Craft & Process: The Making of Brass, Bronze and Zinc
Tradition Meets Technology
The objects we love—doorknobs, sculptures, jewellery or fixtures—often start as molten metal. Although the basics of casting, forming and finishing remain similar to those used thousands of years ago, contemporary artisans combine hand craft with technologies such as CAD, 3‑D printing and CNC machining to refine the craft. This piece explores how brass, bronze and zinc products are created and finished, and how their natural patinas evolve over time.
Lost‑Wax Casting – The Sculptor’s Method
One of the oldest techniques for producing metal objects is lost‑wax casting (also called investment casting). It dates back over 6,000 years and continues to be used for jewellery, statuary and architectural hardware.
Model making – The artist creates a model in wax or another material. Traditional lost‑wax casting carves the design directly in wax; however, if the original must be preserved or reproduced, the artist first sculpts it in clay and then makes a rubber and plaster mould to produce wax duplicates. In bronze casting, the wax model will be destroyed during the process, so duplicates preserve the design.
Creating a mould and wax patterns – A master pattern is used to make a flexible wax mould . Molten wax is injected into the mould to produce multiple wax copies . Sprues (branches) are attached to the wax pieces and assembled into a “tree” that allows molten metal to flow through later .
Investment and burnout – The wax assembly is coated with silica slurry or placed in a flask filled with investment plaster . After the material dries, the flask is heated upside down to melt and burn out the wax . This leaves a cavity that matches the original design.
Pouring and devesting – The heated mould is filled with molten metal—gravity or vacuum pressure helps it fill narrow channels . Once cool, the plaster is quenched and dissolved, sprues are cut off, and the cast parts are cleaned and heat‑treated .
Finishing – Files, grinders and sandblasting remove casting marks, refine details and achieve the desired surface . Today, digital design and 3‑D printing are often used to create precise patterns, reducing manual labor and enabling rapid iteration .
Lost‑wax casting is widely associated with bronze. Instructables notes that humans have been casting bronze using this method for almost 6,000 years and that the “lost” wax model is sacrificed during mould making. The same technique is used for brass; artists often cast handles, knobs or jewellery in brass using wax patterns made by hand or 3‑D printers.
From Alloy to Object – The Brass Journey
Unlike bronze, which is primarily copper mixed with tin, brass is an alloy of copper and zinc. Modern foundries control the copper–zinc ratio (often 60–80 % copper to 20–40 % zinc) to achieve the desired colour and mechanical properties . The process of making brass products is fairly standard:
Melting the metal – Copper and zinc are melted together in furnaces reaching about 1000 °C . The molten alloy is poured into moulds to form ingots.
Shaping – Ingots are processed using several forming techniques :
Casting – Molten brass is poured into moulds to create intricate shapes for sculptures, decorative objects and jewellery .
Forging – Heated brass is hammered or pressed into shape for hardware, tools and instruments where strength is critical .
Extrusion – Molten or hot brass is forced through a shaped die to create uniform profiles such as rods, pipes or wires .
Finishing – After shaping, the surface must be prepared for use. This involves sanding to remove roughness, buffing with polishing wheels and abrasives to achieve a high‑gloss finish, and finally coating the surface to protect against tarnish . These steps remove imperfections and give brass its distinctive sheen .
Brass products are valued for their durability and machinability. They can be recycled repeatedly without losing quality , making brass a sustainable choice for architectural hardware.
Zinc Casting and Fabrication
While brass and bronze are traditionally cast using lost‑wax or sand casting, zinc is often formed through die casting. Molten zinc flows easily and can reproduce intricate details. Deco Products notes that zinc casting produces three‑dimensional parts with features that protrude from surfaces, often eliminating the need for machining or welding. Advantages include:
High fluidity and dimensional accuracy – Molten zinc fills tight tolerances and complex shapes. Zinc castings can be produced at high rates and lower costs than aluminum because zinc melts at lower temperatures.
Durable tooling – Zinc die‑casting tools last up to 10 times longer than aluminum tools, saving money and ensuring consistent quality.
Good surface finish – Zinc castings often emerge with smooth surfaces that require little post‑processing, and almost any finish can be applied to them.
Material efficiency – Thin‑walled zinc castings are strong yet light, using minimal material and producing little scrap. In contrast, stamping sheet metal consumes more material and can result in higher scrap rates, whereas die casting yields complex parts without the need for welding.
Architectural zinc used for roofing and façades develops a matte grey patina when exposed to the elements. RHEINZINK explains that natural weathering produces this protective layer, which shields zinc from weather and corrosion and gives it a service life of up to 200 years . Zinc also “heals” scratches on its surface, making it a maintenance‑free cladding material .
Finishing Techniques: From Burnishing to Mirror Polish
Machine‑Polished Finishes
Metal finishing can dramatically change the appearance and feel of a product. Directional finishes (also called brushed or architectural finishes) are achieved by polishing the metal with a 120–180 grit belt and then softening the lines with finer abrasives . The most efficient way to create a directional finish is by using a wet metal polishing and finishing machine . This produces consistent, linear scratches and a satin sheen.
