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Dealing with "dirt"
What is « dirt »? Anything other than wax in my lost investment mould can be considered dirt and will leave ugly “dirt” traces in the bronze. Typically gathering in the lower points of the mould, they are one of worse recurrent nightmare of casters and artists.
BEFORE POURING: Resumed in 3 main categories, the infinite sources of dirt are: 1) Mould materials; 2) Armature or original model related materials such as wood (becoming ashes), copper (generating metal oxides); 3) Alien materials (from the surroundings)
DURING POURING: 1) Dirt already present in the crucible, 2) Oxidation of the metal as it comes to react with the air. 3) “Entrainnement”. The flow of molten bronze being 8 times heavier than water, it can break and carry along with it parts of the inner walls of the investment mould. Delicate parts, such as lettering or very thin details are very vulnerable…
SOLUTION: PREVENTING “DIRT”
Preventing any alien materials to enter the mould (closing of any orifice during manipulation)
Creating a lost investment mould which can be well cleaned.
That is, provide the statue with a net of channels granting any unwanted particle an easy way out, by aspiration or vacuum. This very net will also perform as an excellent venting system during pouring.
In the crucible: The top layer of the molten bronze is collected and as soon as it reforms itself (because it does!), it is regularly pushed back or away from the pouring angle.
Air oxidation: Reducing as much as possible the distance between the crucible and the feeder in the lost investment mould, that is, not pouring ‘high’.
Did you know?
All casters recycle the feeders & vents from their previous casts. It is common mistake to link “second hand” bronze with the traces of mold material we so too often find on statues. The causes of those plaster pockets is the flow of bronze itself. As it travels through the mould, the flow of molten bronze can cause bakes in the plaster inner walls of the investment mould and carry with it any lost particles.
This phenomenon is called “Entrainement” in French, meaning, caught up or side tracked. The mold material attached to the reused bronze is converted to slag and sulfur dioxide gas during the melting.