"Bronze casting manual - The ceramic shell technique"
by Jill Varani

Making the First Wax
Step One: Sculpting the Original

Step one in the sculpting process is to create your original sculpture. This can be done using any material but traditionally artists use either clay or wax. Both materials have their advantages and drawbacks.

·       Clay is a good medium because it is soft and easy to mold. It is great for working smaller pieces, which is what we are focusing on here. However, there are two major disadvantages: it needs to be kept moist or it will dry out, and it requires an armature to hold its weight.

·       Plasticene is a more expensive kind of clay that won’t dry out, but it also cannot be fired.

·       Wax is also easy to use, and for smaller pieces it doesn’t require an armature because cold wax is strong enough to hold itself up. It is not as soft as clay and therefore more difficult to model. It can be warmed under a lamp until it is soft enough to mold in your hands, and cooled down in order to smooth the piece and add the finer details. Another  advantage of sculpting the original in wax is that you don’t necessarily need to make a mould.

For information on armatures, see:

·       From Clay to Bronze by Tuck Langland

·       http://tx.essortment.com/armaturewire_rkxg.htm

·       You can also sometimes purchase pre-made armatures at Sculpture supply stores. Barnes in Australia, (http://www.barnes.com.au) supplies both pre-made armatures and armature wire. Be aware that the armature wire sold in sculpture supply stores is just a soft aluminum wire that can usually be found at metal merchants for less than a quarter of the price.

 Step Two: Making a Mould of Your Sculpture

The mould is the hollow form into which you will pour molten wax, that will then harden in the shape of your original sculpture. This is made in two parts or layers. The first layer is a silicone-rubber skin that catches the details of your sculpture. The second layer is a plaster backing that supports the silicone.

The silicone component of the mould can be made as a single skin that covers the entire piece, or it can be made in parts. A single skin will need to be carefully cut to free the sculpture; therefore, a complicated piece with many angles, undercuts, and holes would be better off made as a piece mould.

Before Beginning the Single Skin Mould:  

·       You need to set aside a block of time. The silicone must be done all in one go, layer by layer. If you wait too long between layers, it may set and the layers won’t bind together. It takes between three to seven layers of silicone, with at least a half hour wait between each layer. You can extend the half hour safely for up to six hours, but be careful that you don’t wait too long, or you will have to start over.

·       You need a clean space to work. The mould will need time to set overnight, so make sure it is somewhere out of the way. It will be a messy job, so lay down newspaper or plastic.

·       Think about where you want the opening of the mould to go: the opening of the mould goes where you don’t paint silicone or build plaster. If you secure the sculpture to a base and paint silicone around the entire thing, then the opening will be at the bottom of the sculpture, where it is secured to the base. This could be a problem for a piece with a smaller base than peak, for instance, a standing human figure. It would be wise then to make a sprue on the thickest part of your sculpture, and secure the other end of the sprue to the base (for definitions/how to, see the section on “Sprueing your wax”).

·       The sculpture needs to be secured firmly to a base. For small pieces, the best way to do this is to take a flat metal tray and pour a thin layer of molten wax into the tray so that the bottom is covered. Once the wax has cooled very slightly—after only a minute or so—set your sculpture onto the middle of the tray, so that it sinks into the warm wax. Let the wax cool down the rest of the way, and test to make sure the sculpture does not move.

·       It is handy to put the sculpture on a turntable in order to view it from all sides so as not to miss any spots when coating it in rubber.

·       If the sculpture is made of anything besides wax, it should be coated with a release agent to free the mould from the sculpture.

·       You will need the silicone rubber and its component parts: silicone rubber (RTV), the catalyst, plus the thickener, which is useful for the final layers. Barnes (http://www.barnesonline.com.au) in Australia, and TopMark (http://www.topmark.co.nz) in New Zealand supply various silicones and accessories. “Trade 26 Silicone RTV” and the associated catalyst and thickener are a good choice. Be aware that there are less expensive alternatives to silicone for flexible moulds, including latex and some polyurethane rubbers.

·       You will need a scale for measuring and mixing, and disposable plastic cups and a plastic spoon or knife for mixing.

·       You will also need latex gloves and several disposable brushes.  
 

