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Cat and Bird - Bronze

Cat and Bird - Bronze Sculpture

Another sculpture - more abstract than is the wont of CooperToons - quote - "serious" - unquote - art. As with the terra cotta sculpture, you can view the figure from various angles. If you prefer to shift the positions with a click of the mouse (opening in a new window from the General Art page) click here. For an automated page, click here

Now bronze sculpture is something you do not try at home. Most professional artists who routinely create statues in bronze do not do the actual casting themselves. It is a long and at times arduous process and something you never do except at a properly equipped foundry with trained personnel. If you're not expert, you have to be under direct supervision of a qualified teacher. This was cast under the criterion.

And how bronze sculpture done? Well, to paraphrase Captain Mephisto, it's very simple really but again don't try this at home.

First make a statue using a "plastic" medium like clay. This should be made the way you want the bronze to look - perhaps with details made a bit deeper. This original is often made in a water based clay that can harden. But some are made in oil based clays.

Secondly, you make a mold of the original. Traditionally the mold is made from plaster although now latex molds with an outer plaster shell are often preferred since the detail is kept better. But making molds is a complex process and has to be done in parts so you can detach the mold from the original and reassemble it to form a hollow shell with an inlet (but no leaks). Of course, if it's not done properly you can completely destroy your statue or have the mold "lock" on the original.

Third, you make a wax cast from the mold. Small statues can be cast solid but in larger bronze works (when you need the final statue to be hollow) you pour the hot wax into the mold and roll it around to coat the inside. This leaves a thin film of wax on inner surace and you then pour the excess out. After the wax film cools and solidifies you then pour more hot wax in. This builds up the layer of wax, and you repeat the process until you get a wax film of about 3/16 of an inch thickness. You then detach the mold from around the wax model and you have a hollow wax replicate of the original. Needless to say, things like mold release agents and careful technique are important. If the statues are particularly large, they have to be cast in pieces and welded together.

The fourth step is to mount the model on a "sprue" (a fairly short but thick stick of wax) attached to a cup (the thicker paper drinking cups - say 8 to 12 oz - are surprisingly good for statues even of moderate size - but don't use plastic). This gives you a handle to hold the model and also forms an inlet to pour the bronze. Thinner sticks of wax are attached to form vent lines in the proper places (advice from an expert is needed here) and certain parts of the statue may have to be connected by wax rods to prevent pockets of air forming in areas in the mold. Putting a lid over the opening of the cup is helpful when you move to the next step.

Fifth, you dip the wax model - including the cup - into a special solution of colloidal silica material in water. The preparation of the silica is itself pretty involved and requires special ventilation and protective equipment.

Sixth, roll the wax model (now coated with the silica slurry) in very fine sand. The sand must be fine enough to preserve the details of the model. The lid on the cup prevents the sand from getting in side the cup (a little bit doesn't hurt). You also do this in a specially ventilated room and wearing respiratory protection equipment.

Seventh, let the model dry at least three hours with a strong fan blowing over the surface. Then dip it once more in the silica slurry followed by the rolling in the sand. Again let it dry for three hours.

Eighth, you do eight more dips in the silica and sand (ten in all) and every two dips you switch to a coarser grade of sand. Between each dip you again let the model dry three hours. As you may guess, building the shell (as its called) isn't done in a day.

Ninth, you remove the lid from the cup and place the wax model, now covered with the hard (and heavy) ceramic shell, into a special oven that heats up to 1600 degrees Fahrenheit in a couple of minutes. Some of the wax melts (which runs into a special trough and into a receiver) and the rest (such as any that remains in the shell) gets burned out. The paper in the cup also burns away. You now have an empty ceramic shell with an opening.

Tenth, if the mold is set aside for another day it must be preheated before pouring in the bronze. A temperature of 1600 Fahrenheit is again used and this prevents the bronze from solidifying too soon. At this point everyone puts on protective aprons, jackets, gloves, leggings, and face shields. The bronze itself is melted in a blast furnance and the molten bronze held in a graphite/ceramic crucible. You have to use special - bit scarecely high-tech - methods to keep the crucible from sticking to the bottom of the furnace. A small crucible holds around 90 pounds of bronze and with proper equipment a crucible with 200 pounds of bronze can be handled by three people. The molten bronze is not removed from the furnace until it reaches 2200 degrees.

Eleventh, with great care and proper technique (again details are omitted lest the foolish try this on their own), the hot molds are placed upside down in a bed of sand in what in essentially a bath tub without a bottom. The molten bronze is then lifted by a hoist with a special device for pouring. The "pourmaster" directs the operation, the "dead end" holds the other side of the frame, and a crane operator controls the hoist to raise and lower the crucible.

Twelfth and with (obviously) great care, you pour the molten bronze into the molds. You let the bronze cool - usually this takes a couple of days even for small works - and then you break away the mold with a hammer (and hopefully don't damage the statue). You then cut off the vent lines, sprue, and any extra metal. You next sand blast any residual ceramic residue (sometimes high pressure water is used), and grind and polish the surface as desired. The sand blasting can be repeated after the grinding and polishing to produce a "matte" surface.

Most bronze statues are then "patinaed" to - strangely enough - remove the bronze look. This is done by heating them with a blowtorch and brushing on a water solution of inorganic materials. Naturally some rather nasty fumes come off, and you have to work a special table or area of the foundry with an exhaust hood or fume snorkels. Copper nitrate forms copper oxide which is the green color you usually see on bronze statues. Other minerals will give different colors.

The statue shown here was patinaed using "liver of sulfur" or sulfurated potash, which is chemically, a mixture of 25 % potassium thiosulfate hydrate and 75 % potassium trisulfide. When dissolved in water and brushed onto the bronze the copper metal, the thiosulfate and trisulfide becomes the black cupric sulfide. Although it is isn't necessary to use a blowtorch if you patina with liver of sulfur, it does give a much nicer looking surface.

Finally you usually wax the surface. In old days beeswax was commonly used, but today good old car wax is a bit more convenient. Waxing is done best with the metal being hot enough to melt the wax, but not hot enough to burn it or make it smoke. Once the statue has cooled to room temperature, you then buff the surface with a soft cloth. Hey, presto! You have a bronze statue.

Another bronze statue was also cast along with this one. It may be posted at a later date.