A stainless steel flask, filled with investment. Photo: Sara M. Sanford.

The author makes the piece above in “Casting a Gold Pendant” in page 38 of November 2002, Lapidary Journal.

Okay, you have, with the help of the previous articles in this series, created the perfect wax model. Now, how do you transform the wax into metal, avoiding the many disasters that can happen in the next few steps? The short answer is, by knowing not only what to do, but why each step in the process is necessary. In other words, if you follow the prescribed guidelines, trying not to fudge too much, you will be much more likely to come out with a successful casting. But if something does go wrong, you'll be more able to troubleshoot if you know the “why” as well as the “how.”

Two rings by designer Mary Wong, cast in 14K yellow gold, with garnet, green tourmaline, and diamonds. Photo: Daniel Van Rossen.

First, a short review on the process of lost wax casting. A model is created, exactly as you want the finished product to appear, usually out of wax, but sometimes out of another clean-burning material. This model is invested, or coated with a plasterlike material called investment, which is contained within a metal cylinder, or flask. Once the investment sets, it is placed in a kiln, and the wax is burned away — you lose the wax, hence the term “lost wax casting.” Once the wax has been eliminated and the investment cured, the flask is removed from the kiln and molten metal poured, or cast, into the negative space left behind. The metal takes the place of the wax, duplicating the wax model in exact detail (if all goes well).

To prepare your model for casting, you will need a sprue base on which to mount the model, wax sprue wires to connect the model with the sprue base, and a flask to contain the investment.

SPRUE BASES are vinyl or rubber caps which fit snugly over the end of a flask, holding the sprued wax model in place while the investment is first poured into the flask and then hardens. Once the investment sets, the sprue base is removed, leaving the wax model embedded in the investment. There are commercial sprue bases to fit any diameter flask, but Kerr® brand flasks are odd sizes (1-3/4", 2-3/8" or 3-3/8") instead of the more common 1" or 1/2" increment diameters, and require Kerr® sprue bases.

Earrings by Deborah Spencer, cast in 14K yellow gold with Yowah opal, tanzanite, and tsavorite. Photo: Daniel Van Rossen.

Commercial sprue bases have a cone (or a larger hollow ring for mass production “trees”) in the center — this is where you attach the sprues, because the cone forms a funnel into which the molten metal is cast, leading into the negative space where the wax had been. To connect the model to the sprue base, fill the hole in the cone with sprue wax, and firmly attach the sprued model. (See Spruing below for details)

Before flexible rubber sprue bases were commonly available (back in the Dark Ages), sprue bases were made of metal. The problem then was how to prevent the liquid investment from leaking out at the bottom. (You may need to use this method if you don't have the right size sprue base, or if you are using a flask that is oddly shaped or not a standard size.) Plasticine, an oil-based modeling clay, was used to seal the bottom of the flask to the metal sprue base. But unless it was very thoroughly sealed, the investment could still leak out, creating a mess. Ceramic clay can also be used to seal the flask to the base, although clay dries out rapidly if not kept well-wrapped.

FLASKS, sections of steel tubing, are necessary to contain and support the investment while the wax is being burned out and the molten metal poured in. Typically, flasks are made of stainless steel, since regular steel will quickly break down when heated to casting temperatures, and the water-based investment will cause a non-stainless steel flask to rust. Stainless steel flasks are available from jewelry equipment companies, and come in various diameters and lengths. If you know someone who works in an auto repair shop, it may be possible to obtain leftover sections of muffler pipe. Unless you do only one size of casting, it's a good idea to have a variety of flasks (and the matching sprue bases) on hand to accommodate different sizes of wax models.

Quite a few years ago, I had a friend who was an engineer at a nuclear power plant (which has since closed). Some of the cooling pipes were being replaced, and she offered to get me a few short sections to use as casting flasks. They were made of titanium — perfect for lost wax casting. Titanium has a very high melting temperature and is much lighter than steel. In the larger sizes, steel flasks are quite heavy, and using titanium considerably lightened the total weight of the flask while casting. I can only assume that titanium tubing is too expensive for commercial jewelry suppliers to consider, but I think it would be a great idea!

Occasionally you might need to cast a model that won't fit into a standard flask. For this special circumstance, you could use a “tin” can (actually steel) and shape it to your needs. (Be sure you do use steel and not an actual aluminum can — test it with a magnet; aluminum will not be magnetic.)