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Making the Skin Mould:

·       Begin by mixing the silicone and catalyst according to the product instructions; do not mix in the thickener yet. If you are working with a small sculpture (Approximately 20x10x10 cm or less), then only mix 50g of silicone. For sculptures that are a bit bigger, mix 100g, and beyond that, use your best judgment, keeping in mind that you want a layer of silicone no thicker than 1 cm. Also, if you are making a mould for a very large piece, you could try mixing 100g at a time and painting it on in sections.

·       Paint a very thin layer of the silicone mixture onto the sculpture, beginning from the top. The silicone will slide to the bottom until it has cured. It may be necessary to keep moving silicone back to the top to keep an even coating over the entire surface. If you see bubbles forming, you can help free the air by prodding the bubbles with a needle or with your brush.

·       Wait approximately 30 minutes until the first layer begins to set. Test the layer before painting the next: if the silicone comes away from the piece when touched, it needs more time. If the silicone gently smoothes back into place, then move onto the next layer.

·       For the second layer, repeat the procedure for the first layer, keeping a thin even coating over the entire piece.

·       For the third layer, mix twice as much silicone with a proportionate amount of catalyst, and mix in the recommended amount of thickener.

·       Paint on an even coating of silicone. The third and following layers are meant to thicken the silicone rubber mold. The thicker the mold, the stronger and also less flexible it will be. For a small piece, you want 1 to 2 cm thickness. For a larger piece, you will want a thicker mold. The main thing is to reduce or preferably eliminate all undercuts on the mold so that you need less plaster parts in the next step.

·       Continue this process until you have reached the desired thickness of silicone rubber.

·       Let the silicone cure completely. This amount of time will vary depending on the type of silicone rubber RTV you are using, so refer to the product instructions.

               Making the Plaster Support

·       Start by cleaning the space around the mold, and removing any unwanted silicone bits from around the mould. Cut around the base of the mould to remove the excess silicone rubber.

·       For the plaster mould, you will need water, plaster, a bowl or bucket and a plastic spoon for mixing the plaster. You’ll need enough clay to cover half the silicone covered sculpture, a rolling pin or bottle for rolling the clay, and a knife for cutting the clay. You will also need a small brush, some petroleum jelly, a marker, and a sponge.

·       Begin by deciding how you will build your plaster parts so that they can be easily taken apart. Notice any undercuts or places where the plaster will get stuck trying to come off the mould. If it helps, mark the line where the plaster will separate.

·       Roll out the clay and cut it into strips no more than 3 cm wide.

·       Use the clay strips to build a wall along the plaster parting line. Use the excess clay to lump behind the wall, leaving one plaster section free of clay. The wall should be firm so that it won’t move when the plaster piece is built against it.

·       Use the back of a pen or marker to press “keys” into the clay wall. The keys are small indentations that will help the plaster pieces refit together easily once the mould comes apart.

·       Mix the plaster according to the package instructions. Mix enough for at least 3 cm thickness of plaster where you are working on the mould.

·       When the plaster is still liquid, fill the key holes and any areas that might prove difficult to fill later on. Add more plaster as it begins to set.

·       Build an even section of plaster. Take special care to make the edges of the section thick enough. Before it hardens, make sure the top of the clay wall is visible. Run a knife along the parting line.

·       Add knobs of plaster to the section: these will help hold the rubber bands in place when putting the mould back together.

·       The final step is to smooth out the section of plaster. The easiest way to do this is take a damp sponge to smooth out the plaster.

·       Let it set completely.

·       If you have more than two sections of plaster to build, then continue this process of building a clay wall along the parting line. Remove the clay wall from the plaster section previously built, and use the petroleum jelly to act as a release agent between plaster sections (paint it on with a small brush, and thin it with turpentine if necessary). Otherwise, simply remove the clay from the mould, paint the petroleum jelly on the side of the plaster section, and build the second section the same as the first.

Making a Piece Mould

·       If you have a complicated sculpture with too many undercuts and deep areas to make a simple single skin mould, it might be easier to do the silicone in parts.

·       This is done the same way as the plaster parts: simply build a clay wall around the parting line of the sculpture and paint on the layers of silicone in sections.

·       This method takes much longer; but when it comes to taking apart the mould, it allows you to avoid the difficult job of cutting the silicone off the sculpture without ruining it.  

Cutting the Silicone and Removing the Mould  

·       Take off the plaster parts of the mould. If you have made it properly, the plaster sections should come off easily. If any sections break, they can be rebuilt or it may be advantageous to redo the plaster completely, using a different parting line.

