Procedures For Preparing A Wax Cast In Jewelry & Sculptures

The pictured piece is my first project for the Fall 2010 semester in my Metalwork & Jewelry Beginner II class at Montclair State University (Montclair, NJ). We're learning wax casting.

Creating A Wax Casting Model

As you may recall, I made a piece last semester with a cuttlebone cast. In that procedure, you scrape away the areas you want to fill with metal from the cuttlebone. In wax casting, you create your piece entirely of wax -- there are three kinds we use in class: wire wax, which are thin pieces resembling wire; block wax, which is a solid piece of wax, either in a rectangular shape or you can also get it with bracelet and ring cut-out sections pre-made; and sheet wax, which are thin layers of wax. We are required to use at least two out of three in our early projects. This ring was created in block wax with a precut finger hole in it. I melted a solid piece of the wax, and then turned it around in my fingers to create the "dripping" effect. The more I melted it, the more it took on a warped shape. When I finished, I realized I had something resembling a heart. I added more block wax to create the ring shank -- the part that goes around your finger. The pieces of wax are adhered to each other using small dental-like tools and heat. We use alcohol burners at school -- I have also used a candle at home -- and hold the tool over the heat until it gets hot enough to melt the wax. We slide the tool between the two pieces of wax to be affixed, and hold them in place as the melted wax hardens. This happens fairly quickly, especially with smaller and thinner pieces of wax. The good thing about this technique is that you can "rebuild" a section of your piece if it melts away or doesn't look the way you want. Once the heart was affixed to the shank, I had to carve out some of the wax from behind the heart to make the piece lighter. If you fill a solid area with metal, it will be approximately 10 times the weight of the wax figure. That could be a lot of metal! If you're using gold, the weight is approximately the amount of the karat weight times the wax. For example, if you're using 10K gold, it will be 10 times the weight of the wax piece...if you are using 14K gold, it will be 14 times, etc. Carving out the back of the heart was how it got its "battered" look. In some cases, my heated tool broke through to the front of the wax. I kind of liked the effect, so I left it that way and oxidized the piece later to darken the spaces. I also had to file the inside of the shank to make it fit my pinky. The piece of wax I'd used to put the shank on was a spare piece I'd been playing with earlier, but it was rather small. I figured this was my first attempt, so I wasn't too concerned with the fit...I was more concerned that the piece came out correctly! You can also use power tools to shape the wax. We use a flex-shaft at the studio on campus, and I use a Dremel® 300 at home. Just be careful when using the power tools, as they heat up rather quickly and will melt the wax more than or differently than you intended! And don’t forget your safety glasses, as you may end up with melted wax squirting up toward your eyes!

Spruing Your Wax Casting Model

Once you have your wax model the way you want it, you have to attach wax sprues, which are thick rod-like pieces of wax that will create the passage for the metal to fill the cast. Often only one is required, but that depends on the size and configuration of the piece. The sprue(s) should to be attached to the area of the piece that will require the most metal. If you have more than one area that is dense, and you think the main sprue will not enable the metal to reach those points sufficiently, affix multiple sprues to those other areas, having them branch out of a main central sprue. Be sure the sprues are positioned so that all of the wax in your model is above the top end of a sprue, meaning that the point where the sprue attaches to the model should be positioned as the lowest point of the model. Metal can only flow in one direction; the metal is melted in a crucible with Borax, and then it is spun into the cast, which is set horizontally in a centrifuge. If you think about the metal flowing into the horizontal cast through the canal created by the sprue, you realize that any openings in the investment behind where the metal flows into it will remain empty. The force of the centrifuge will propel the metal forward only, not backward.

Once you have attached the sprue(s) -- make sure you have a good bond, so that the model stays in place -- you have to weigh your wax model to calculate how much metal will be required to cast it. Keep notes, so you will have an easier time remembering the figures (I got scolded for that recently!)...BTW, the measurements we use at school are grams, but you can use any measurement as long as you are consistent between the wax and metal weights. The scale used must be calibrated before weighing the wax and then again before weighing the metal. Add an additional 25 to 40 percent of the wax weight to account for the sprue and a button. The easiest way to do this is to multiply the wax weight times 1.25 or 1.40. A miscalculation could ruin your whole piece! A 0.1 gram error in wax is minimal, but the smaller the piece, the higher the error rate. And as with the cuttlebone cast, you only get one shot at this!

