Lost Wax Casting

A while ago, I delved into the world of casting with my attempts at creating greensand. When the geometry of the parts I needed to cast were unable to be used with this medium, I turned to the more conventional plaster investment.

Before even beginning any of the casting itself, the object to be casted had to be carved. There are a few different materials that can be used for this, depending on the type of casting being done. For lost wax casting, the material is obviously wax, although other things such as styrofoam can be burned out of the investment as well. For non-destructive casting, the master can be virtually anything that will hold its shape, although the geometry usually limits this sort. The undercuts of the wax I carved prohibited it from being used in a pressed mould (where the top is open to the environment) or any sort of sand (where the mould is made in two halves, each done separately and then stacked when the part is going to be cast to create a void representing the whole object).

Not knowing much about casting, I went with a generic 'purple' wax, which has decent hardness and carvability. There are other varieties, ranging from green to blue to red, which all have different uses. One melts more easily and is more malleable, other extremely hard for machining, etc. This wax came from RioGrande, although there are several suppliers online and, if you are fortunate, in local jewellery outlets.

The entirety of the shaping was done with three tools. A coping saw to cut the block into pieces close to the size I would need, a small drill bit to start the holes for the blade's tang, and a 120 grit flap disk bit for my rotary tool. That is it. The miniature flap disk has a convenient radius and leaves a surprisingly smooth finish. Although a bit expensive, it was well worth the investment (when I got it for a different project years ago).

After all the pieces are carved and situated with the antler handle counterparts (to finalize the spacing and position), I made any final adjustments. This was important because I will be making a silicon mould of these carvings with which to make additional waxes. Those in turn will be invested in plaster and burned out, being destroyed in the process. Because my setup is extremely crude and my experience limited, I knew I would not have a perfect success rate. With vacuum or centrifugal casting machines, it is much easier to produce good castings, and often the process of making duplicates of the models is unnecessary. 

From my recycling bin, I fashioned a home-made vacuum chamber. It consists of a jar with a hole cut in the bottom, lined with duct tape. The hose of my floor vacuum fits over the hole and seals along the duct tape, and I can suck the air out. This was meant to help the silicon settle and remove the air bubbles as it began to cure. 

To make the silicon moulds, I used a 2 part silicon compound (any will probably do) and submerged the waxes in them. A few lessons learned-

     -Pour a layer in the bottom of whatever you make these in (probably about 1/5th of the total height) and allow it to begin curing before putting the piece in. The wax is more buoyant than the silicon, so it floats to the top. By having the layer in the bottom, you can adhere the wax to it enough to keep it from rising.

     -The vacuum chamber was probably useless but I pretend like it was a good idea. The silicon is so viscous that the air bubbles will be unaffected unless you want to burn out your vacuum trying. An actual vacuum chamber might work better.

     -Do not be afraid to mix the two parts of the compound thoroughly. I had a small pocket of unhardened silicon trapped in one of the moulds, and even months later it is still fluid inside. While this did not effect the parts I needed to be solid, it very nearly did.

     -Similarly, trust the working time listed on the package. Use as much as you need to ensure the hardener is mixed in homogeneously.

     -Cure it in a toaster oven set a little higher than the higher of the two 'recommended curing temperatures'. I had it sit in at the higher of the two temps listed for quite a while before bumping it up another 20 degrees or so. That did the trick and the silicon began to firm up nicely. (The silicon is rated to much higher temps than this).

From the silicon, I was able to pour in a pink injection wax. Heated gently over the stove in a pot to the point where it is almost smoking and releasing air bubbles, it pours like a dream. There is no need to vacuum the copies, as it flows into every, and I mean every, detail of the silicon. Each of the copies needed only a little cleaning before being read to cast in the investment. I removed the seams and added sprues to the cone left by pouring it into the silicon. This was the best part, as I had to make over half a dozen of the rightmost wax before getting one in bronze that I was happy with.

The investment I used was Satin Cast 20, also from RioGrande. Word of warning, it comes in a bag in a box, which is only bad if you have a violent time opening it. The instructions give you the ratios to mix it with water by weight, so I used a small postage scale to see the consistency I needed. In the subsequent ones,  I more or less winged it, which worked out fine. 

