This project requires some knowledge of the casting process and also access to casting equipment. I wouldn’t recommend going to the expense of buying all the equipment until you know this is a process you enjoy and want to continue to explore.
A working knowledge of wax carving, hand tools for finishing, polishing equipment, comfort using a torch are needed. For the sake of simplicity, this project focuses only on the casting process. You will need to carve a wax and do the finish work yourself.
Think safety first! Work in a well-ventilated area and wear appropriate clothing: nothing baggy, loose fitting, or highly flammable. When working with investment, casting, and polishing, tie your hair back and wear a shop apron, safety glasses, and a dust mask. This is quite a fashion statement. Keep a fully charged fire extinguisher mounted within easy reach of the casting station. Use common sense and think the process through before starting.
If casting different metals and alloys, a separate crucible and stir stick for each will be needed. Otherwise the melt can become contaminated, affecting the quality of the alloys.
To cure a crucible, start by filling 1/3 of the crucible with powdered borax, place it in a kiln, and heat it until the borax melts. Wearing safety glasses and a heatproof glove, remove the crucible using casting tongs. Move the crucible around until the inside is well coated with flux. Do this over the centrifuge tub so that any spills are contained and can be easily cleaned when cooled. Remember, it’s hot. If the flux gets hard too soon, put the crucible back into the kiln and heat it again until the flux melts.
To prep your sprue base, use the wax pen or alcohol lamp to melt sprue wax into the center cavity until it’s filled. Let it cool. Continue to build up a smooth cone shape with more sprue wax and coat the surface with a layer of sticky wax.
Spruing the wax model is a major key to successful casting. The sprues must be large enough and placed in such a manner as to carry the molten metal easily into the mold cavity with as little turbulence as possible.
Using the wax pen, melt a small amount of sticky wax and dab it onto the heaviest area of the wax model. Attach 1/2″ length of 6 gauge sprue wax. Make sure the seam is smooth and clean with no bubbles trapped inside. Select the temperature that is most appropriate to the wax being used. This is determined by experimenting with the wax and wax pen. Some wax manufactures will supply the melting temperature, although most wax pens don’t have a temperature gauge. If it is too cold, the sprue won’t stick or seal properly; too hot and the sprue may flop over, cracking the seam, or the wax may burn leaving black or brown residue, causing porosity in the cast. For this project, use 2 main sprues and because there is a narrow area between the nose and tail, also attach a small auxiliary sprue between them on the inside. These will all be removed upon finishing.
Once all the sprues are attached, weigh the wax. Write the weight down because you will need it later for the metal weight.
Attach the model with sprues to the base. If there is more than 1 piece, the models can be temporarily attached. Don’t use the wax pen. Just put the models in place leaving ample room between them, keeping in mind they will be sawed off once cast. When you have worked out the best pattern for a successful cast, use the wax pen to seal the attachments. Remember that metal will not flow backwards. Any area below the sprue attachment won’t fill.
When everything is attached and cool, hold the base at eye level to check for possible problem areas, tight spots, back flow, bubbles, poor sprue attachments, or melted spots on the model where it was accidentally hit with the wax pen. Fix those things now. The quality of the wax model is important, and a clean wax means less metal cleanup. I actually polish my waxes to a shiny finish.
Put the flask securely in the rubber base making sure there is at least 1/4″ of clearance between the model and the top of the flask. If there is more than 1 flask, it is very helpful to mark the type of metal and total weight of each with a light-colored pencil (model weight + sprue + button i.e.: sterling 20 dwt.) That way you don’t confuse the flasks when it’s time to cast. This will be explained again in STEP 9.
I use distilled water at room temperature to invest the mold. If you want to use tap water, put it in a covered container and let it sit for a couple of days before using to get rid of air bubbles. The investment should be at room temperature as well.
Always wear a dust mask, safety glasses, and apron when measuring and mixing investment. Make sure the tools are clean and free of dust and old investment debris. Follow supplier directions for the water/investment ratio (Ransom & Randolph is 40/100; for a 2.5″ x 2.5″ flask, that’s 3 fl. oz. water/9 oz. powder). Always pour powder gently into water and mix thoroughly. You will have about 8 minutes of working time, so stay focused. If the slurry is too thin (looks like heavy cream), add a tad more powder. If it’s too thick (clumps like sour cream), add a tad more water and mix well make sure that no lumps remain.
When it is time to vacuum the flask, place it under the bell jar, making sure the rim is sealed. Hold it at 28lbs. for about 2 minutes. Every machine is different, so look for the investment to rise and then fall. I tap the base of my machine a few times for good measure.
Turn the machine off at the same time that you release the pressure valve. Hold the flask at an angle to gently pour slurry onto the inside of the flask to fill it from the bottom up. Stop when the model is barely covered, leaving room for the slurry to expand again. Repeat the vacuuming process. Fill flask evenly to the brim and don’t move it for at least 1 hour. I generally do my investing at night so the flask can go right in the oven first thing in the morning after it has had several hours to cure. Since I live in a fairly humid environment, this works well.
