Chemical coloring of metals

Chemical coloring of metals is the process of changing the color of metal surfaces with different chemical solutions.

See also: Patina
Henry Moore, Family Group (1950), patinated bronze
Bull Head Attachment, about 700-600 BC, Urartian, northwest Iran, bronze - Cleveland Museum of Art, early example of metal coloring
Metallocromia - Nobilis colored rings, Museo Galileo; Florence

The chemical coloring of metals can be split into two types:

Chemically coloring a metal is distinct from simply coating it using a method such as gilding or mercury silvering, because chemical coloring involves a chemical reaction, whereas simple coating does not.

History

The processes of chemical coloring of metals are as old as metalworking technology. Some of the earliest known examples of colored metal objects are about 5000 years old. They are bronze casts with some silver-colored parts, which originate from the Anatolian region. Similar processes can be found on some ancient Egyptian copper sheets.[2] Another example of early chemical coloring of metals is the Nebra sky disk, which has a green patina and gold inlays.

Pliny the Elder mentioned the distinction between naturally occurring and artificial patina in the first century C.E.[3] Another ancient document about the chemical coloring of metals is the Leyden papyrus X (3rd century C.E.).

Two important sources from the Middle Ages on chemically colored metals are the Mappae clavicula, which was dated between the 9th and 12th centuries, and Theophilus Presbyter's work De Diversis Artibus, which was dated to the 12th century.

At the time of the Renaissance, the most significant documents were the Treatise on Goldsmithing and the Treatise on Sculpture by the famous Italian mannerist, sculptor and goldsmith Benvenuto Cellini.[4] Patination is also briefly mentioned by Italian painter and writer Giorgio Vasari and by Pomponius Gauricus in his work De Sculptura 1504. André Felibien also briefly mentions some techniques for patination of bronze sculptures in his work Principes in 1699.[5]

The beginning of modern science-based chemical or electrochemical coloring of metals is marked by Leopoldo Nobili's (1784 - 1835) discovery of Nobilis colored rings in 1826.[6] Leonhard Elsner, Alexander Watt, Antoine César Becquerel (1788 - 1878) and Rudolf Christian Böttger (1806 - 1881) are also important people in the early history of electrochemical coloring of metals. George Richards Elkington (1801 - 1865), known for his patent for the electroplating of silver and gold (1840), had patented at least one electrochemical metal coloring process.[7] In the 19th century, the first manuals dedicated exclusively to the chemical coloring of metals were published.[8] In 1868, Puscher reported on the application of multicolored or lustre patina based on sodium thiosulphate and lead acetate for the first time.[9]

Since the end of the 18th century, chemical coloring of metals has been a regular topic of various collections of chemical technology recipes, and from the mid-19th century onwards, this topic was included in most electroplating manuals and handbooks of goldsmiths and silversmiths.

Great progress was made in the industrial application of chemical coloring of metals in the early 20th century. For example, around 1905, the first patents for black nickel (German patents DRP 183972 and DRP 201663) and black oxide (circa 1915 - 1922, German patents DRP 292603, DRP 357198, DRP 368548) were made. Between 1923 and 1927, the first UK patents relating to oxidised aluminium were published.[10][11], and black chromium was developed in 1929 (German patent GP 607, 420).

After the Second World War, there was a growing interest in green patinated copper sheets, which were intended primarily for architectural use. Technologies for anodic oxidation of titanium, and later niobium and tantalum, have evolved since the mid-1960s. Technology for the anodic oxidation of stainless steel was developed too in 1957. (patent US 2957812A).[12][13][14]

Now, the possibilities of using bacterial cultures in the patination of copper and iron are being investigated, and laser-induced staining of copper and its alloys, niobium, stainless steel, and chromium plated objects, are being tested.[15][16]

Uses

Chemical coloring of metals is primarily used in the manufacture of sculptures, jewelry, badges, medals, and decorations. It is also used in architecture, in the manufacture of metal furniture, and for military purposes as well as decorative vessels. It is used in the restoration and conservation of metals to some extent.

