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The difference between foundry bronze and bronze resin
If you are considering commissioning a bronze sculpture it’s important to understand the differences between foundry bronze and bronze resin. Having just completed Maelstrom in both foundry bronze and bronze resin it became apparent that a blog on the differences would be helpful information to share with you. They both involve the process of casting but by looking at the technique and materials used in more detail, it will help you make a more informed decision about buying or commissioning a piece in bronze.
Bronze over the ages
Bronze has been used for over 6000 years, first to provide harder substitutes to stone and copper for objects such as tools and weapons and due to its durability and its attractive colour it became a popular choice for decorative objects. It is essentially a metal alloy comprising predominantly of copper (approximately 90%) and whilst tin is the other most recognised metal used over the centuries, today it can include other materials such as aluminium, lead, nickel, iron, and silicon. The ancient objects are referred to quite simply as bronze but as the 20th century brought us advanced new composite materials then greater clarity in the terminology is required.
Foundry bronze and the lost wax process
Foundry bronze, also known as hot cast bronze clearly hints at how the object has been created. In order to cast a ‘pure’ bronze artefact, one of the most common techniques used is the lost wax process. After creating a mould (commonly in silicone) from a master copy of an object, molten wax is poured into the mould and once it solidifies again, the wax copy is carefully removed and then encased in ceramic. When the ceramic sets the whole piece is placed in a furnace and the resulting molten wax is allowed to drain away. Finally bronze ingots are melted down (at around 1000 C) and the cavity left by the wax is then filled with molten bronze. Once this has cooled down the ceramic is chipped away and the final bronze piece is then touched up to remove any traces of the casting process.
Molten bronze
Whilst the finished piece can be created either as a solid or hollow cast, the material itself is bonded together at a molecular level and is therefore very durable. Please note that whilst some people refer to foundry bronze as solid bronze, given that the piece may be solid or hollow, I have avoided this term to avoid confusion.
Bronze resin
Bronze resin or resin bronze is a term used to describe a composite material which has been created with bronze powder mixed with a liquid resin and a catalyst which then solidify in a mould. As it doesn’t require any great heat in the casting process it also known as cold cast bronze. In fact the chemical reaction generated by the catalyst creates heat and this needs to be kept to a minimum to avoid cracking and distortion in the final cast. For very small objects this exothermic reaction is relatively negligible and therefore the piece can be created with a bronze resin mixture through and through. However as the quantity of casting material is increased (for a larger product) the exothermic reaction increases exponentially and if combined with a conductive metal powder, this is a recipe for failure. Therefore most cold cast bronze castings are created with an initial coat of a resin mixed with bronze powder which is usually between 1 and 3mm thick. If the initial resin bronze mix is too heavy with metal powders, it will not only be very difficult to pour or brush into the mould but it will also be particularly brittle. Too little metal powder will make the piece look artificial. Once this layer starts to set it is either reinforced with a backfill mix or fibreglass. The backfill mix is often packed out with fillers such as stone powders which will limit the exothermic reaction. This technique is occasionally referred to as bonded bronze and the bond itself is mechanical rather than molecular and is therefore not as strong as foundry bronze.
Bronze finishes
Whilst this blog focuses on the casting processes, I wanted to distinguish cast bronze from a bronze finish. Both casting processes involve pouring/brushing either a molten or cold liquid into a mould. With a metal finish the surface of an object is covered with a metal coating which will usually be sprayed on. Typically a coat of paint will be less than a 1/10 of a millimetre thick and whilst thicker products can be sprayed on, the viscosity is considerably thinner than when applying a resin bronze mix to a mould and therefore the metal content will also be lower. Ultrafine powders which float when added to a liquid medium are also sold as ‘bronze powder’ but typically are made primarily of zinc with elements of copper, therefore a brass powder by definition. Finally a ‘metallic’ finish will be made using a shiny metal substitute such as mica powder.
A direct comparison
If you are considering buying a ready-made product but are unsure if it is foundry bronze or resin bronze then the easiest way is tell is to tap the piece (gently) with something metal and you will hear whether the piece is metal or not. Weight is not ideal as a metric for comparison because bronze resin pieces are often filled with marble powder which is heavy, although not as heavy as bronze. So unless you have two identical pieces in foundry bronze and bronze resin side by side to weigh, this may not answer your query.
Exposure to the elements
Indoors, both forms will last indefinitely, but outdoors, bronze resin will not cope as well with exposure to the elements. This is particularly the case if there are areas on the surface of the sculpture where water can pool, combined with freezing conditions. The shiny surface or patina on both can be protected from the elements for some time with a lacquer, oil or a wax but some maintenance will be necessary outdoors. As the resin expands and contracts continually over the years the mechanical bonds will eventually be susceptible to cracking and require repair work. This may be a question of a couple of decades however some repair work will be necessary during a typical lifetime. Fortunately, bronze resin is far easier to repair than foundry bronze. Foundry bronze may lose its lustre or its patina may fade, but if cared for it will still remain structurally sound for centuries.
Conclusion
Foundry bronze involves a labour intensive process which requires a large amount of energy to melt the ingots and it will contain a larger amount of bronze than cold cast bronze. The casting process (both hot and cold) always leaves imperfections that need repairing. As foundry bronze is extremely hard in comparison with bronze resin, then this process requires much more time and this is amplified when a brushed (shiny but not mirrored) finish is requested. All of these differences are reflected in the comparative prices of the techniques/products.
Whilst foundry bronze can still be scratched, chipping or even breaking it requires a concerted effort. The nearest comparison for a bronze resin piece it terms of its brittleness would be ceramic which can be easily chipped.
One benefit of bronze resin can be seen with larger sculptures. By way of example, Maelstrom which is 50cm in diameter, weighs 15kg in foundry bronze. A similar sculpture, if a metre wide would most probably require mechanical lifting apparatus. Bronze resin, particularly when reinforced with fibreglass is as a considerably lighter option and will usually solve this problem.
My limited edition sculptures come with certificates of authenticity and this will verify the material it is made in as well as the edition number. However before you get to this stage you may wish to discuss which process would best suit your needs so please feel free to get in touch.