·       You need to cut the single skin of silicone to remove the sculpture, preferably without breaking it. You want to cut as little silicone as possible. The beauty of silicone rubber is that it can be stretched and pulled gently, almost peeled off the sculpture in places. Therefore, try cutting as little as possible and then removing the sculpture with a little manipulation. You will have to cut where there are holes or gaps in the sculpture. Take this job slowly because hasty cuts can ruin a mould.  

Step Three: Pouring the Wax

The process of pouring the wax into the mould is called “slushing.” The method is simple but will vary from sculpture to sculpture, and it takes a small amount of practice to figure out the best way for a specific piece. To pour any wax, you need at least enough molten wax to fill the mould. Wax can be purchased at foundries or sculpture suppliers. A traditional wax is Victory Brown, which is a soft enough wax to sculpt in your hands, but hard enough to hold it’s own weight for very small pieces.

·       The simplest way to pour into the mould is to set it up so it won’t tip, and fill the mould with molten wax. This often results in lots of bubbles throughout the wax, which is the reason most waxes, solid and hollow, are poured in layers.

·       To pour a first layer of wax, use wax at a temperature of 105-110° C, ladle wax into the mould until it is at least 2/3 full, and roll it around slowly, coating the entire inside of the mould with wax. Slowly pour it out. This is the slushing method, which is commonly recommended by literature on the subject because it is believed to be the best way to avoid bubbles in the first layer. However, another method is to completely ignore the slow slushing rule, fill the mould 2/3 full of wax, and shake it vigorously until the bubbles float to the surface.

·       Do the following layers by slushing the wax slowly. The second layer should be wax at a temperature of approximately 90° C. The third layer should be 80° C, the fourth at 70°C and so on. If you intend to fill the mould for a solid wax, you probably only need one or two layers. However, the more layers, the less chance of defects and bubbles, but if you pour the solid core too cold, it may not fill small thin sections properly.

·       If you are pouring a hollow wax, try to get an even thickness of wax throughout the piece. This will make the bronze casting go more smoothly.

·       Let the wax cool before opening the mould.

Preparing the Wax
Step One: Cleaning the Wax Sculpture

What is in the wax will be in the bronze, therefore, the sculpture should be cleaned to your liking.

·       Look for seam lines (where the cuts were made in the silicone usually results in a line of wax that isn’t part of the original; this is the seam line), bubbles, holes, miscasts (what happens when the wax doesn’t flow into all areas of the mould), and anything else that isn’t part of the original or to your liking.

·       To fill bubble holes in wax: Use an oil burner that uses a candle to heat the oil: put wax where the oil should go. You may want to melt the wax with a small torch, because it will take a candle a long time to melt the wax. Take a small brush—the best ones for the job have fiberglass bristles that can be lit to heat the brush and smooth the wax with the warmed brush. You can also used a needle for this job. Pick up a small amount of melted wax on the needle or brush end and drop it over the bubble.

·       If you have a surface that is covered in bubbles, try rubbing a wax paste onto the surface to smooth out the texture: you can simply warm some wax and rub it over the surface, or you can make a wax paste out of normal victory brown and beeswax.

·       To smooth seam lines: again, the best tool for this job is a fiberglass brush. Simply warm the brush in a candle flame and smooth the wax with the heated brush.

·       To imitate the original texture of the sculpture try a variety of metal tools that can be heated in a flame, including basic silverware. A butter-knife is ideal. Dentistry tools are great.

·       There are some chemicals that soften wax. Turpentine will dissolve wax: dip a scouring pad into a little bit of turpentine and rub the wax with it. Wipe away the dissolved wax/turpentine with your finger or soft cloth. There is a product called WaxKleen that has a similar effect.

·       Practice and patience are the best tools for cleaning wax.

·       Many sculptors sign their work at this stage, although it can wait. The signature might also be erased in the chasing process.

 Step Two: Sprueing the Wax

Sprueing is the term used for adding sprues, vents, and cups to the wax piece. Bronze is poured into the cup, sprues are the channels through which the molten bronze will flow, and vents are the channels through which trapped air can escape.

Therefore, sprueing the wax is essentially the plan of how the metal will flow into the shell.

