BRONZE CASTING

Bronze casting, step by step...

 

1 Spruing

The model is prepared to answer the demands of molten bronze flowing… A precise and sometimes long conversation must take place. The most serious foundries even have 'committees' in order to come to a final decision regarding the spruing of a wax model! All revolves around 3 priorities: Bronze must flow in as freely as possible while air must be given all possible chances to escape, formation of gas must be prevented. Any trapped bubble of air mercilessly results in missing bronze. Any formation of gas will make the bronze look like the surface of the Moon… The making of the silicone mould already took care of many technical necessities required when casting wax (in-gates for material coming in and vents for air to escape) but casting bronze asks for a 'more' you do not want to ignore if your aim is to have a bronze version of your creation! . For instance, casting 'hollow' requires the making of a core. This core needs to be secured and I must 'pierce' my statue with spikes or needles. This core might also need to be vented and I might have to insert a copper pipe very deep inside my statue. It might also occur that the feeding channels and the air channels I created for my silicone mould might need to be modified to provide the best possible bronze circulation-flow in the investment mould. The French like to 'force' the flow of bronze till it reaches the bottom of the sculpture before channeling it up through the sculpture and when I apply this method, I find myself creating an amazingly beautiful very complex carefully planned new net of channels in wax which I attach (wax welding) to the foundry model. This technique is extremely demanding but allows the casting of very thin bronzes of exceptional quality. The Dutch, more practical, create a more simple net which has the bronze flowing directly into the sculpture. Depending on the project, I use one of the 2 traditions.

 

2 Lost mould for Solid Investment

The sprued foundry model is 'invested' in solid mould which purpose is double: allow the wax to melt away (therefore the name 'Cire Perdue' in French or Lost Wax in English) for the bronze to take its place. It is a one and unique time mould which I have to break in order to extract the rough cast version of my statue… The more modern foundries use a technique called 'ceramic shell' although the shell is not made of ceramic but of a slurry of silica and sand like stucco but, so far, I always preferred the traditional plaster and gravel mixture:1/2 water, 1/4 plaster and 1/4 gravel (brick dust has excellent refractory qualities). The proportions are important. If my mixture is too liquid, my mould might not be able to sustain the pressure of the bronze flow and crack open resulting in 'flashes' which can very well, when thick, cause irreparable deformations. When too hard, the mixture might not stick accurately to the surface of my model, also resulting in extra volumes which eventually can ruin my statue. This mixture must generously cover the entire outside form of the foundry model, including the net of channels and paths. The first layers I carefully apply with a brush to make sure I capture every detail of the statue. The rest, I apply by hand-projection or by creating a 'wall' around my piece which can allow me to simply pour into my wall as much mixture as I need. Between the projected layer and the poured layer, I might want to re-enforce my investment mould by creating a wall of chicken wire. My mould will then have more chances to do well when it will experience the high pressure of the molten bronze flow. How thick should my mould be? Generally speaking, my wax should be covered by a minimum 5 cm thick 'shell'. I can give any external form to my investment mould, but often, it is cylindrical or rectangular.

 

3 Creating a core or cast solid bronze?

To each artist his own but I prefer, when possible, to cast hollow and as thin as possible! Not to economise bronze but for elegance and refinement. Thin is a sign of quality, just like it is in glass, crystal and porcelain...To allow my statue to be hollow, the inner part of the sculpture must be filled with a refractory mixture. Called 'core' (or 'anima'), this inner part of the mould must be carefully secured (by nails or metallic pins of sort) because, once the wax is gone, it will have to 'float', so to speak, but not move in the void. Failing to obtain a perfectly stable core has terrible effects… If the core comes to touch any part of the mould, the bronze will stop flowing resulting in 'missing' parts. If the core comes too close to any other part, its flow is so reduced that my statue offers paper thin (unworkable) skin.

