Pouring bronze into the final ceramic mold.

One way to sculpt in bronze is the technique of Lost-Wax casting.  This involves a wax sculpture as the starting point.  The wax is encased in a fireproof mould such as ceramic.  Once the wax gets melted off it reveals a cavity.  The bronze is then poured into this cavity to make the sculpture.  Each step represents a different copy of the original model. These copies alternate between "positives," or duplicates of the model that look just like the model itself, and "negatives," or molds of the model that fit the model like a glove on a hand.[2]

The final clay form - we'll cover this with latex or other mold compound.

Working with modelling wax is fun, but it can be difficult to get the shape that you want.  In order to work with it you need to heat the wax over a stove or have internally heated tools.  The wax sticks to everything and the tools you use to work with it need to be constantly cleaned or you won't be able to form intricate details.  If it gets on your clothes it will likely never come out.  Even cleaning it off counter tops and stovetops can be a real chore.  Using a camp stove outdoors or in the garage is a good alternative to messing up the kitchen.  Overall it's a difficult medium to work with if you want to create a lot of detail, but it's cheap and doesn't require any special equipment.

Instead of trying to do intricate designs directly in wax, you can instead start with another sculpting material like clay or even wood.  It depends on what medium you're comfortable working with, but most find modeling clay easy to work with because it doesn't dry out, it keeps its form pretty well and it can easily be formed by hand or with whatever tools you have.  To the right is the clay version of the finished product. 

Once the clay sculpture is made, a rubber mold is formed around it to capture the detail.   There are a lot of different kinds of rubbers out there so it can be difficult to figure out the best type of rubber to use.  You can buy rubber from a lot of different places.  A common choice is smooth-on because they have a lot of learning resources, tutorials, good customer support and a wide selection of products.  The choice to use depends on how much you want to spend and how you would like to apply it.  I chose a cheap brush-on rubber which will later be reinforced with plaster.  Below you can see a few examples of what the clay sculpture looks like after it has been coated in the rubber compound.

Covering with a latext brush on mold.

Over the cut line make sure to build up the mold material so that you can put it back together after taking the piece out.

Once the rubber has hardened, or cured, you can apply the plaster layer.  Plaster is cheap and it's simple to work with.  I won't go into the details of working with plaster, but basically you have to set up the mould in several segments so they can get removed later without the need of a hammer.  This stage doesn't have to be perfect because it's just a holding mould which will keep everything in place.  It isn't responsible for capturing any details. 

After all the plaster parts are formed you take it all apart and remove the rubber piece.  Make a seam along the length of it so that the inner clay piece can be removed.  Take the clay original out of the rubber and the result will be a rubber suit with a hollow mould cavity.  The Plaster pieces are put back together around the now empty rubber mould.    In this case it was planned out to pour the wax into the feet of the sculpture.  This way if there are any resulting imperfections at the pour point we can ignore them since they're on the bottom of the sculpture.  Wax is poured into the mould cavity and allowed to harden. 

Adding plaster to keep everything rigid during wax pouring.

After cutting the lines and taking the wax version of the sculpture out, we have an empty cavity to pour wax into.

Pouring wax into the feet first - that will keep any imperfections on the bottom.

When pouring the wax into a mould like this you need to make sure the wax is hot enough to get to all the crevices.  Once the wax melts you should cook it a bit longer to make sure it's hot enough to make it through the mould when you pour it.  After pouring, wait a half hour or so to make sure the wax solidifies all the way through.  Remove the outer plaster portions to get to the rubber mould.  Once you remove the rubber mould the result SHOULD be a perfect wax replica of the original.

The image below on the left is an example of wax that wasn't hot enough when poured.  It resulted in an incomplete casting.   This was, of course, useless and needed to be scrapped.  The next attempt used hotter wax and ended up being successful.  I would like to check the temperature of the wax to see the correct temperature next time.  It will also depend on the type of wax used. 