·       Sprues, vents, and cups can all be made in wax: To make a cup, take a plaster mould of any cup shape around the outside of the cup. Once the plaster has set, put it in water for twenty minutes (otherwise, the wax will not come off the plaster). Pour wax into the cup and let it cool around the edges (you can see the molten wax dull slightly as it cools), then pour it out, leaving a layer of wax on the inside of the mould. Let this cool and then take it out. This is the cup. Methods for Modern Sculptors, by…, recommends making square cups and square sprues, because it may reduce turbulence in the metal as it flows from the cup into the sprue. Round cups also work well, however.

·       Sprues can be made from rectangular or cylindrical shapes, but should be made as thick as the thickest part of your sculpture. Again, Methods recommends rectangular sprues to reduce turbulence. Vents can be any kind of shape; they do not need to be very thick. Make the sprues and vents using plaster moulds of these shapes. Let the plaster moulds sit in water for twenty minutes. Pour wax into the moulds, and let them cool. You’ll probably need several vents for each sculpture, so make a lot.

·       You want to attach the sprue to the thickest part of your sculpture, where the most bronze will be. This is done for multiple reasons; mainly, you want the metal to flow to all parts of the sculpture, so if you attach the main sprue to a big area, more metal can flow into the sculpture all at once and out into the smaller areas more easily. If you attached the main sprue to a thin area, the bronze might freeze before filling the entire sculpture, resulting in a miscast.

·       Attach a wax sprue by welding the end of the sprue to the sculpture. Welding wax is simple, although delicate: take a metal knife and heat it with a small torch. The knife is hot enough when it melts the wax instantly upon coming into contact with it. Take the hot knife between the two wax ends that you want to weld. Put the ends together on the knife and gently slide the knife out, holding the two waxes together. Test the bond once it has cooled to make sure it is strong enough to hold the sculpture with the sprue.

·       You want to attach the sprue at an angle that will allow the bronze to flow gently into the sculpture. It is often recommended to attach the sprue at such an angle that the metal flows up from the ground to reduce turbulence and encourage a smoother flow of metal. This method of sprueing from the bottom works best when the heaviest part of the sculpture is at the bottom and it might be too complicated for other pieces.

·       Weld the bottom of the cup to the end of the main sprue.

·       If you have two thick sections on the sculpture, connected together by a thinner section, called a neck, you may want to add a feed, which is simply a secondary sprue into those other large sections. Weld all feeds to the main sprue, not the cup, to ensure that the metal flows smoothly. Feeds should also be welded at an angle so that the metal flows up into the feed as it is flowing into the sculpture. This ensures that two flows of metal aren’t created, which sometimes results in miscasts.

·       Attach vents at all appendages where air might get trapped. It is good to attach vents from the appendage to the cup to allow easier dewaxing, but it doesn’t usually make a difference how long the vent is when pouring the bronze.

·       Weigh the final piece and mark the number down for reference when pouring the metal.

 The Investment Process

"Investment Casting" is another name for Lost Wax Casting. Investment is the term used for coating the wax in a refractory ceramic shell. Once the shell has dried, the wax is then melted out in a process called dewaxing. 

        The Primary Layer(s)

·       Before shelling up, clean the wax with a mixture of dish soap and isopropyl alcohol at a ratio of 50:50. You can also use shellac over the wax. Brush on the mixture and let the wax dry.

·       Throughout the entire investment process, you will make a slurry: a mixture of molochite flour and colloidal silica. For the first layer, you want a thin creamy slurry.

·       Paint the slurry onto the wax. You can also dip the wax into a bucket of the slurry if you have made a lot. Make sure the entire wax is covered, including vents, cups, and sprues.

·       Apply fine sand over the wax. For the first layer, use zircon sand. The thin slurry and very fine sand will catch the details of the piece.

·       Let the wax dry. Set it up in front of a fan to accelerate drying times. Drying times vary depending on the temperature and humidity of the area.

The Secondary Layer(s)

·       Make a thin slurry; more milky than creamy.

·       Apply the slurry to the piece: if the previous layer comes away, it is not dry enough.

·       Apply molochite 30-80, which is heavier than zircon sand, to the entire piece and set it aside to dry.

           The Final Layers

·       These layers are for adding thickness to the ceramic shell. Therefore, make thicker creamier slurry.

·       Apply heavier molochite sand.