So why not use “tin” cans regularly? They are basically free, and can be discarded after one use. The problem is that both centrifugal and recessed vacuum casting machines are made to accept only round flasks, in a limited range of sizes, so when using a “tin” can, you may have to rely on a gravity pour. Also, commercial flasks are made of a much heavier gauge of metal, and therefore are more stable throughout the investing, burn-out, and casting procedures.

If you use a recessed vacuum assisted casting system, you will need flasks that are perforated, so that the vacuum can pull the air through the investment from every direction. These special flasks require rubber sleeves when pouring investment into them. Table top, or non-recessed, vacuum casting uses regular solid-wall flasks.

SPRUING. In order for the wax to get out of the investment and for the molten metal to replace the wax once it has been burned out, you will need to add wires called sprues to the model. The sprues (also called “gates” in large-scale casting) will join together at a point where there is an opening to the outside of the investment. These sprues allow the metal to flow in and fill up the void left by the burned-out wax.

Wax made specifically for spruing is available from jewelry suppliers in several diameter sizes — from 6- to 14-gauge, and in lengths from 4" to 24"; or as one continuous wire on a spool. I recommend 8-gauge wire for the primary sprue; for additional sprues, I usually use 10-gauge, switching to 12-gauge if the length is short and to a small area. If your model is very massive, a larger main sprue might be necessary. Conversely, a very tiny wax could require only a 10- or even 12-gauge sprue wire. Sprue wax is quite soft and flexible, and rather sticky, or tacky. It has a relatively low melting temperature and easily attaches to most wax models. However, since carving wax has a much higher melting temperature than sprue wax, you must be sure that the two waxes are firmly connected.

If you do a lot of casting, you may want to investigate fillet wax. This wax is sold through industrial suppliers, to pattern shops which make wood prototypes for steel parts. It comes coiled in approximately 2 lb. boxes, and in several diameters: 1/8", 3/16", 1/4", and larger. It has much the same characteristics as jeweler's sprue wax, works well, and is less expensive, although you have to purchase a full box. You can also use the same wax wire that is used to make models in the first place, but that particular wax wire tends to be more expensive than sprue wax.

Sprues are attached to your wax (or other material) model with the same tools and heat source you used to create the model itself: alcohol lamp and biology probe; or an electric hot point. (See “Tools for Wax,” July 2002 for details.)

The strategic placement of sprues is crucial to the success of your casting. Here are a few guidelines to help you:

Figure 1

• Determine the most efficient size of flask to use — using a flask larger than is required will only waste investment. But you will need to allow a certain amount of space between the wax model, the sides of the flask, and the top of the investment. Especially in centrifugal casting, the metal will enter the negative space with quite some force, and will break through sections of investment that are too thin. A good rule of thumb is to leave at least 3/16" between the wax and the side of the flask, and 1/4" on the top (Fig. 1). If your wax model is massive, more space should be allowed, particularly at the top, because a larger volume of metal will be more likely to break through than a small amount would. Attach the sprued model to the sprue base which fits your chosen flask.

Figure 2

• Facilitate the flow of the metal — think about how the metal flows through the sprues to reach the cavity, use the shortest sprues you can, and make them as straight as possible (Fig. 2). If the metal has to travel long distances or turn sharp corners, this can cause porosity and incomplete castings. Since molten metal will want to travel forward only, don't have any sprues or parts of the model which depend on a backward flow of metal — attach an auxiliary sprue if necessary.

• Avoid attaching sprues to very delicate or textured areas — remember that you will have to cut the sprue off after casting, and it may be difficult, if not impossible, to restore the texture or recreate a delicate pattern.

This may mean attaching sprues to the inside of a ring shank, somewhat contorting the sprue wire (Fig. 8), or attaching several small sprues to a delicate area instead of one heavy one.

Wherever possible, attach sprues to the back or underside of the wax model, since the point at which a sprue is attached is a common place for porosity to occur.

• Center the model and sprues — make sure that the model and sprues will be centered in the flask. Commercial sprue bases do this automatically, but if you use another system, place the sprued model as near the center of the flask as possible, so that the entrance to the negative space will line up properly with the crucible on the casting machine you will be using. If the sprue hole is off center, chances are that the molten metal will be blocked from entering and filling the cavity.