Examples of coloring processes
Interference colors obtained by heating steel to a certain temperature. The same colors can also be obtained by chemical or electrochemical means
Anodized titanium colors chart

Before a metal is colored, it must be cleaned of oxide and grease using a strong detergent solution or pickled in acid. The person performing the coloring should wear protective clothing, gloves, and safety glasses, and work in a well-ventilated area.

Black for silver

The objects are immersed in a 2.5% solution of potassium or sodium sulphide. After the appearance of the color, the silver objects are thoroughly washed and protected by waxing or varnishing.

Green for copper and alloys

The objects are painted or sprayed with a solution of 250 grams of ammonium carbonate / 250 grams of ammonium chloride / 1 liter of water. Each layer is dried for 24 hours. After reaching the desired tone, the material is waxed or varnished. If the amount of chloride is reduced, the color will be more turquoise. If the concentration of ammonium carbonate is reduced, the color will be more chartreuse.

Black for copper

The copper objects are immersed in a potassium or sodium polysulfide solution (2.5%). Alternatively, sulfurated potash can be used. After the color is developed, the objects must be rinsed, dried, and waxed or varnished.

Brown for copper

The copper objects are boiled in a solution of 12% copper sulfate and water that is at least 3 days old. After the color is developed, the material is rinsed, dried, and waxed or varnished.

Black for iron

The objects are very thinly coated with linseed oil, then gradually heated up to 300 - 400 °C. If necessary, the process is repeated. This process may be used on any metal except lead, tin and its alloys, that can be heated to the temperature mentioned.

Brown for iron

The object is coated with a 5% aqueous solution of ferric chloride. After 24 hours it is rubbed with a coarse cloth or finest steel wool. Subsequently, the process is repeated at least 3 times. Finally, the material must be wiped with a greasy cloth.

Gray for tin

The objects are immersed in a 20% aqueous solution of ferric chloride and then rinsed, dried and waxed or varnished, when colored.

Gray-black for zinc

The objects are immersed in a 20% aqueous solution of ferric chloride for 20 minutes. After the desired color has been reached, the objects must be washed, dried and waxed or varnished.

Black for aluminum

The objects are immersed in a boiling solution of 20 g of ammonium molybdate and 5 g of sodium thiosulphate in a liter of water. It is necessary to rinse, dry and wax or varnish the objects after the development of color.

Lustre colors patina (interference colors)

A solution of 280 grams of sodium thiosulphate, 25 grams of copper acetate and 30 grams of citric acid can be used on copper and its alloys, silver, nickel, iron, and gold. The color depends on the duration of immersion, the sequence of colors on brass: golden yellow-copper-purple-dark blue-light blue-chrome-nickel-red-gray, only blue and gray-black on iron or carbon steel

Different colors on titanium

3% trisodium phosphate solution, a stainless steel cathode and an object as anode can be used for a simple electrolyte.

Many other electrolytes can be used - supposedly even Coca Cola.

The colors depend on the DC voltage.

Color Voltage
Straw yellow 10 V
Purple 29 V
Blue 30 V
Blue green 45 V
Light green 55 V
Purple red 75 V
Gray 110 V

It is mandatory that this process is performed while wearing rubber gloves because of the potentially dangerous voltage.

Different colors on stainless steel 18 Cr/8 Ni

- 250 gms chromic acid, 500 mll sulphuric acid, 500 mll water, lead cathode, object=anode, temp.80 C[17]

The color depends on the duration of immersion (5 - 50 minutes). The sequence of colors is brown, blue, purple, green. After that step, immersion in a 50 - 100 gms potassium dichromate bath must be used (80 - 90 C, pH 4-5, 15-20 min.).[18]

Further reading

In English
  • Hiorns, A.: Metal Colouring and Bronzing, London 1892.
  • Field, S., Bonney,S.R.: The chemical coloring of metals and allied processes, New York 1925.
  • Angier, R.H.: Firearm Blueing and Browning, Onslow County 1936.
  • Fishlock, David: Metal Colouring, Teddington 1962.
  • Hughes, R.; Rowe, M.: The Colouring, Bronzing and Patination of Metals, London 1982.
  • LaNiece, Susan, Craddock, Paul: Metal Plating and Patination: Cultural,Technical and Historical Developments, Boston 1993.
  • Sugimori, E.: Japanese patinas, Brunswick 2004.

