Monday, April 9, 2012

The Lost Wax Process Using Ceramic Shell Investment

This project is my response to the millions of gallons of oil spilled into the Gulf of Mexico and the recent nuclear catastrophe at Fukushima Daiichi  in Japan.

It's about a legacy; an effect we are having.

The original solid clay pattern was thrown using a steel armature and jig frame built over a Brent electric wheel.  Pictured is the seven plaster molds taken from the original clay form.   

The wax patterns gated and vented.  Indirect pour sprueing systems deliver the bronze to the pattern quite effectively as gravity builds the pressure inside the shells from the weight of the metal behind it coming from the crucible.  Indirect systems also delivers the metal from the bottom up allowing the air in the shells to exit smoothly. 

This is a photo of me applying the fused silica powder to the wax pattern prior to submerging them into a colloidal silica based slurry. 

After the fourth dip and sand, wrap fiberglass strand around the patterns.  The fiberglass reinforces the shells and provides strength to hold the weight of the metal inside while it is molten.  I've also used paper clips on the edges for extra strength, and fiberglass sheet strips to repair big areas.
 I stopped at 8 total dips, with a final slurry dip without sand to seal in the layers.  If this last seal coat is not applied the shells will flake silica sand when ever it is handled.  Very messy!
The shells cup lips are ground down with a grinder, revealing the wax, as well as vent slits are also cut at the very bottom.  These vent slits assist with releasing vapors and pressure from the expanding and melting wax.  Wrap the shells with black bailing wire and hang upside down in the wax burnout kiln.  Our burnout kiln is a steel 55gal drum lined with kaowool suspended over a water basin. 
Heat the shells slowly in the burnout kiln allowing the wax to melt out.  When the wax cup melts the inside shell portion drops out and wax will begin to consistently pour out.  Continue to increase the heat in the kiln until no more wax is dripping out, and the shells and interior of the kiln have a red glow.  I let the shells cool and pulled them from the kiln
At this point the wax is burned out, but there still is residual carbon left behind.  This carbon will create swiss cheese vapor inclusion in the metal castings.  To get rid of the carbon and strengthen the shells completely, load the shells (cup up) into a gas or electric kiln and heat to 1550F and hold for an hour.  Allow the shells to cool, unload and patch the vent slits, any cracks that occurred, and  reattach any broken sections using a high temp refractory cement and a little hot glue.  The hot glue is used to temporarily attach the pieces while the refractory cement is drying.  The type of refractory cement I use is called Econoset 50.
After all patching and repairs are made, the shells are reheated to 1300F in an electric or gas kiln, while the metal is being heated in our Speedy Melt furnace in a silicon carbide crucible.  Right before the metal is ready (bronze = 1950F), pull the heated shells from the kiln and partially bury the shells upright in sand to hold them upright and provide additional support during the pour.  Pictured is the shells with the bronze cooling inside.
As the metal cools the shells begin to crack under the pressure of the shrinking metal.  Once they are cool to the touch chop off the majority of the ceramic shell using a hammer. 

These are the bronze castings with all the ceramic shell cleaned off.

I then cut off and ground flush the gates, and sand-blasted the remaining shell off, leaving a very clean and uniform metal surface.

Pictured is the final bronze pieces being welded together using a TIG welder.  I had to tack-weld the whole piece together first and weld 4" sections at a time and change locations frequently so as to keep the sculpture from warping under the heat expansion/shrinkage of the welding process.

A link to an article focused on Lake Tahoe Community College Metal Arts Program:
Hard Work: Metal casting and fabricating classes at college turn out works of art

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