The centrifuge should be balanced before putting the flask in the oven. Remove the rubber base and gently place the flask in the sleeve where it normally sits, push the crucible up to it, and loosen the center screw. Gently hold the swing arm straight. If a side drops heavily, adjust the weights accordingly until a more balanced position is achieved. Tighten the center screw. Don’t forget to put the flask in the oven and start the burnout.
Turn the vent hood on and leave it running during the entire burnout cycle. Place the flask in the oven with the sprue cavity facing down. Cure the flask slowly.
Set the oven to heat up and hold at 400°F for at least an hour, 750°F for an hour, 1000°F for an hour, and hold the temperature at 1300°F for an hour. Then turn the oven down, drop and hold the temperature at 900°F for an hour. It takes my oven about 20 minutes to drop to 900°. Because my oven and centrifuge are some distance apart (in separate rooms), I hold it at 900°F for 30 minutes instead of an hour. I also do a quick visual check of the inside of the oven at 1300° and another when it’s cooling from 1000°. Sometimes the pyrometer sticks, so the visual is a double check. Even with a computerized system, it helps to do a visual. While the flask is burning out, weigh the metal and set up the centrifuge.
Find the paper on which you wrote the weight of the wax model (including the sprue). To get the total metal weight, multiply the weight of the wax model by 10.6 (SS) and add 15 to 20 dwt. for the sprue bottom (e.g., 0.7x 10.6=7.42+ 15=22.41).
Metal conversions: sterling silver = 10.6; 14K gold = 13.6; 18K gold = 16. To convert pennyweights (dwt.) to grams, multiply by 1.555. To convert grams to dwts., multiply by 0.643.
Remember to use at least 50% fresh metal with each casting. If reusing old sprues, make sure they are clean and free of investment and flux. If using scrap, make sure it is solder free. As suggested earlier, the total weight can be written on each flask before it goes in the oven. This will prevent confusion if there is more than 1 flask.
To setup the centrifuge, wind the arm clockwise 2 1/2-3 revolutions. Set the hold pin. Put the metal in the crucible and check the oven temperature. When it’s time to cast, it is helpful to have a buddy who can bring the hot flask and place it in the sleeve while you are melting the metal. That way the flask won’t cool down too much before the melt is ready.
For casting, set the propane to 8-10 psi and the oxygen somewhat lower 5-7 psi. Use a big bushy “reducing” flame, which has a lot of gas and just enough oxygen to get rid of any yellow color in the flame.
If it’s hissing loudly, then there’s too much oxygen and the oxygen needs to be reduced slightly.
Heat the metal and the whole interior of the crucible at the same time. Rotate the flame around evenly. Don’t be in a hurry. The metal will melt. Make sure to heat all the way into the crucible orifice. If the orifice is too cool, a lot of metal will stick to it on the way through. As the metal melts, it will start to flow together. At this point, your buddy should bring the flask and set it in the centrifuge sleeve. Using the stir rod in your gloved left hand, push the crucible forward (don’t take the flame off it, keep rotating), locking the crucible and flask together. Breathe!
The metal will look melted before it truly is. When the metal is shiny and has pooled together, use the stir rod. Gently drag it through the melt, from the front of the crucible towards you. The rod does several things: it mixes the metal, cleans the melt and indicates when the metal is at flow temperature. Continue dragging until the metal surface becomes shiny, swirling, and is fluid throughout. Quickly set the rod on a heatproof surface without removing the flame. At the same time, pull the arm back to drop the pin, remove the flame, and let go!
Let the machine spin down by itself until it stops. While you wait for it to stop spinning, turn off the tanks, drain the hoses, and back off the regulators. Tidy up the workspace, and then put the #1 blade in the saw frame. Wait 10-20 minutes before quenching the flask in the 5-gallon bucket of water. Use an old toothbrush to scrub plaster off the casting. I also use an old dental tool to pick out hard-to-reach places. Do this cleaning over a bucket, not the sink, and remove as much excess as possible. Place the whole thing in the pickle pot for a while to remove oxidation and flux.
Before cutting off the sprue, critique the casting. Look for any problem signs: flashing on the button rim means the metal was too hot. Flashing or water marks on the ring mean the slurry was too thin and/or the flask was cured too quickly. Obvious porosity may be the result of bad spruing or placement, a bad melt, air bubbles in the wax model, or small bits of plaster carried in with the metal from a dirty sprue button. If it’s not too serious, the area can be drilled and soldered, or wire can be fused into it when finishing. Cut the sprue off as close to the ring as possible and complete the finishing process. It’s not unusual to have some minute surface porosity that is easily removed by filing and sanding.
However, if it continues through the entire piece (more sanding revealing more porosity), consider the flask and/or metal temperature too hot. Learn from every casting. Be patient, stay focused, and have fun.
Lapidary Journal, February 2001