 

4 Burnout

The investment mould is placed feeding-cup (main in-gate) down, in a huge oven where it must stay for several days (depending on its mass), slowly reaching a good 750 degrees. 2 reasons for this: a) the wax will melt and flow out the investment mould and be collected, somewhere outside the oven, in special 'drawers. The wax being gone I have… void! It is precisely in this void that the bronze will flow and solidify. If the wax could not burn or melt away properly and entirely, gas will form during the casting and my bronze surfaces will have an ugly Moon skin... full of craters. b) If water does not evaporate entirely from the mixture, I will also verify some gas traces. No wonder founders are always very serious about their ovens…

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5 Bury the mould

Bury my mould in a special sand (or other method) so that it can resist the pressure the pressure of a flow of 1200 degrees molten bronze.

 

6 Pouring molten bronze

Most impressive photos we see everywhere! The investment mould, free from any traces of wax or water and vacuum cleaned of any possible impurity (like little particle of plaster), is made furthermore ready to receive the molten bronze by being turned bottom side up to expose the feeding cup in which the bronze will flow. The cleaned downside up is now buried into a special container (metallic ring or tube filled with what is called Belgium sand (also used in the sand casting technique) which must literally hold the mould into place. To that effect, the sand is firmly pressed all around the mould (air compressed hammering is perfect tool for the job). The precision with which this hammering is done matters a great deal for making sure the mould will not move but also not crack under the pressure of the bronze flow resulting in very damaging 'flashes' which solidify outside the mould but are attached to the statue inside. Some flashes can be beautiful by themselves though and I used to collect some of them (when I would see a form) with the intention to, someday, exhibit them under the title 'Accidental Art'! Important too is to sustain the effort to keep the inside of the mould clean. The vacuuming of any possible plaster impurity was phase 1, protecting the feeding cup from receiving any new impurity, like Belgium sand during its placing or hammering, phase 2. I seal in any way possible (taping pater the feeding cup looks marvel) till the very moment the bronze will flow. All foreign elements I could could not catch or prevent to enter, I will find back, in the bronze or better say, instead of bronze. Additionally, I will find this impurities where I less want them… in extremities like fingers, noses, hair, toes… in other words, very delicate points. It happened to me more than I can appreciate to have to re-work entirely 'cold' some part of my statues. This can immensely satisfies the worker in me (some repairs are so beautiful that I record them in a file I named 'Miracles'!) but I do resent the statue having to go through so much 'surgery' as if becoming bronze was not demanding enough already! While the cleaning, turning, burying, pressing is happening, a burning furnace with a crucible in it brings the bronze to its desired 1200 degrees temperature. After this, I feel the most impressive phase of a casting session opens up: silence, cooperation and all effort honouring one purpose: serving! Silence because it is imperative to 'hear' the bronze as it 'talks' and informs how it flows in each and every mould. If there was a radio, it is put off, if there was a discussion or an exchange, it is suspended… Cooperation because safely serving the task prevails! While the air turns blue and opaque due to the fumes, with a crucible itself or a huge spoon, the molten bronze reaches the most inner parts of the moulds. The investment mould is allowed to cool down before it is 'sacrificed'....

 

7 Releasing

The investment mould is hammered to liberate the rough cast. Big bangs, at the first, followed by more gentle hammering soon after. Eventually, I will soft-brush most of the remaining plaster. The beauty of the statue, in its roughest stages is so very touching, tender and almost confusing experience because I know how much more brutality this piece and any piece after this will endure before it can enter my own showroom, a gallery or the customer's kingdom. I never fail to take photos of this temporary blessed phase.

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8 Sand Blasting

This procedure allows the rough bronze to be cleaned from all traces of plaster. The water reveals some colours the oxidised skin did not reveal before: Yellow's, purple's, light grey's and black's, just to mention the most amazing tones. Light's generosity is always difficult to describe, let alone to let go of because time will paint this skin all new simply by making it dry. This cleaning phase is a Be in the Moment master class! And I took photos just to cherish the fact they are frustrating and totally unable to convey the richness of what, through the eyes, I experienced…

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9 Metal chasing

Everything which is not the sculpture is cut off and every 'wound' taken care of by either taking away (chisel, rasps, etc…) or by adding (welding).

Given the precision with which I work, the 'surface work' or what I call 'satinatura' is definitely the most demanding of all. I can spend an enormous amount of time (my record being 1200 hours) with rasps, sand papers and all sorts of abrasives materials to obtain the desired results. For more on the CHISELING process, visit the corresponding page in PROCESS.