What is Bronze ? Bronze is an alloy primarily made from copper and tin although sometimes other materials are added such as manganese, lead or phosphorous, dependant on the required properties of the finished piece. The bronze used for the casting of sculptures and statues would generally contain a small amount of lead as this makes for a softer alloy, hence this is a more malleable and workable material. Typically 7% tin, 3% lead and the rest copper would be a good general mix but the amounts of the various materials can be juggled vastly to acheive widely differing properties making this a very versatile material for casting bronze sculptures.
Bronze Casting - The Lost Wax Method
Foundry bronze is normally made using the lost wax method of casting. The ancient lost wax method has been around for what is thought to be over 1000 years although the exact date is unknown. It is believed that the first people to use this method began with a piece of clay roughly the same size as the object to be sculpted, they covered this with a layer of wax and then finalised the detail. This was then covered with another layer of clay, once this was heated the wax core ran into the bottom of the oven and was in effect 'Lost wax' and it is thought this was where the name came from. The clay outer hardened and into that went the bronze, once the bronze hardened the clay was broken off leaving a bronze casting. This method basically remains unchanged to this day, with just adaptations of the method.
Modern day manufacture would begin with an original sculpture being made from clay or wax, this would then have a release agent applied (to allow removal from the mold), this would then have several thin layers of silicone painted onto the sculpture. Once the layer of silicone is thick enough it is then carefully peeled back from the original sculpture. This silicone mold can then be used to make unlimited wax replicas of the original. To make a shell, the wax castings are then dipped in a 'slurry' which hardens as it dries, the number of times the casting needs to be dipped will vary dependant upon the size or weight of the item to be cast. Once the shell has reached the required thickness it will be left to dry and then heated in an oven upside down which allows the shell to cure and the wax to run out. These shells will then be checked for flaws and cracks. They are then heated back up to around 1800 degrees centigrade before being placed in a pouring pit (which is like a sand pit allowing the molds to be stood upright for pouring). The molten bronze is then poured into the molds and allowed to cool, once cool they are 'knocked out' where the outer casing is broken off to reveal the bare casting. The casting is then ready for finishing where any remnants of the shell will be removed, rough areas may be sanded or welded where required and finally the desired patina will be applied. The main finishes are Verdigris (green), antique brown and museum brown although many other variations exist.
Bronze Casting - Cold Cast Bronze
Cold cast bronze is usually a composite material made from resin and bronze metal powder. Generally this would be around 55% resin and 45% metal powder which could be bronze, aluminium, brass or copper depending on the characteristics you require. Higher percentages of metal powders can be used which produces a higher quality finish and a higher price tag. The mixtures is then poured into the mould before being turned out and chased & finished in the same way as hot cast bronze. Certain variations of cold cast bronze also afford traditional patination to be applied in the same way as hot cast bronzes. This material is much quicker and easier to work with hence the finished sculptures are almost always much lower in price. Cold cast bronze has brought the availability of bronze sculptures within reach of anyone, previously a collection of bronze statues would have been only possible for the wealthy but now, fortunately, fine bronze artistry is more widely affordable.
It is possible to make cold cast bronze without using any resin at all. Genesis Fine Arts of Ireland use a patented method of cold-casting which uses bronze metal powder and a tiny amount of barium sulphate, this is then subjected to massive pressure which cause a chemical reaction bonding the bronze particles together. This creates a superior bronze finish and results in the highest quality cold bronze available. To see my Genesis Fine Arts / Heredities sculptures click here
Caring for your Bronze
The level and type of care your bronze sculpture will require naturally depends on where your bronzes are kept, and also what you are hoping to achieve from the ageing process with regard to the appearance of your bronze. Bronze is typically a very easy metal to care for as we will go some way to explaining below;
For bronze statues kept inside the home in a warm and relatively dry atmosphere it is generally sufficient to clean the bronze with a soft cloth regularly, and apply treatment of wax about once a year. This will be ample care to keep your bronze in tip top condition.
For bronze statuary kept outside, in the garden or perhaps the orangery, we will need to adopt a differing approach. If one is hoping to maintain a lustrous brown finish then it is essential that we keep the bronze well protected from the elements. Clean the bronze regularly with water and a soft brush to keep the surface free of grime and soiling. Dry the broze thoroughly, taking care to get all the nooks and crannies (a hairdryer can be handy for this). Once the statue is dry we can than apply a coating of wax to the surface.
For outdoor bronze sculpture where you are hoping to acheive a weathered appearance simply keep the surface clean and free from soiling and bird droppings etc by cleaning with a soft brush and clean water.