In the interests of efficiency, you might want to sprue more than one model in a single flask, if all are to be cast in the same metal. This is fine, as long as enough space (at least 1/8 ") is left between the wax models. If the investment area between them is too thin, not only can metal break through and fuse the two models together, but bits of investment will become embedded in the metal, creating pits in the casting.

Investment used for jewelry casting is formulated to be somewhat porous, so that the air left after the wax has been burned out can be absorbed into the investment as the metal is cast. If you are using a gravity pour method other than vacuum assisted casting, you will want to add some air vents. These are additional sprues which extend out from the wax model, or even bend back and come out at the bottom end of the flask, separate from the sprue (Fig. 9).

The purpose of an air vent is to allow the air, which occupies the negative space, a place to go as the molten metal rushes in, thus preventing an incomplete casting due to air pockets. Adding air vents is not usually necessary when using a casting method with some force behind it, like centrifugal casting. Although vacuum casting is in essence a gravity pour, the vacuum works to suck the air out through the investment; that's why the flasks for recessed vacuum casting are perforated. However, if you have a very large or massive wax model, you may want to add a few air vents even if you use a centrifuge.

Figure 10

Some technique books recommend adding a ball of wax, a “reservoir,” to the main sprue just below the model (Fig. 10).

The theory is that this reservoir will take longer to cool, thus relieving some of the stresses on the metal when it is solidifying. I have tried it both ways — using a reservoir, and not using it. I can't say I noticed any difference between the two methods in the final casting, but you might like to experiment with this technique.

Wax model (red) sprued (black) with pearl and heads in place. Photo: Sara M. Sanford. Finished cast piece is shown at beginning of article.

If I have used a softer wax in a model, such as sheet or wire, I like to leave any stones in place while I sprue the model, as those waxes can be easily distorted when adding sprues. But I do remove the stones before investing. Which brings up another point: what stones can be left in the model through the burnout and casting processes? Obviously, heat-sensitive stones like turquoise, opal, or emerald should never be cast in place. In fact, I recommend that no stone be cast in place unless you can afford to replace it. The best stones to try are those with the fewest internal stresses or inclusions. Synthetic stones fit this requirement to a T. Diamonds can also be successfully cast in place, but again, if they have fractures or inclusions, they may not survive the stress of being heated to burnout temperatures and having hot metal hit them. Coat any stones you plan to cast in place with a solution of boric acid and alcohol, such as is used when soldering without removing diamonds. And don't forget that a lot of stones are treated in some way — dyed, heat-treated, or irradiated. Heating to burnout temperatures probably will alter a treated stone in some way, usually not to its advantage.

I do occasionally cast metal findings, such as prong heads, in place. I like to put them in the model as I work on it, and then I know they will fit correctly after the wax has been cast. But if there are many, or they would be difficult to solder on after the piece is cast, I will cast them in place. To do this, melt a small amount of solder, preferably one with a relatively low melting temperature, on the finding where it will be attached to the model. After pickling, clean the soldered area thoroughly, and brush a little liquid flux on it. When the flux has dried, attach it to the wax model. The theory is that the molten metal will be hot enough to melt and fuse the solder. In fact, this has worked for me only about half the times I have tried it; but with the solder already in place, and a perfect fit, it is relatively easy to do the soldering after casting.

WEIGHING THE WAX MODEL.
Once you have your wax model securely sprued, you need to weigh it so that you can determine how much metal you require for casting. The easiest way is with a good scale, like a triple beam balance. Weighing the wax with the sprue base, then subtracting the weight of the base itself, will give you the most accurate measurement of the model, including the sprues. Just weighing the wax model by itself means you have to “guesstimate” how much metal to add for sprues.

I have marked all my sprue bases with their individual weights, and given each a unique letter. (I avoided using numbers so I wouldn't confuse them with the weight.) When weighing, I write down the letter of the base and the total weight (model and base) on a small sticky note. Then I subtract the weight of the base, and multiply the result by whatever factor I need for the metal I am going to cast into that flask. I also write down which metal I will be using. Once the investment has set, I scribe the top of the investment with the letter from the sprue base, so that I know exactly which metal and how much to melt for the model in that particular flask.