·       Depending on the size of the piece and amount of metal to be poured, the number of layers needed will vary. For smaller pieces, a minimum of five layers is sufficient. For larger pieces, up to eight layers is standard. If the shell is too thick, the metal will not “breathe” and trapped air won’t be able to escape through the shell. If the shell is too thin, it may crack or break during the dewaxing.

·       Let the shell dry at least 24 hours before attempting to dewax.

          Dewaxing

·       In a foundry, dewaxing is done in a furnace at such high temperature that the wax burns out, leaving the empty shell. This is done in a matter of a few minutes. You can build a furnace for dewaxing, leaving the shelled piece standing on a grate, with a tray a little below to catch the wax as it drains from the shell. Another easy way of dewaxing is standing the pieces on a grate with a tray or metal pan below to catch the wax, and using a large torch to heat the piece. An advantage of draining the wax from the piece and catching it in a metal pan is that the wax can then be reused.

·       Take care to set the piece up (cup-side down) so that it won’t fall over and break. Once the shell is empty and before it is baked, it is at its most fragile stage.

·       If you are using a torch to melt the wax, it is best to heat from the bottom and move the torch slowly to the top of the piece. This ensures that the wax has somewhere to go when it starts to melt. The wax, when heated, expands. If it has no space to breathe, it expands against the rigid shell, which cracks. Most shell failures occur at this stage.

·       Continue to torch the shell until there is no more wax melting out of the bottom of the piece, and no more flames indicating burning wax.

·       Gently set the shell up somewhere safe to cool.

Reinforcing the Shell

·       Often during dewaxing, the shell cracks. These areas must be reinforced so that the metal doesn’t leak out of the shell. There are also areas on the shell that will experience more shock as the metal pours into the shell. These areas should also be reinforced to ensure a strong shell capable of withstanding the pour.

·       Mix a very thick slurry.

·       To fix visible cracks, take a small torch and heat the slurry as you paint a little bit at a time over the crack. This will make sure the slurry dries before dripping into the shell.

·       To reinforce a general area, paint on slurry. Over this slurry, lay strips of fiberglass cloth and paint slurry over the fiberglass. Sprues should be reinforced as a rule, as well as the areas where the metal will fall first.

·       To fix clean breaks where the shell hasn’t shattered: put the two pieces together at the break and paint slurry as you heat it, taking care not to drip slurry into the shell. Once the break line is covered with dry slurry, it should hold enough to where you can reinforce it with fiberglass.

·       Let the reinforced shell dry for two or more hours.

The Pour
Baking the Shell

The shell should be baked in a furnace. A simple furnace can be made from refractory bricks lined with "k" wool. K wool is toxic to the lungs, so it's best to avoid it.

·       The shell must be baked at high enough temperature so that the shell remains red hot for at least twenty minutes. This process strengthens the shell enough to withstand the contact with molten metal, and it ensures that no moisture remains in the shell. You can tell that the shell is hot enough when it has a red almost transparent appearance.

·       Let the shell cool down slowly. Leave it in the furnace until the metal is almost ready to pour.

·       Set up the shell to pour. It must be placed in sand, with the cups up. The sand is a safety measure against spills or leaks. Take care not to get any sand or other material into the open shell. Set the shell up so that it is easily accessible when you are ready to pour. When you pour, you should take the crucible from the furnace and easily pour into the shell without having to walk backwards and without having to move around too much. Think about how you will have to pour the metal, and set up the shells to allow for the smoothest possible operation.

·       Another option is to bake the shells until the metal is ready to pour, and then to pour the metal into red hot shells. This method captures very fine details but often results in miscasts. Most literature on the subject says to pour into very hot shells, but past experience has proved more luck with cooler shells. It is something to experiment with.

Pouring the Metal
    To pour molten metal into the baked shells, you need a furnace, a crucible, a pair of tongs for lifting the crucible, and something to hold the crucible when you pour. This is typically a long metal handle with a metal ring on one end that holds a certain sized crucible. A manageable size for a single person is a 10K crucible. Above that you may need another pair of hands to help carry the weight. Then you would also have a long metal handle with a ring in the center and a person at each end lifting the molten metal. Also, each person involved needs a pair of heavy gloves, a face shield, long sleeves and pants, and appropriate footwear. No skin should show when you pour hot metal. It is a good idea to have a bucket of cold water nearby.
     Gas furnaces are more highly recommended because they melt metal quicker, although there are charcoal furnaces that work. If you are building a furnace, there are some things to keep in mind: It should be cylindrical to allow for even heating throughout, with the torch entering the furnace near the bottom. Use a propane torch. See Methods for Modern Sculptors for some good tips and ideas. Also there is a good reference online at http://www.artmetal.com/project/TOC/proces/cast/ag_cast.html

·       Load a crucible to the desired amount of metal. If you weighed the piece before investing, then you know about how much you need, but it is wise to have a little extra.

