Bronze is a term used to describe a metal alloy of around 88% copper and 12% tin and forms of the alloy have been around since 3500BC. Other minerals may be added to give bronze various other properties. Bronze has less metal-to-metal friction than most other metals, making it ideal for items with moving parts such as rowlocks. Bronze is also aesthetically pleasing and it weathers well even in harsh marine environments.
Because of bronze's high melting point it cannot be cast in metal moulds as steel is. It must be cast in sand moulds, adding to the cost as the sand moulds can only be used once and cannot be automated.
Different Types of Bronze
Most Classic Boat Supplies' bronze hardware is made with a variety of bronze known as Gunmetal (otherwise called Leaded Gunmetal or LG2 for short). Gunmetal contains approximately 85% copper, 5% zinc, 5% lead and 5% tin, giving it ideal properties for a salt-water environment and allowing the finished product to be machined and polished to a rich gold colour.
The remainder of our bronze hardware is made using Aluminium Bronze (or AB2 for short). Aluminium bronze contains around 81% copper, 10% aluminium, 5% nickel and 4% iron. Aluminium bronze is ideal for products exposed to heavy loads and friction and those requiring additional strength. Classic Boat Supplies' stocks of bronze rowlocks are made from aluminium bronze.
For marine use it is important that the quantities of ingredients in each bronze are strictly controlled to derive the required properties and our suppliers adhere to strict guidelines to guarantee the highest quality product.
The Bronze Casting Process
Our bronze hardware is created from sand casts. This involves packing oiled sand around two halves of a male mould within a solid box. The moulds are then carefully removed. Being damp with oil, the sand holds its shape well.
A hole through which to pour the molten bronze is then pressed through the sand to the mould chamber. Smaller holes are made where required to let air escape.
The two halves of the box are then joined and molten bronze is poured into the cavity and cooled. The cooling process is very important as if it is not done correctly, desirable properties in the finished product, such as strength, can be reduced.
Once cooled, the solid bronze is removed from the mould and cleaned up. Some products are left in their natural state and have a rumbled finish. Others are polished using a buffing wheel along with a waxy abrasive compound. Hand polishing with finer abrasive compounds is then needed to bring out the best colour in the bronze. The polishing process is time-consuming but essential to get right to create the best finished product.