If you don't have an accurate scale, there is another, more primitive way to determine the amount of metal you need — water displacement. Use as narrow a transparent container as the model will fit into completely, such as a milliliter measuring graduated cylinder, or a standard glass measuring cup for larger models, and fill it with water to within an inch of the top. Then submerge the wax model, including the sprues. Since wax will float, you will have to hold it down in the water. This is very easy to do if you simply grasp the sprue base, turn it upside down, and submerge the model plus sprues in the water. Note how far the water rises, and mark that level on the container or write it down. Once the wax has been removed, fill the container with your chosen metal until the water rises to the same level as the wax did — you are simply replacing the volume of wax with the corresponding volume of metal.

CALCULATING THE METAL NEEDED.
While you are weighing the wax, you may as well weigh out the metal you will need to cast. I like to put the metal I have weighed out into small individual containers, with sticky notes telling me which flask they will be cast into. If your model is not wax, but another material — styrofoam or organic material — the water displacement method will be the best way to calculate the metal needed.

WAX WEIGHT TO METAL WEIGHT RATIO.
The specific gravity of a substance is the ratio of its weight to the weight of the same volume of water. Wax has a specific gravity of slightly less than one, which means that you can simply multiply the weight of the wax model by the specific gravity of the metal you're using to get the amount of metal you'll need to melt for the casting. (Note: the specific gravity charts I consulted varied slightly in their numbers — when there was a variance I used the highest number, since it's better to have too much metal than not enough. Different alloys will have slightly different specific gravities depending on the proportion of the metals used.)

Multiply the weight of wax by the specific gravity (SG) to figure amount of metal needed.

METALS FOR LOST WAX CASTING.
But what metals are appropriate for lost wax casting? The main limitation may be your torch, or whatever source of heat you will be using to melt the metal. Certain metals, like platinum, have a high melting temperature and require a specific formulation of investment as well. (Platinum casting is a special process, outside the scope of this series.) Some metals may require more heat than others, regardless of similar melting temperatures. For instance, sterling silver melts at 1640°F, and brass at about the same, depending on the alloy used. But you'll need nearly three times the heat to melt brass than you will to melt silver. Again, it depends on the alloy, but 14-karat yellow gold melts at close to 1475°F, while 14-karat white gold is molten at about 1700°F. The heat source you use will also determine the amount, or volume, of metal you can melt. Small, single gas torches such as a propane torch are hard pressed to melt even a single ounce of sterling silver. A mixed gas torch, preferably with oxygen, will give you far more versatility in casting a variety of metals as well as different amounts. ( See “The Heat Is On,” July 1999, for a discussion on torches.)

So which metals can you cast? Assuming you have the appropriate heat, any metal with a melting point under 2000°F should be okay. This includes silver, both fine (pure) and sterling, and all the alloys of gold (unless it is a very exotic mixture). Brass and bronze, which are alloys of copper with zinc (brass) or tin (bronze), should be fine, although they require more heat to melt. Copper itself will not cast successfully with most small-shop casting equipment, even though its melting temperature is 1981°F — when melted it will be very brittle and porous. Two other possibilities are aluminum, which melts at 1218°F, and pewter, with a very low melting temperature of under 450°F. White metal alloys, used for mass-produced costume jewelry, usually will melt under 750°F. Tin, the major component in pewter, can be easily melted, but is so soft that it is impractical for most jewelry use. (Under no circumstances would I recommend casting lead! The fumes are highly toxic.)

The question now arises of how much, if any, “old” metal you can use when casting. Melting the metal for casting seems like an efficient way to use the scraps we all have in abundance. But unless the scrap is totally solder-free, and not previously cast, keep the proportion of old metal to less than 50 percent of the total being cast (I usually use less than 40 percent old metal). I don't recommend using filings at all, because there will be tiny bits of stray material in them, broken saw blades for instance, which will cause pits in the casting. Some jewelers who are fanatical about the quality of their work use only new metal, fresh from the refiner, for casting. All major precious metal suppliers offer refining services, and most will trade your scrap for fresh metal.

Why not use more previously cast metal? When precious metals are heated to melting temperatures, they undergo changes in their molecular structure. Metal that is repeatedly melted and cast becomes very brittle. It will need to be broken down, practically to its constituent atoms, before it regains its malleability and is once more suitable for casting. Regardless of which metal you choose to cast, and whatever the percentage of old metal, all the metal you melt must be completely clean and free of old investment, solder, or any other contaminant, or you run the risk of porosity or incomplete castings.

Which metal or metals you use will help determine several other factors later on in the process of lost wax casting, such as how long the flasks remain in the burn-out kiln, and what the flask temperature should be when the casting is done.

Sara M. Sanford