·       Place the crucible in the furnace

·       Safety check: make sure there is nothing in or around the furnace, including the area above, that might catch fire. Check the gas connectors and regulators against gas leaks. Make sure safety equipment is nearby. If you happen to get burned during the process, immediately immerse the burn in cold water.

·       Begin melting the metal.

·       If you are using a gas powered furnace, it will take approximately twenty minutes to melt the metal. This time varies from furnace to furnace, and takes longer for charcoal furnaces. For bronze to melt, it needs to reach a temperature of approximately 1300°C.

·       Check to see that the metal is the correct temperature: there are expensive electronic devices called pyrometers that can tell you to the exact degree the temperature of the bronze, but, lacking this, you can check it with a steel rod. Dip the rod into the crucible and wait five seconds. If the rod comes out clean, the metal is ready. If the rod has metal stuck to it when you pull it out, it needs more time.

·       When you are ready to pour, turn off the gas, and put on gloves and face shield.

·       Pull the crucible out of the furnace with heavy tongs, and either place the crucible on the ground if you will be using the tongs to pour, or place the crucible into the metal handle. Remove the "slag" with a metal scoop. The slag is the term for the impurities in the metal that float to the top when the bronze has melted.

Jillian Varani pouring molten bronze under guidance of Olivier Duhamel
(Photo Maeva Bodin - © 2008)

·       Pour the bronze and pour any extra into a ready made refractory cup. Lay the crucible on its side to make cleaning the crucible easier.

·       If there is a leak in a shell while pouring, leave it and move to the next one. You can try to salvage the leak, but it helps to have someone else do this while you pour: take a wet cloth and press it against the metal leaking from the shell. This will freeze the metal coming from the shell and allow you to continue pouring into the shell until the metal fills the cup. This is very dangerous.

·       Let the bronze cool fifteen minutes or more. Many experts say to let the shell cool down slowly over a matter of hours or even days. However, after fifteen minutes the metal has hardened, even though it is still hot. Take a pair of tongs and drop the piece into a bucket of cold water. Again, most experts say not to do this, but nobody seems to know why not. If you know why, please send me an email.

·       To get the ceramic shell off the bronze, knock the sprues with a hammer. The vibration should be enough to get most of the shell off the bronze. In deep areas, it may be necessary to take a chisel and carefully knock away at it. As you get into smaller crevices, smaller files, picks, and even needles come in handy.

Chasing and Patina
      The final steps in the bronze process are cleaning the metal (chasing) and coloring the bronze with oxides (patination). Often the bronze comes out of the shell full of defects, effects in the bronze that weren't in the original wax. These include everything from surface problems to holes and miscasts, where the metal hasn't run fully into the piece. Most of these problems can be fixed with a bit of chasing. Once the piece is chased to an acceptable level, the sculptor either does a patina or sends it to a patina master.
    Chasing

        Tools for chasing include sandpaper or a course pad such as scotch brite, files of various sizes, and chisels and punches. A very handy tool is a dremmel or handheld rotari tool. These typically come with many useful attachments for cleaning very small places. You will also need a saw to cut the sprues and vents, a sander to flatten the bottom of the piece, and grinding wheels of various size and grit. The best tool to have, if you can afford it, is a sandblaster.

·       Cut the sprues and vents with the saw. If the sprue was attached to the sculpture at a flat area such as the bottom, then simply sand it flat. If it was in a rounded area you will need to reshape it with grinding belts and then sandpaper and a dremmel.

    List of Common Problems/Defects

·       Miscast: Often unfixable, this happens when bronze doesn't fully flow into the empty shell. Possible reasons include gas trapped in the shell, or more likely, a poorly sprued system. Try recasting with a different sprue system. If the missing area is very simple in shape and small enough in size, if may be possible to weld a lump to the sculpture and shape it back in the metal. The problem with welding/braising is that it requires another whole set of skills and knowledge, and the metal won't be quite the same alloy as the bronze, which will result in different color effects during patination.