For each of the three castings (upper guard, lower guard, and the two spacers together), I made two flasks. The size was about perfect for the purpose, but will likely be a bit small for anything else. Be sure to measure what you are casting before deciding on a size! The bottoms are rubber and have a cone in them to attach the sprue to. I would recommend a type that has a solid rubber cone with a straight hole drilled in it to accept the sprue rather than the type I have where the walls of the cone are a uniform thickness and the hole widens the deeper it goes. This made it difficult to remove the rubber bases without damaging the plaster. To work around that, I filled the cone, allowed it to dry, then carefully 'welded' the wax to that in a few key spots so it would hold for the plaster but break away easily when pulled off.

Sprues themselves are best left as big as possible. I learned that the hard way. Especially with high surface tension metals like bronze, small sprues will not fill properly in solely gravity-based casting systems. Larger sprues let more metal flow into the hole, faster. Think of a mercury thermometer. The meniscus is gigantic compared to a water or alcohol thermometer. The larger the difference between where the metal adheres to the walls of the shaft and the top/bottom of the curve connecting those two points, the worse off you are. So, if that did not make sense, trust me when I say bigger is better (within reason, of course). I think 1/8"~3/16" would have been better than the ~1/16" sprue I used (until I stuck a bunch of them together). Anything over 1/4" for this size of a casting would be excessive (in my limited and uninformed opinion).

Once the wax was affixed to the rubber base, the flask (nothing more than a piece of pipe really) was inserted around it. Some prefer to create a plaster 'shell' around the wax before pouring in the investment in an effort to prevent air bubbles from adhering to the wax, while others place the plaster in a vacuum chamber. I did neither of these. Instead, I was careful not to aerate the plaster when mixing, and poured it slowly into the flask. The only point where I had to tend specially to it was with the holes in the waxes for the tang of the blade. I filled it about 3/4 of the way up the height of the hole, then tipped the flask until the trapped air could escape. Then a little more, and a little more, repeating the tipping action, until the hole was fully submerged and air free. Poking the hole with the tip of a knife made sure there was nothing in there but investment. From there, I topped it off and let it sit.

Curing these is fairly simple. Depending on the type of investment, there will be a sit time associated with it (assuming you mixed it properly). I let mine sit over night, having coincidentally poured them all in the evening. The next day, I set them upright (so the wax would stay in the flask, not pour out everywhere) in the trusty toaster oven at ~300-350 degrees (F) for an hour or so. When the wax was fully liquid, I poured as much out as possible, then returned it to the oven. After that hour, I bumped the temperature up to the max, which was around 450~500 for another hour~hour and a half. This cures the plaster and helps expunge any water trapped inside. Read, do not heat them too fast! (ask me how I know there was water in there...)

Finally, I put the moulds in the forge at about 2000 degrees (F) for an hour. This atomizes any remaining wax and ensures a sterile environment for the bronze to inhabit. Any wax in the cavity can turn to gasses which prevent the metal from filling the space, cause pocked holes on the surface, large voids in the metal, or a complete failure like you will see later. This is the time where you want to be overly careful. A few ways to check the moulds-

     -Take them out of the heat. Are they burning like a candle? If yes, there is still wax inside. If no, there might still be anyway.

     -Is the plaster blackened? This indicates there is still wax lingering where it should not. Once it is completely gone, the plaster will become white again.

     -How hot is the inside of the void? If it is still at a black heat (no visible colour), then it is not hot. Mine were glowing when I took it out and allowed it to cool a little.

While the moulds are curing, I began to prepare the bronze. Mine is in the form of casting grain, although it also comes in ingots. I prefer this, as it generally melts faster. I used both clay crucibles (above) and graphite foundry crucibles. The clay were easier to use, as you can see the surface of the bronze without removing it from the heat source (in my setup anyway). There are a few ways of melting the metal. The simplest, at least in my shop, was to simply put it in the forge and wait. Under the burner cone, the metal easily reached pouring temperature, which is around 1905 degrees (F) for this alloy. Beware! The pouring temperature and melting point of the metal are NOT the same. In the case of this particular grain, the difference was about 175 degrees.

