I've been regularly asked about the lost was lost-wax bronze process. Common on every continent, except Australia, the lost-wax method dates from the Red millennium BC and has sustained few changes since then. Here is a simplified summary of this process I been learning at Stuart Roussel's facility.
1. Original clay figure
I quickly modeled this woman's face specifically for this exercice that requires a simple and small piece.
2. Application of the silicon rubber mold
Excellent surface replication of the original can be achieved with a high-quality silicon rubber (RTV70), that will pick up the details of the work to the nearest micron.
This mixture is applied directly to the surface of the clay, starting by the underside of it, using a brush to actually "paint" it onto the surface. Special attention needs to be given to avoidance of air bubbles that may be trapped within the rubber mixture.
3. Laying cutting wires
Wires are used to cut the plaster shell that will clamp the RTV mold. We use yarn for the boat sail repairs due to its resistance.
The number of cutting depends on the complexity of the piece, here on very simple shape like this face, a cut in four pieces will be sufficient.
4. Plaster shell
Once the coat of silicon rubber has dried, a firm outer "shell" is made to help retain the shape of the more flexible rubber mold for pouring the wax replica.
The wires are pulled before the shell has completely hardened to ensure separation of the various parts. Then the shell is allowed to finish hardening.
We also make sure to flatten the top that will become the base of the shell.
5. Opening of the shell
The shell is opened according to the previously made cuts and the original piece is removed from the mold. It is recommended to number the different parts of the shell before opening it, especially for complex sculptures that require many cuts, in order to facilitate its reconstitution.
6. Silicon rubber old removal
The silicon mold is removed like a glove. The sculpture has now gone from a positive form to a negative form.
Then it is carefully cleaned to get rid of any impurity that would be imprinted on the wax and consequently later on the bronze.
7. Completing the "mother mold"
We put the mold back in its carefully closed shell in order to avoid any distortion during the wax casting process.
This is where the flat base is coming in handy.
8. Ready for the wax
A positive form can now be generated by pouring a wax replica in the negative form of the "mother mold"
9. Wax casting
The wax is poured in three stages with temperatures of respectively 75°C, 68°C and 60°C.
The ideal thickness of the wax is of 2 to 3 mm, which will be that of the final bronze.
10. Building the edge of the mold.
We brush lukewarm wax around the opening to build an edge on which fine sprues and gates will be welded.
11. Cleaning of wax drips
We carefully clean the wax drips making sure no debris is falling in the opening.
12. Installation of the sprues and gates
This is a most important step.
A pouring cup and a network of wax pipes, called sprues and gates, are attached to the wax ledge previously built. These pipes first will allow the wax to escape as it melts. Later, they will enable the molten metal to flow evenly throughout the mold and also let gases air escape as the metal is poured.
To consolidate the wax copy of our sculpture, we pour a mix of plaster and chamotte inside the wax shell and around the sprues and gates, being careful to let their ends free of plaster (here the bad pupil had to clean it a bit).
14. Wax copy of the original sculpture
Once the plaster mix has hardened, the shell is opened and the silicon mold removed to free the wax positive replica.
At this stage, it is possible to repair all of the imperfections (few air bubbles, seams and mold lines) created during the pouring process to maintain the original details within the final mold. It is usually removed by using small scrapping tools and a hot tool to fill in the "holes".
15. Installation of the last vents
We install the gates on the risky parts of the piece (here nose, chin and chignon) where a bubble of air might otherwise be trapped.
These rods will be welded to the standby wax rods that have been clamped in the plaster mix.
The base will then be "hammered" and wet to get an irregular surface on which the refractory mix will better stick.
16. Applying the refractory mix
We apply on the wax figure a mix of plaster and alumina which will constitute the fire resistant negative mold capable of supporting the molten bronze.
This operation is very delicate because this new mold is the that will give its final shape to our bronze sculpture. It is applied by brushing it on the wax replica making sure to avoid any air bubble that will prevent the fireproof mold to take the perfect impression of the figure.
17. Finalization of the cylinder
A plaster/chamotte cladding comes around the refractory mold which is very fragile when not fired.
The presence of a dog is optional.....
18. Dewaxing process
The cylinders are laid upside down or on their side so that nothing obstructs the visible wax pipes. The wax will melt and flow out of the mold, leaving a space between the fire resistant clay mold and the central plaster mix. The molten bronze will later occupy this space.
This is why this method is called the lost wax process.
This step also hardens the refractory shell in order to prepare it for the extreme temperature of the molten bronze.
The cylinders will be fired for 15 hours at 750°C. The whole process of slowly reaching the requested temperature, firing the molds and cooling down to 250°C, take more than 24hrs.
The bronze lingots are not very attractive at this stage.
The melting temperature is 950°C (beginning of the liquid state) but it will not be poured before 1150°C.
20. Bronze melting
Wonderful show during which we keep busy.
21. Positioning of the cylinders
Still hot cylinders (250°C) are placed in containers and wedged with tightly packed sand to prevent them from moving when the bronze is casted.
The openings of the gates and sprues are obviously placed on top to received the molten bronze.
When handling the cylinder, we must make sure that no dust or impurity enters the holes as they would later be imprinted on the bronze sculpture.
22. Removing the crucible from the melting furnace
When the bronze has reached the correct temperature, the incandescent crucible full of liquid bronze is removed from the furnace and mounted on a frame positioned at a specified angle to let the metal run off into the mold.
23. Bronze casting
The moment we've been waiting for : our first bronze casting !
Molten bronze is poured in the space left by the "lost" wax.
A whole week of hard work for a few seconds of pure magic !
24. After the casting
The magnificent flow of liquid metal is cooling and slowly becoming blackish.
25. Extraction of the bronze statue
This process needs to be done very cautiously in order not to cause unwarranted damage to the metal surface, very fragile while still hot.
It is then immersed in cold water to cool it down too a more "handy" temperature. Any remains of the plaster shell and fireproof clay mold left inside the hollow bronze are now removed with high pressure treatment. The professional foundries prefer sand blasting process.
27. Raw bronze
The bronze sculpture and its sprues and gates are an exact reproduction of the wax in step 15.
28. Feeds/vents cutting
Once the bronze has been cleaned, the sprues and gates which have now become metal, must also be cut away or sawed off.
The most obvious faults are removed before chasing operations.
30. Problem !
Here it is lacking bronze, probably because of too thin a layer of wax at this spot. Similarly, small bubbles or air on the nose and forehead will lead to some repairs.
31. Welding repairs
We will repair by welding or braising process. Heating the sculpture again and using bronze rods. Professional foundries use the much quicker arc welding process.
32. Done !
The lady's nice hairdo is back.
33. Bronze chasing
The surface of the bronze is then polished, chiseled and filed so that trace of the sprues, gates and repairs can be seen. This process of hand-finished the bronze to perfection is called chasing.
A patina not only protects the sculptures, but also gives it color. It is a step in the making of the finished bronze wherein hot or cold oxides are applied to the surface of the metal, creating a thin layer of corrosion. This layer -of various colors from slightly brown to green or even blue) is called the patina.
The patina protects and enlivens the surface of the bronze.
Here is the bronze replica of my original clay sculpture. The patina is not quite what I had in mind, but patina is an art no one can master that easily.
A word about patina : it is extremely important for the enhancement of a sculpture.
Just as a framing can spoil or enhance a painting, a wrong patina can "kill" a sculpture and a right one can highlight impressively a piece of art.
Moreover, many artists try and develop their own patina which contributes to the immediate recognition of their work.
Of course professional founders have experienced specialists who will suggest and make wonderful patinas.