Metal-based medals are often made of more than one metal mixed together, called an alloy. Alloys are not limited to metal alone, but can be a mixture of metals and non-metals. To create an alloy, metals are heated and mixed in their liquid state. The resulting alloy often has different properties than the individual metals it is made of. For example, some alloys are harder and stronger than their components.1 This is true for alloys such as steel, which is made of carbon and iron. Steel is an interstitial alloy where a smaller atom, carbon, fills the spaces between a larger atom, iron. Alloys such as brass are substitutional alloys, where the atomic radius of zinc and copper are similar. Increased hardness is also a property chosen for in alloys commonly used in the production of art medals, such as bronze. Bronze is 5% tin and 95% copper, and is less brittle than other metals, making it ideal for medal shaping.
Oftentimes in the production of metal-based medals, thin green or brown layers are added to the medal’s surface, to give an appearance of age. These are typically a result of chemical oxidation, which is a finish given to medals called a patina. Not to be confused with rust, which is always destructive to the surface it forms on, patinas tend to protect the surface they form on. The statue of liberty is an example of copper metal that has formed a protective green patina due to multiple cupric ion compounds depositing on the surface, preventing further corrosion.2 Patina on any metal can be made from the metal’s ions mixing with oxides, carbonates, hydroxides, sulfides, and sulfates.3 While patinas can be admired and desired for their beauty, they can come naturally to the metal over time, in addition to manually by a patina finisher. Patina finishers can add virtually any color of patina to a medal by treating it with chemicals such as potassium nitrate, combined with additional methods such as a blowtorch to heat the metal to a specific temperature.4 Metal-based medals can also be electroplated; a process often taught in general chemistry courses. Metals such as silver and gold can be plated onto other metallic surfaces, to give them the appearance of the more valuable metal.5 This process is common with Olympic gold medals, although they are over 90% silver, they have 6 grams of gold plated onto them to give them their shiny golden appearance.6
In addition to metal, some artists prefer to non-metal materials to construct their medals. For example, many use porcelain to create art medals. Porcelain includes a broad range of ceramic products, and is made of clay, feldspar, silica, and flint.7 Variations of porcelain can be made by changing the ratio of the aforementioned materials, and medals are hardened by heating the components to temperatures of 1200 – 1400 °C.8 Porcelain is a rigid material that is resistant to both thermal and chemical shock, making it difficult to manufacture, compared to its counterparts such as stoneware and earthenware. In addition to porcelain, resin is a non-metal material that can be used for the creation of medals. Resin has many uses in industry including glue, electrical insulators, and foam. Resins contain epoxide groups that upon the addition of a hardening agent, or catalyst, initiate polymerization and hardening of the material.9
REFERENCES:
(1) Armstrong, L. Handbook of Alloys; WILLFORD Press, 2022.
(2) Why the statue of liberty is colored blue green?. worldofchemicals.com. https://www.worldofchemicals.com/560/chemistry-articles/why-the-statue-of-liberty-is-colored-blue-green.html (accessed 2022-07-05).
(3) Kuntzleman, T. S.; Cullen, D. M.; Milam, S.; Ragan, D. Rapid Formation of Copper Patinas: A Simple Chemical Demonstration of Why the Statue of Liberty Is Green. J. Chem. Educ. 2020, 97 (8), 2244–2248. https://doi.org/10.1021/acs.jchemed.0c00417.
(4) Newman Numismatic Portal at Washington University in St. Louis | Comprehensive Research & Reference for U.S. Coinage. https://nnp.wustl.edu/library/dictionarydetail/516463 (accessed 2022-07-05).
(5) Metal Finishing or Plating for Medals| FIA : Gold or Silver Plated Medals. FIA-MEDALS.COM.
(6) How Much Gold Is in an Olympic Gold Medal? – The New York Times. https://www.nytimes.com/2022/02/10/sports/olympics/gold-medal-price-value.html (accessed 2022-07-05).
(7) How porcelain is made – material, making, used, processing, parts, components, composition, steps. http://www.madehow.com/Volume-1/Porcelain.html (accessed 2022-07-06).
(8) What Is Porcelain And How Is It Made. African Pegmatite, 2020.
(9) acrylgiessen. What Is Resin? – Learn What Is Resin Made Of. acrylgiessen.com – Creative Magazine, 2020.