Non‑directional finishes are created using orbital sanders or automatic finishing machines with fine or medium‑grade sanding discs . These finishes conceal scratches and give a uniform matte appearance. Mill finishes (also called matte finishes) result from rolling or drawing the metal through mills, leaving a smooth but dull surface . For a mirror polish, the surface is ground and buffed repeatedly to achieve a highly reflective, mirror‑like look. Mirror finishes are labor‑intensive when done by hand but can be achieved more efficiently with wet or dry finishing machines .
Automated finishing equipment offers advantages such as consistent quality and cost‑efficiency; machines can process both sides of a workpiece in one pass . However, artisans still value hand finishing for its ability to highlight subtle details and impart individuality.
Burnished Finishes – Hand and Machine
Burnishing involves polishing the surface with a series of abrasives to create a soft, warm sheen. Chicago Brass notes that a burnished finish may be produced either mechanically or by hand; the metal is polished with abrasives, yielding subtle variations in tone and a surface that is less reflective than a high polish but feels lived‑in. Burnished finishes are prized for their timeless appeal, warmth and character and are durable enough to withstand years of wear. No two burnished pieces are exactly alike because the process leaves unique variations.
Different metals respond uniquely to burnishing:
Burnished brass is a polished brass finish that has been further burnished to achieve a smoother, more lustrous appearance. It often shows a warm golden hue and a reflective surface, giving a slightly aged or antique look.
Burnished bronze is created by applying burnishing techniques to bronze; the process produces an antiqued finish with a mixture of dark and light tones, adding depth and sophistication.
Burnished zinc and copper (noted in the same article) also exhibit warm, refined hues, each with subtle tonal variations.
Burnishing can be executed by hand, where artisans polish with small circular motions, or by machine, which uses vibratory tumblers or rotary tools. Hand burnishing allows the craftsperson to control highlights and shading, while machines provide uniformity for high‑volume production.
Patinas and the Evolution of Metal
Brass Patina
Patination is a chemical reaction between metal and its environment. Joseph Giles explains that natural brass patination occurs when the copper and zinc in the alloy react with air, moisture and other environmental factors, forming a layer of copper salts on the surface . The colour of this patina depends on the alloy composition and environment: high humidity or saltwater exposures can produce green patinas, whereas polluted urban air tends to create black or brown patinas . Brass typically develops its maximum patination after about two years, though protective coatings and regular cleaning can slow the process, while harsh chemicals or abrasive materials accelerate it . The rate of patination also varies with finish: darker waxed finishes may show wear back to the base brass faster than mid‑antique finishes, and exterior hardware patinates faster due to moisture and pollution . Frequent use transfers oils from hands to the metal, speeding oxidation .
Bronze Patina
Bronze patina forms when the copper in the alloy (copper and tin) reacts with oxygen, carbon dioxide, salts and moisture. Impact Signs notes that patina is a thin layer of copper carbonate or copper chloride that forms on bronze surfaces exposed to air or seawater . This tarnish—typically green or blue‑green—gives bronze plaques and statues their characteristic hue and also protects against further corrosion . Far from being damage, a well‑formed bronze patina is coveted for its beauty and is seen as a mark of age and prestige . Preservationists recommend gentle cleaning with mild, non‑abrasive materials to remove harmful deposits while retaining the patina . Waxes or oils can enhance the patina’s appearance and provide additional protection . Painted finishes do not evolve like patina and may flake over time; in contrast, a naturally aged patina forms a stable layer that matures with the metal and contributes to its longevity . For those wanting an aged look more quickly, hand‑rubbed patina finishes can accelerate natural ageing .
Zinc Patina
Unlike the warm browns and greens of copper‑based alloys, zinc develops a matte grey patina. RHEINZINK describes how natural weathering causes zinc to form a grey protective layer that shields the material from weather and corrosion . This patina evolves slowly and gives zinc cladding a lifespan of up to 200 years . Because scratches on zinc re‑oxidize to form fresh patina, zinc roofs and façades are largely maintenance‑free . Manufacturers also offer pre‑patinated products that replicate the matte grey look at delivery, bypassing years of natural weathering .
Caring for and Celebrating Metal Surfaces
The way a piece of hardware or art is finished—polished, burnished or allowed to patinate—affects not only its appearance but also how it will age. Light polishing and protective coatings maintain shine but delay patina. Burnishing brings out warmth and subtle variation while providing durability. Allowing a natural patina to develop acknowledges the passage of time and gives a piece individuality. Whichever finish is chosen, regular maintenance should be gentle: clean with soft cloths and mild, non‑abrasive products ; avoid harsh chemicals that strip the surface . Applying wax or oil coatings can both slow corrosion and enrich colour . For zinc surfaces, simply allowing the grey patina to form and leaving it untouched is often the best approach .
Conclusion
Crafting objects from brass, bronze and zinc involves a delicate balance of tradition and innovation. The lost‑wax casting process still follows the same steps used millennia ago , yet artists now create models with 3‑D printers . Brass must be alloyed, cast or forged and meticulously finished ; zinc is transformed through high‑precision die casting and then protected by its own patina . Burnished and machine‑polished finishes offer different aesthetics, while natural patinas tell a story of age and environment. Understanding these processes helps us appreciate the craftsmanship behind everyday hardware and the enduring beauty of metals that change gracefully over time.