An important factor to remember is how much the metal will shrink on cooling.  When casting a thick piece of metal it might shrink enough to crack.  Anything more than about an inch thick should be considered at risk.  For that reason you must hollow out the thick areas of a sculpture before pouring the bronze.  One way to make a hollow wax piece is to use the slush casting technique.  Here you would pour the wax into your mould cavity then allow it to solidify for a short amount of time.  Before the inside solidifies you pour out the molten wax on the inside.  This leaves a layer of wax along the inner wall of the mould cavity.  In the case of this sculpture it would be difficult to approach the problem in this manner because the thin leg portions of the sculpture would solidify too quickly and block the exit route for the molten wax.  Another approach, the one used here, is to cast a solid wax piece and just hollow out the thick areas afterwards.  On the image below to the right you can see a line has been drawn with an awl or knife which will shows where the back will be cut off the sculpture. 

Incorrect wax pour - the wax was not hot enough.

Perfect wax casting!

Here I drew a line where I want to cut the back off. The inside must be hollowed out since the metal will contract.

Cutting the back off reveals the inner portions of the thickest part of the sculpture.  Now you are free to gouge out the insides with whatever means at your disposal.  It doesn't have to be pretty because in the end it will never be seen by human eyes.  The thickness of the skin should be at least a quarter of an inch.  This will allow for the bronze to flow through the mould cavity without getting rapidly cooled on both sides and creating a blockage. 

After you have removed a satisfactory amount of material try placing the two pieces back together to confirm a proper fit.  The edges don't have to be perfect as you will eventually need to weld along the seam to seal the two pieces together.

The back piece will be poured in bronze separately. They will be welded together after casting.

All hollowed out. Ready for the ceramic coating.

The wax workstation with a bunch of tools. Anything works from x-acto knives to spoons.

Now you need to encase the wax piece in a fireproof mould.  Once it gets encased the wax will be melted out to result in a cavity to pour the bronze into.  There are a few options for materials when creating the fireproof mould.  The ceramic method will be described here.  There are certain types of clay that work well, but nothing beats ceramic for capturing precise detail.  Another benefit of ceramic is that it is light weight so you can move it around easily throughout the pro

Below to the left you can see a full setup for doing ceramic dip moulds.  The yellow barrel is filled with a liquid compound which is used like a glue.  The other barrelt is full of a ceramic sand.  The images on the right show what it looks like on the inside.  Compressed air is pushed into a mesh on the bottom of the barrel.  This areates the sand and allows you to actually dip a solid object into the sand much like you can dip something in water. 

After applying the yellow glue layer to the wax piece you dip it into the sand.  This is then allowed to hang from a drying rack for about 10 to 15 minutes until it is dry enough for another dip.  After several layers have been applied to the piece you need to let it dry overnight to allow for the ceramic to properly solidify. 

The ceramic station. The yellow stuff is all the ceramic mixture and the barrel to the left is the sand.

Just a bucket of sand.

After the air is turned on the sand starts bubbling!

Multiple layers are applied in this fashion until the desired thickness is achieved.  It can take 20 to 30 layers to get the ceramic thick enough for the final bronze casting.  The thickness necessary depends on the size of the piece.  Larger pieces need thicker ceramic moulds because they need to hold a larger amount of bronze metal.  The weight of the bronze could crack the mould if it is too thin.  After the disired thickness is obtained you need to dip it one last time in the glue liquid.  This will create a final smooth layer over the entire casting.

Below are examples of what the piece looks like after the ceramic layers have been applied to all the pieces.  You will also notice a lot of extra pieces branching off of the main piece.  Just with the wax pour, when pouring bronze you want to pour into the bottom of the piece in some situations to prevent imperfections from showing up on a place that is visible.  All the pieces also get topped with a wax cup which will end up being a cavity to pour the bronze into.  This was formed by pouring wax into a paper cup.  A wire clothes hanger is curved into a hook and placed into the molten wax of the paper cup before it solidifies.  This allows you to easily hang the piece from the drying rack after applying the ceramic layers.  Large conduits of wax are also connected between the cup and the sculpture.  These help the bronze flow into all the different areas of the sculpture during the pour.  These conduits are easily removed and ground off with an angle grinder after the bronze sculpture has cooled.