Once you have the weight figured out, you attach the bottom of the sprue to the bottom of a casting flask with more wax. The bottom is a round piece of rubber with shallow sides and a bump in the center of it...this "bump" will create a button of metal at the end of the sprue canal. This excess metal will be cut off the finished piece, but it is necessary to ensure the entire cast has been filled with metal. You can reuse it in another piece, though, so you are better off allocating more metal than less in your calculations. The sprue is adhered to the center "bump" with wax. Again, make sure you have a good bond, so that the model stays in place. Your piece will now be standing upright or at an angle -- depending on how it is configured -- kind of like it is sprouting out of the center of the rubber piece. You can also place multiple pieces in the same flask for investing, which is more economical. Naturally, all of the pieces will be cast in the same one kind of metal, as you can’t cast different metals within one mold. Be sure the pieces are not touching (a minimum of 1/8-inch apart is optimal), or they will be merged with metal, as the wax models creating the form will not be separated by investment. Investment is a plaster-like substance that is added to the flask to create the cast. Also, be sure the angles are such that all of the pieces will receive metal...meaning that none of them can be leaning over and dipping below the top end of its sprue. There should be room for at least a 1/4-inch of investment on the sides and the top when covering the piece. Thinner walls of investment may cause the metal to burst through them, ruining the model and your piece.

Investing Your Wax Cast

Once your piece is ready, place the top of the flask over the rubber bottom. The flask is a cylindrical piece of metal that creates the walls for your cast. It fits snugly within the shallow rubber walls of the base. Spray de-bubble on the wax piece to avoid surface tension, so that the investment won't bead on the piece. Any bubbles in the investment will create weak points in the mold. Use a rubber collar around the top of the cast, so you can fill it all the way to the top and scrape the excess investment off to create a flat surface for the bottom of the cast. You are really creating the cast upside down in this procedure, as the area where the button and sprue are will be the top/front of the cast where the metal will enter it. Once you have the flask all set up, you can measure out your investment powder and water. Investment is hydroscopic, meaning that it absorbs water, so the source container must be kept covered to keep it dry. There is a formula to use for the correct amounts of each, based on the diameter and height of the flask you are using. We have a chart in the studio, but I'm sure investment comes with one when you purchase it. The weight of the powder is measured in grams, and the amount of water is measured in milliliters. Pour the water into a large rubber bowl and have a spatula nearby for mixing. Once you add the powder to the water -- you always add the powder to the water, and not the other way around -- you have approximately seven (7) minutes to complete the procedure before the investment starts to harden. Add the powder to the water in the rubber bowl and mix it thoroughly. It should be creamy and thick, with no lumps that can cause weak spots in the cast. Place the rubber bowl in a vacuum machine to make the bubbles rise to the surface. This should take about 23 to 25 seconds according to our instructor, but the textbook we use said it could take as long as 1-1/2 minutes. I’ve personally experienced it taking a bit longer. Take the bowl out of the vacuum machine and quickly pour it into your flask at an angle. Try not to pour it directly onto the model, sprues, or wax foundation, as it could loosen the model and knock it over. Pour it down the side like pouring a glass of beer (my teacher's analogy, not mine! LOL). Once you've got the flask almost full -- not completely full, since the investment will overflow in the next step if you do -- put the flask in the vacuum machine get all of the air bubbles out of the mixture again. After another 23 seconds to 1-1/2 minutes, take the flask out and top off the mixture to the very top edge of the flask. If using a rubber collar, you can overshoot it a bit without making a mess. Just be sure to have a clean/flat surface remaining when you take the collar off. Use a knife along the top edge of the flask to smooth out the investment. Let the investment set until it hardens -- as I said earlier, approximately seven (7) minutes total. The investment turns a different shade of white and the edges kind of turn brownish or grayish, so you can usually tell when it is hard enough. This is caused by the water in the mixture evaporating, and is called "glossing off." If you're not sure, touch it with your finger. If no investment sticks to your skin, it's good to go.

Burning Out Your Wax Cast

If you are not burning out the cast yourself, be sure to carve your name or initials in the flat surface, as well as the weight in wax or your chosen metal (two teachers in our department have differing preferences, so we received conflicting instructions). Be sure to indicate which weight you are using, so that you will know whether or not to make calculations when weighing out your metal later! Take the bottom rubber piece off the flask, and you should see a domed convex opening in the center of the cast, created by the "bump" in the rubber. You may also see some residual wax, but this will be eliminated once the cast is burned out. This procedure will create a mold of your wax model. If you are not burning out your cast that same day, place it in a plastic bag to preserve its dampness. The investment must be damp when it is burned out in a kiln. It will crack if it is dried out, plus the water helps to disperse the heat evenly. Place it on a non-vibrating flat surface such as a windowsill, and let it sit for at least 25 minutes to cure.

Our teachers handle the burning out part, so I don't know the exact procedure for it. The basic idea is to place the invested cast in a kiln and heat it so that the wax melts, creating open spaces in the investment to be filled with metal. Once the cast has been burned out, you are ready to cast your piece in the centrifuge. Keep the flask in the hot kiln until you are ready to cast.

I will explore the actual casting and post-casting procedures in a future piece, as this one is pretty long already. :)