About Copper

This section will briefly discuss why copper and copper alloys are the best material choice for art and sculpture, as well as other technical issues regading the selection of the proper alloys, colouring techniques, and finishes.

A. Why and How Copper is Used

B. Main Processes Used to Make Sculptures

C. Copper Alloys

D. Properties

E. Colouring Concerns

F. Chemical Colouring Techniques

G. Finishes


A. Why and How Copper Is Used

Copper and copper alloys are the best materials to use for sculpture for many reasons. They can be shaped to any desired form, providing unlimited artistic possibilities. Copper parts can be joined or assembled mechanically by bolting or bonded by welding, allowing for works of massive size. Copper can accept a wide variety of finishes to show any desired texture. It can be worked to show very fine surface detail. Copper is resistant to corrosion, weathering well, even beautifully, instead of rusting. It has been said that copper sculpture can last for a thousand years. Other materials do not have this wide range of properties or are as flexible when it comes to joining techniques, finishing, and colouring, important factors when fabricating a sculpture or other work of art..

Copper is a malleable red-brown metal that occurs in nature and can be refined from copper ores. Yet copper is difficult to cast without the addition of other metals that slow its oxidation rate and improve the flow of the molten metal. Copper alloys, or substances made by combining copper with other metals, are therefore used for sculpture. Copper and tin create bronzes, and copper and zinc create brasses.

It should be noted that just about all cast metal sculptures are often referred to as bronzes, despite what they are actually made of.


B. Main Processes Used to Make Sculptures

There are a number of ways to fabricate bronze sculptures. These include:

1. Mechanical Joining – the assembly of metal components by bolting, crimping, staking or riveting

2. Welding – a high-temperature and pressure-joining or bonding process involving the fusion of the base metals with, or without the addition of a filler metal.



3. Brazing is an intermediate temperature-joining process using a nonferrous filler metal.

4. Soldering is a relatively low-temperature joining process using solder filler metal.



5. Lost-Wax Technique – this is a very common technique for fabricating sculptures, but does need some explanation.

The lost-wax process of creating a sculpted piece of art gets its name from a wax duplicate of the sculpture that must be melted away in order to leave room for the molten metal to take the desired shape.

There are many steps in this process.

Step 1: The artist provides the original piece of art, in actual size, that is to become a bronze sculpture. The art can be in any material: clay, plasticine, plaster, wood, wax or styrofoam.



Step 2: A rubber mould is made of the original unless it is made of wax or styrofoam. A liquid rubber compound is poured over the original, capturing every bit of surface texture.



Step 3: Once the rubber has set, the original art is removed. Molten wax is then poured into the rubber mould to make a duplicate of the original. This is the wax duplicate that will later be melted away or “lost.”

Step 4: Wax bars are joined to the wax duplicate to form the “sprue system,” which consists of empty channels that will direct the molten metal into the mould to form the sculpture.



Step 5: The wax duplicate with its attached wax sprue system is then heavily coated with a ceramic slurry and stucco. This encases the duplicate in a hard protective shell. At this point it may be difficult to distinguish the actual piece of art.



Step 6: This ceramic shell is heated by steam until the wax duplicate melts away, leaving a hollow mould. The shell is then prepared for casting.

Step 7: Molten metal is poured into the ceramic shell to form the sculpture. The shell is left to cool, and then the sprue system is taken off and the ceramic is chipped away, revealing the cast piece of art. This piece is always unique, because the mould that made it has to be destroyed in the process.



Step 8: The final step focuses on restoring the original texture and polishing the surface or adding a patina, as the artist specifies.




C. Copper Alloys

Copper alloys are metallic substances made by combining copper with one or more other metals. Copper alloys can be made with zinc, tin, nickel and lead to form bronzes and brasses. Alloys of copper and tin are bronzes. Those of copper and zinc are brasses. Yet the word “bronze” is used for a variety of copper alloys, even without tin.



The following are the common names of copper alloys and the colour they attain when weathered:

Group A: Copper

If the primary material consists of more than 99% copper, it is considered to be pure. Alloy C11000 is 99.99% pure copper. Other specialty alloys in this range, such as alloy C10500, which is oxygen-free copper with silver, contains 99.95% copper and minute amounts of silver and oxygen. These specialty copper alloys are not used for sculptural applications or such other common applications as architecture or tubes. These develop a reddish brown to grey-green patina over time.



Group B: Architectural Bronze/Common Brasses

The following alloys are commonly referred to as Architectural Bronzes even if technically they are not bronze. Their colour can vary considerably, from red to yellow to silver to white, all due to their different amounts of copper, which can be from 57 to 90%, and their companion elements, which can be zinc or nickel. The fact that many of these alloys contain zinc as their primary additional element, technically makes them brasses, which can be confusing.

Group C: Statuary Bronze



These alloys are very specialized. There are special bronze alloys that are used for casting statues and sculptures. A typical bronze for statuary work is silicon bronze, alloy C87300, which contains 94% copper, 3.5 to 4.5% lead and the remainder 3% zinc, iron and other alloys. There are other silicon bronzes that can be used for this application as well. The other two alloys are primarily used for architectural interiors, as well as roofing projects where a unique colour with the performance of copper is required.