Lots of good solid channels were added to let the bronze pour.

Pieces are dunked in the ceramic liquid then in the sand then they are hung out to dry on racks.

The back piece - you can see the cup and channels were added to allow for the bronze to pour into the mold cavity.

On the images below you can see some of the details of the finer points of the sculpture.  Notice that a loop is placed on the details like the fingers and horn tips.  These will help to allow the bronze to flow into all these details and it will prevent air bubbles from getting caught in these areas which will create another type of imperfection on the final product.  The image below to the right shows the final glue layer applied to both pieces.

Small endpoints like the fingers got an extra bit of wax loop to let the bronze flow properly.

When the ceramic moulds have finished drying it's time to lose that wax.  The wax is "lost" or melted off by putting it into a kiln or oven.  The ceramic mould is placed in the kiln up-side-down so that the wax pours out of what was the top cup.  It's important to make sure no wax chunks get super-heated in a remote part of the mould.  This could cause it to boil and build up pressure.  This pressure will cause a crack to form in the ceramic. 

It is easy to spot these cracks by the wax which will leak out.  It will leave a burn streak down the side at that point.  If this happens, try repositioning the mould to allow the wax to get out of that point in a more efficient way.  Once the majority of the wax has flowed out of the top cup turn it up-right again.  This will allow the smoke from the burning wax to escape through the top cup.  The kiln is kept pretty hot during the burn-off.  This will cause any residual wax to just get burned off and evaporated in the case that some of it gets stuck inside the mould.  Smoke will continue to escape through the top cup until all the wax has burned away.  Heating for about 20 minutes should be sufficient.

Tubs or pails are prepared for the moulds by filling them partially with sand.  The sand will catch any remnants of molten bronze that don't make it into the moulds.  The sand also keeps the mould in place so they are stable for the bronze pour. 

Once the wax is fully removed from the mould it is placed onto the bed of sand in the tub.  Tin foil is wrapped over the top to prevent debris from falling into the mould.  Anything that falls into the mould would be encased in bronze and will likely end up being visible on the final product.  After wrapping in tinfoil, sand is poured into the tubs to surround the ceramic moulds.  This will keep them stable for the bronze pour.

In addition to the sand placed inside the tubs, the tubs themselves sit on a bet of sand or earth.  This serves to capture any bronze that escapes from the tub.  If the ceramic mould breaks during the bronze pour this could result in a fairly large pool of metal on the ground.  If you are working on a concrete surface this can be very dangerous.  Concrete will always have some amount of residual moisture trapped in it from environmental humidity.  When molten metal superheats that moisture it can cause the concrete to explode.  Molten bronze and concrete shards flying through the air could potentially hurt someone or fall into your coffee cup - both serious problems.

Once everything is set in sand and stable we are ready to melt the bronze.  If you are well organized the metal would probably have started heating before you are finished with the sand.

The bronze is melted in either a gas furnace or an induction furnace.  The type of furnace used here is an induction furnace.  These furnaces use alternating electric fields to induce an electromagnetic field inside the metal being melted.  Flipping the magnetic field back and forth causes the metal to heat up and eventually turn to a liquid.  The technique requires a lot of electric power and the coils delivering the magnetic field need to be water cooled using a cooling tower.  Because of all this, the process takes some special equipment and knowledge to achieve the correct results.

There were two designs of induction furnaces used on this pour.   One is a tilt pour which has the coil and crucible all in one (see the image below to the left).  With this one you can pour the metal into a secondary crucible or directly into the mould.  The other style, in the images below on the right, is a drop-over type which has a stand for the crucible to sit on and a hydrolic oporated lift which lowers the coils around the crucible.  In the one shown here you have a crucible stand on both sides to allow you to continuously get up metal in preparation for pouring.

