucible of molten metal and prepare to pour. Now, as briskly as possible, put the flask flat side down on the countertop or active vacuum table and then pour in the molten metal until the flask fills completely, including the funnel portion; the sprue tunnels inside will fill with molten metal and are still called sprue, and the roughly conical funnel part will form what’s called the button. If the plaster or flask might fail, this is when: it’s important to avoid being splashed with the hot metal and to turn off the vacuum if this occurs. Allow the hot flask to cool off until a dull orange, at which point you can turn off any vacuum pump used and use the tongs to transfer it to a countertop to further cool. If you have additional flasks, repeat the process, changing crucibles for different metals as necessary. Best practice is to have one crucible per relatively similar class of metals or alloys: when I was involved in casts, we had one for copper and bronze, one for brass, and one for fine, sterling, and argentium silver.

As the last major step, wait for the metal to cool to a dim red only visible with lights off. At that point, use the tongs one last time to plunge the flask into a bucket of water to quench the remaining heat, taking care to hold the hot flask well below the surface to avoid splashing, and keeping clear of the top of the bucket to avoid steam burns. At some point, the bubbling will die down and a distinct clunk will be felt: this is the feeling of enough of the plaster having been destroyed to free the finished piece, which can be removed from the warm water for inspection after the bubbling stops; hopefully the metal has successfully filled the entire mold. Bolt cutters or a hacksaw will likely be necessary to separate the finished piece from the main sprue and button, but snips are generally sufficient to remove smaller sprue. The casting scrap, sprue, and button can all be melted back down, possibly immediately if necessary; I considered it best practice to do so and to have a small rough ingot mold on hand. At this point, standard pickling, polishing, and other fining techniques can be employed. You now have a beautiful or useful piece of metal of the desired form!


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