The following terms are used to refer to certain metals and the processes used for these alloys:

Bronze generally refers to commercial bronze, red brass, Muntz metal, architectural bronze and the silicon bronzes.

White bronze generally means the nickel silvers.

Yellow bronze is a term that covers the other alloys.

Statuary bronze describes the naturally weathered or chemically oxidized brown to black surfaces.



Green bronze refers to natural and artificial patinas.


D. Properties

There are three key properties of silicon bronzes and other copper alloys that make them especially attractive for sculpture and art. These are their workability, strength and resistance to corrosion.

1. Castability and Workability

Silicon bronzes are easily melted and worked. They hold heat well, allowing the molten metal to flow into all the cracks, crevices and other small details, such as surface patterns and reliefs, even as detailed as fingerprints. They can replicate all the details that artists can incorporate into their work. In addition, once formed, they are easily worked and formed in any state, up to and including a cool temperature. These alloys are easily joined either mechanically or by such processes as welding, with the joints being very strong, which makes possible the fabrication of large-scale works.



2. High Strength

Once formed, the alloys have very high strength, which results in a durable and lasting work. Bronze sculptures many thousands of years old still exist with little deterioration of the bronze itself. The relatively light weight of the bronze compared to other materials is a distinct advantage for copper alloys in terms of cost and the time needed to fabricate them. The strength of bronze also means that the works are less susceptible to mechanical damage or vandalism, unless very deliberately caused.

3. Corrosion Resistance

This is the key to the durability of works of copper and its alloys. All coppers form a durable patina upon exposure to the atmosphere. Cities’ outdoor sculptures that have turned very dark brown or black show the alloy’s way of protecting against atmospheric attack due to pollution and other airborne impurities.

Copper’s patina is very tenacious, durable and effective. If formed properly, it will protect the underlying alloy as long as it is in place. It doesn’t come off and cannot easily be removed. The colour it acquires over time is part of its beauty. The patina will continue to evolve slowly over the life of the statue.


E. Colouring Concerns

As noted above, copper and its alloys are relatively active metals which tend to oxidize when left unprotected. After many years, exposure to the atmosphere in most cases leads to the formation of a naturally protective grey-green patina. Because copper and its alloys can develop a broad range of colours, chemicals are used to speed up natural weathering, or bright natural colours are preserved with clear protective coatings. It should be noted that these treatments cannot be expected to last the life of the work. In many cases, regular maintenance is required, which can be difficult for works that are expected to last for hundreds of years.



Natural weathering is a result of airborne sulphur compounds that combine with water vapour to form acids that react with copper surfaces. In industrial and coastal areas, the natural patina usually forms in five to seven years. In rural areas where there is less airborne sulphur, the patina may take 10 to 15 years to form. In arid areas with less moist air, the basic patina may never form. Natural weathering eventually produces a uniform appearance. Such natural patinas are called “statuary” or “oxidized” finishes.

Coatings can be transparent or opaque. Transparent coatings preserve the distinctive colours of the copper alloys, while opaque coatings can provide a specific colour other than the naturally occurring ones.

In many cases, these colours and treatments can be dramatic and a key feature of the work. Often, these treatments are used for works that will be located indoors, where, without outdoor atmospheric effects, the treatments can be more effective and last much longer.


F. Chemical Colouring Techniques



Bronzes are often given a final colour by the application of chemicals. Time, temperature, surface preparation and humidity are variables that can affect chemical colouring. The reason for using chemicals is to speed up the natural weathering of the metal by exposure to the elements.



By chemical colouring, the surface metal is converted into a protective film, usually an oxide or sulphide of the metal, or a compound is created, forming a surface film. These conversion treatments produce patinas and statuary finishes.

These patinas, also known as “verde antiques,” can be developed from acid chloride or acid sulphate treatments. Problems with chemically induced patinas can be lack of adhesion, excessive staining of neighbouring materials, and a lack of colour uniformity over large surface areas.

Here are some of the treatments used to achieve desired colours:

Ammonium chloride: A saturated solution of commercial sal ammoniac is brush- or spray-applied. Several applications may be needed.

Cuprous chloride hydrochloric acid: This is applied by spray, brush or stippling.

Ammonium sulphate: This involves six to eight applications applied by spray.

As well, statuary (oxidized) finishes are light, medium and dark colours depending on the concentration and number of applications of the chemical colouring solutions. Two to 10% aqueous solutions of ammonium sulphide, potassium sulphide or sodium sulphide are swabbed or brushed on the surfaces to be treated, producing the statuary (oxidized) finishes from light to dark. An oxide pretreatment may help the chemical to adhere to the surface. Final hand toning and blending may be needed.




G. Surface Finishes and Textures

While the colouring and patination of the surface may be the most noticeable feature of a bronze sculpture, the actual texture of the material itself is a key component in the composition.