In German
  • Buchner, G.: Die Metallfärbung und deren Ausführung, Berlin 1891.
  • Beutel, E. Bewahrte arbeitsweisen der metallfaerbung, Wien 1913.
  • Krause, H. Metallfaerbung, Berlin 1922.
  • Krämer, O.P. and Jelinek, T.W. Rezepte für die Metallfärbung Chemische Metallfärbung und farbige Metallschichten, Saulgau 2007.
  • Berger, D. : Bronzezeitliche Färbetechniken an Metallobjekten nördlich der Alpen, Halle 2012.

In Italian
  • Ghersi, I. : Metallocromia. Colorazione e decolorazione dei metalli per via chimica ed elettrica., Milan 1898.

In Spanish
  • Castells, F. : “Coloración de los metales”, Viladrau Girona 1992.

In Russian
  • Jampolskij, L. Voronenije-Prakticheskoe rukovodstvo po himicheskom okrashevaniju zheleznogo metalla, Leningrad 1929.
  • Odnoralov, N.V. Dekorativnaja otdelka skulpturi i hudozhestvenih izdelij iz metalla, Moscow 1989.
  • Bobrikova, I.G.; Selivanov, V.N. Tehnologii elektrohimicheskoi i himicheskoi hudozhestvenno dekorativnoi obrabotki metalov i ih splavov, Novocherkask 2009.

In English

In French

In German

In Spanish

In Russian

In Croatian

References
  1. Fishlock, David: Metal Colouring, Teddington 1962., p.8
  2. Hughes,R.;Rowe,M. The Colouring,Bronzing and Patination of Metals, London ,1982, page 10
  3. http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Books/Leyden%20&%20Stockholm%20Papyri.pdf Retrieved 1.01.2018.
  4. https://archive.org/stream/bub_gb_Gsh2BJGzZLEC#page/n67/mode/2up Retrieved 2.01.2018.
  5. http://northernlightstudio.com/docs/Patina.doc Retrieved 26.01.2018.
  6. L. Nobili: Sui colori in generale ed in particolare sopra una nuova scala cromatica dedotta dalla metallocromia ad uso delle scienze e delle arti, Antologia, 39, 117, 1830 e su Bibl. Univ. 15, 337, 1830; e 16, 35, 1830
  7. Fishlock, David : Metal Colouring,Teddington 1962., page 126
  8. http://gallica.bnf.fr/ark:/12148/bpt6k6218907f.r=+patines+du+bronze.langFR Retrieved 2.01.2018.
  9. http://dingler.culture.hu-berlin.de/article/pj190/ar190108 Retrieved 7.02. 2018.
  10. UK patent 290901, 1927.
  11. UK patent 223994, 1923
  12. US Patent Office, http://www.reflectionandrefraction.co.uk/history.html Retrieved 19.01.2018.
  13. https://pdfpiw.uspto.gov/.piw?Docid=03804730&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D3804730.PN.%2526OS%3DPN%2F3804730%2526RS%3DPN%2F3804730&PageNum=&Rtype=&SectionNum=&idkey=NONE&Input=View+first+page .Retrieved May 5th 2020
  14. https://patents.google.com/patent/US2957812A/en
  15. https://www.up2europe.eu/european/projects/biological-patina-for-archaeological-and-artistic-metal-artefacts_8707.html Retrieved 15.01.2018.
  16. B.A. Dajnowski, J. Marczak, A. Sarzyński, M. Strzelec, J.L. Mass, A. Lins, S.I. Shah, R. Murray,T.P. Beebe Jr., Z. Voras Creating Laser Patinas on Copper Alloys: Origins of Colors and Their Implications on Copper Alloys, METAL 2016,New Delhi 2017.,Conference Proceedings ,pages.153 - 160
  17. https://patents.justia.com/patent/4071416
  18. http://tf.npi-tu.ru/assets/tf/kht/files/posobie-po-txdo.pdf


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