A really great device to have is a ceiling mounted crane to help lift the crucibles (and for other steps in the process).   In the image below you can see it being used to lift a bucket of sand.  This crane is suspended from the ceiling from a roller.  It can be manually pushed in any direction and the buttons are used to raise and lower the hook.  In this picture you can also get a better look at the induction furnace ventilation and the water cooling/control towers against the wall (green boxes).

Once the bronze is melted down and ready for pouring it is mounted into a two-person clamp.  This particular clamp is seen sitting in a crane mounted support to keep the weight off the people.  It's tough work pouring this much metal into the small opening of the mould.  It's heavy for one but you want to be able to focus on being precise so you don't waste any metal.  

In this configuration you need a 3-person crew.  Two people will control the tilting of the crucible and one person will stand in front with a scraping tool.  The scraper keeps any sludge and other debris from flowing into the mould.  That sort of thing could ruin the final product if it got on the visible outside of the sculpture.

When you are done pouring the metal is allowed to cool for at least an hour.  Taking it out of the mould too soon could result in dents and abrasions on the sculpture as a result of the metal being too soft.

To get the sculpture out of the mould you crack the white ceramic mould with a hammer.  This seems a bit violent at first considering the care and patience you have taken to create the sculpture, but the finished product in bronze is strong enough to take the abuse.  Of course be careful on any places that have fine detail as a piece like this could easily break off with the appropriate strike from a hammer.  In cases that require a little more finesse, a dremel or rotary tool is helpful.

Once the sculpture has been removed from the mould you need to inspect it for any problems.  Make sure the bronze flowed into all the appropriate gates.  If you look at the image below on the right you can see what looks like a drip of bronze on the tip of the horn.  This is what's left of the loop that I put on there to keep the bronze flowing.  It didn't work quite as I had planned, but without it I probably would have had a missing horn tip instead.

To remove those sprues that are connected to the arms you will need to use a cutting disc on an angle grinder.  After these parts are removed you'll need to grind off the remaining rough parts using a file or rotary tool to clean it up. 

In this sculpture a TIG welder was needed to re-attach the back piece to the main body.  The TIG welder uses a tungsten rod as the electrode and melts a rod of the desired metal as you feed it into the target.  Since the back and main body pieces were confirmed to fit correctly before creating the ceramic moulds they fit together like puzzle pieces even now after all the additional steps since the wax version was created.  After cutting off the sprues and fitting the two pieces together they are clamped in place in preparation for welding.  

The TIG welder is used along the seam as you insert a rod of bronze.  Iinitially only 3 or 4 points of contact are welded together at opposite sides of the seam.  This ensures that the pieces do not end up getting lopsided after finishing one side.  After it is fastened in place at a few points, you continuously feed the bronze in as you run along the entire seam.   Try to use as little material as possible on the connecting seam.  This said, the seam does not have to look perfect because it will be ground down with a rotary tool once it's finished getting connected.

In the case that you have some missing bronze somewhere, such as a pit or a trapped air bubble, it's possible to actually add bronze onto the the piece using the TIG welder.  Just feed in the bronze rod as you apply current to the area. 

Once everything is cleaned off the sculpture and the seams are ground off and leveled, run over the whole piece with sand paper to make everything uniformly smooth.  This can also help you find imperfections that will show up later.  If you have finished off the surface you may want to apply a patina at this time.  A patina is a chemical treatment applied to the surface of the metal which changes its appearance by altering the color and/or the texture.  There are many different patina techniques and recipes out there and books devoted to the subject so there is ample information out there to base your decisions off of.  Usually the patina is applied by bringing the surface up to a high temperature using a sunflower tip on an oxy-acetalene torch to distribute the heat.  Then a spray bottle is used to apply a uniform coating of the dilute chemical.

This piece does not have a patina, but is instead the natural color of bronze.  This may change in the future but for now I like it.