·       Fin/"Flashing": This occurs when there is a crack in the shell before the pour. It is easily fixed by carefully using a hammer and chisel or using the grinder wheel on the rotary tool.

·       Protruding Rough Surface: Often this happens because either the shell is too hot at the time of the pour, or too cold. It's one of life's mysteries, but fortunately also easily fixed with sand paper or again, the rotary tool.

·       Bubbles/depressions: These can appear as a result of gas or other organic material trapped in the shell. Sometimes, these defects can be ignored or hidden under a dark patina, or if they're really obnoxious, then braising is an option. Braising bronze takes a little practice but the technique can be learned quite easily: heat the entire sculpture: put a little flux on the hole. Then take a bronze rod, of thickness depending on the size of the hole you are filling, and melt a little of the end into the hole/bubble, until it is filled. Let the sculpture cool and chase back the excess braised metal until you have the surface you want. It may take a few tries. Like welds, braised spots will also show a different color to most patinas than the bronze sculpture.

·       It's difficult to pinpoint the cause of most defects. As with all skills, practice much, have patience, and soon you'll know the best techniques for bronze casting.

    Patina
    Patination is the chemical process that oxidizes the bronze to a specific color. Many colors and effects can be achieved with a few chemical mixtures and a little practice. The basic technique for any patina involves a propane torch, a brush, the chemical, water, and a soft cloth.
    List of Chemicals, Techniques, and Effects:

·       Liver of Sulfur: Used for the base coat and dark browns to black, liver of sulfur is usually purchased in its unmixed form, as small rocks. The mixture ratio is approximately 5mL liver of sulfur to 1L water. To make a good light grey base coat, submerge the bronze in the liver of sulfur mixture for half a minute, or paint it on until the color change is apparent all over. Rinse the sculpture in water. Use the torch to evenly heat the entire piece. You will notice the moisture evaporate first, then the color should go almost bright blue. Using a clean brush, polish the blue while heating to make a beautiful silvery grey base coat. To make a dark black, keep applying layers of liver of sulfur, although you don't need to keep brushing the heated sculpture.

·       Ferric Nitrate: Used for browns, oranges, reds, ferric nitrate is usually purchased in its liquid form and applied diluted with water. It is applied hot or cold. To apply it cold, brush it on all over the sculpture and let it dry or dry it by heating. To apply it hot, heat the sculpture and use a brush or a spray bottle to apply it. It should sizzle as it touches the sculpture. If it burns the brush, the sculpture is too hot, if the liquid runs without sizzling, the sculpture is too cold. To achieve red and dark browns, you need more layers. Experiment to achieve different colors.

·       Cupric Nitrate: Used for blues and greens, it is usually purchased liquid and applied hot or cold.

·       Bismuth Nitrate: Used for tan and white. For tan, mix with ferric nitrate. You need very little bismuth to have a noticeable effect.

·       Experiment to achieve different colors. There are a huge amount of other chemicals, some are specific brands sold as "blue" or "tan," and some you can find pure chemicals. With just liver of sulfur and ferric nitrate, you can achieve a spectacular range.

·       For spots, dab the chemical onto a hot sculpture.

·       For rings, use a circular shaped brush.

·       For a granite effect with many little spots, try using a spray bottle onto a hot sculpture.

·       Try fading from one color to another.

·       Safety: For any hot application, use a mask. To throw away waste chemical, mix with plaster before dumping.

    Finishing Touches: Signing and Sealing
    Once you have achieved a patina to your liking, sign the piece if you haven't already.

·       To sign the metal, you can use a little engraving attachment for the rotary tool. If you are making a series, decide beforehand how many are to be made, and make sure you put the correct number onto the piece. It is helpful to make a log of all previous sculptures to keep track. Series are signed "number of edition/number in the series." If you don't limit the number in the series, put UL for unlimited.

·       Usually artists use a hard wax or neutral shoe polish to seal the patina. Remember that a wax sealant may slightly change the appearance of the patina. Heat the sculpture evenly and brush on melted shoe polish over the entire sculpture, even the bottom. Let it dry before touching it.

Making the First Wax
   

Preparing the Wax
    

The Investment Process
    

The Pour
     

Chasing and Patina
       

Finishing Touches

   Signing and Sealing

 

© Copyright Olivier Duhamel 2003-2009