On top of the grain, I sprinkled a little flux. This forms a glassy layer over the surface of the liquid metal and prevents oxidation (at least in crucible steels). This is not really necessary, but I was weary of these crucibles and the other things that I had already melted in them. An important note- do not add flux to cold things. It will eject everywhere the instant you introduce it to the heat (if it is not electric), as it will not have had time to melt and adhere to the things inside the crucible. 5~10psi is plenty to shoot the powder in all directions, and it is not something you want to breathe.

If you are using clay crucibles, it is critical that you coat them with borax or comparable flux before using them the first time. As with any crucible, you also need to fire it before use. This drives out any moisture that may be trapped inside from manufacturing and sitting on a shelf. If you are curious about this, there is plenty more information elsewhere on the internet.

When you have all the preparation done, it is time to begin melting the metal. Bronze, brass, copper, aluminium, lead, tin, pewter, silver, gold, whatever it may be, it will take time and patience. There are a few methods of melting, summarized here for your convenience-

    -In the forge. As a smith, this is the most readily accessible. I slid the crucible into my smaller forge so it sat directly under the burner.

     -Torches. If you have access to an acetylene torch, this works too. It is faster because the flame is hotter. This provides the clearest view of the metal as it melts. For zinc containing metals, however, and other things that burn off at lower temperatures, it is more important to not overheat it this way, as the flame is well above the temperature of evaporation.

     -Electric melting furnaces. These are the easiest and most foolproof, yet most specialized. Simply set a temperature (the pouring temp of the metal) and wait until it tells you it is good to pour. If I had one, I would use it, but I don't...

     -Other melting furnaces. Like electric but not. Likely crude and with little to no temperature control other than in the fuel line. No PIDs, unless there is a thermocouple in there somewhere, and probably little to no view into the crucible without opening it. More useful for melting steel or larger quantities of casting metals. For the size of this project, absolutely overkill.

     -Things that I don't know about.

This is the product of the first melt. There were still gasses trapped in the void, so it created a pocket that would not allow more bronze to enter the mould. Also, the bronze was not hot enough. It was liquid but not quite there for pouring. Back to the drawing board.

As the carnage of the first melt re-melts, you can see the two moulds staying warm on the other side of the forge. It is important that they are not cold when the bronze goes in, as the thermal shock will either break the mould or solidify the bronze or both. Burnout for the wax happens around 1800~2000 degrees while pouring in the bronze is as low as 1000~1400 degrees.

One way to tell that the metal is ready to pour is how the surface looks. If your equipment allows you to see the surface of the liquid metal, you will notice a series of wavelike ripples on the surface when it is close. I also had a non-contaminating rod (you can use either graphite or a pure bar of one of the predominating metals in the alloy, or a bar of the alloy itself) I could use to stir it. When it is ready to be poured, the metal will roll off the rod like water. Finally, if you have a thermocouple that can be submerged in the liquid metal, that is the easiest way to indisputably read the temperature. If it is below the pouring temp, it is not hot enough yet.

Melting temperature strikes again. This was the pour I used the graphite crucible for, and the result is clearly half baked (bad pun intended). The fresh casting grain melted fine, but the larger chunks from old attempts did not fully liquefy. As a result, the semisolid bronze clogged the sprue, resulting in this thing. 

A general note, it is advised that a mixture of new and recycled bronze (or otherwise) be used if you are remelting any previously used. This prevents brittleness in the piece. If I remember properly, the numbers are 50/50, but I could be wrong. 

This is the result of the 5th cast, finally a good one. The 4th was very close, but I heated the plaster too quickly any boiled the trapped water out of it. That deformed the interior of the void beyond repair. Other than that, the cast was good, which was a shame.

After a little cleaning and removing the sprue, I could fix the geometry and begin the patina. There are a thousand different ways to patina bronze, or any metal for that matter.

In the end, I came out with a good cast and the experience to finish the remaining pieces. Soon, I will finally be able to finish this project that has sat on the bench for nearly a year now.

Happy casting!