Henry Moore, Family Group (1950), patinated bronzeBull Head Attachment, about 700-600 BCE, Urartian, northwest Iran, bronze – Cleveland Museum of Art, early example of metal coloringMetallocromia – Nobilis colored rings, Museo Galileo; Florence
Chemical coloring of metals is the process of changing the color of metal surfaces with different chemical solutions.
anodizing – electrolytic passivation process used to increase the thickness of the natural oxide layer, producing a porous surface which can accept organic or inorganic dyes easily. In the case of titanium, niobium, and stainless steel, the colour formed is dependent on the thickness of the oxide (which is determined by the anodizing voltage).
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 5,000 years old. They are bronze casts with some silver-colored parts, which originate from the Anatolian region.[2] Similar processes can be found on some ancient Egyptian copper sheets.[3] Another example of early chemical coloring of metals is the Nebra sky disk, which has a green patina and gold inlays. An early example of black colored iron is the famous Celtic spearhead found in the River Thames and dated between 200 and 50 BC.[4][5]
Pliny the Elder mentioned the distinction between naturally occurring and artificial patina in the first century CE.[6] Another ancient document about the chemical coloring of metals is the Leyden papyrus X (3rd century CE). Zosimos of Panopolis, a Greek-Egyptian alchemist who lived in the late 3rd and early 4th centuries,is also extremely important, since to him attributed texts are considered to be the oldest written recipes for chemical coloring of metals.[7]
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.[8] 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.[9]
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.[10] 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 ( his procedure was further elaborated and perfected by the American J.E. Stareck 1937. ).[11][12] In the 19th century, the first manuals dedicated exclusively to the chemical coloring of metals were published.[13]
In 1868, Puscher reported on the application of multicolored or lustre patina based on sodium thiosulphate and lead acetate for the first time ( contemporary recipes for that patina uses copper compounds instead toxic lead).[14][15]
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.,[16][17] 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.[18][19][20] 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).[21][22][23]
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.[24][25]
Uses
Chemical coloring of metals is primarily used in the manufacture of sculptures, jewelry, badges, medals,clocks, watches and decorations. It is also used in architecture, metallography, 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
The metal to color should be completely free from oxide and grease. Protective clothing, gloves and goggles should be used in a well-ventilated area or outdoors.
Black for silver
Items are immersed in a 2.5% solution of potassium or sodium sulfide, after the appearance of the color wash objects well and wax or varnish it.[26]
Green for copper and its alloys
Paint or spray objects with a solution of 250 grams of ammonium carbonate / 250 grams of ammonium chloride / 1 litre of water, each layer is dried for 24 hours, after reaching the desired shade wax or lacquer it. Brown or black can be used as a base color for copper patina. If the amount of chlorides decreases the color will be more bluish-green, if carbonate decreases, more yellow-green.[27]
Black for copper
Solution of sodium polysulfide 2.5%, items must be submerged in the solution after color developing, wash, dry and wax or varnish colored object.[27]
Brown for copper
Items are boiled in at least 3-day-old water solution of 12% copper sulfate, after color being developed, the material is washed, dried and waxed or varnished.[27]
Coat object with a very thin layer of linseed oil, then gradually heat it to 300–400°C, repeat the procedure if necessary, this process can be used on any metal, which can be heated to the temperature mentioned (except lead, tin and its alloys).[28]
Brown for iron
Use a 5% aqueous solution of ferric chloride. The object is coated with a solution, after 24 hours it is rubbed with coarse cloth or finest steel wool, the process is repeated at least three times, finally, the material should be wiped with a greasy rag.[29]
Gray for tin or pewter
Use 20% aqueous solution of ferric chloride, it is necessary to immerse the objects in solution, dry and wax or varnish.[30]
Gray-black for zinc
Use 20% aqueous solution of ferric chloride, the objects are immersed for 20 minutes, after the appearance of colour, objects should be washed, dried and waxed or varnished.[30]
Black for aluminum
Boil a solution of 20 g ammonium molybdate and 5 g of sodium thiosulfate in a liter of water, immerse the objects, rinse, dry, wax or lacquer after the development of color.[31]
Lustre colours
Use a solution of 280 g of sodium thiosulphate, 25 g of cupric acetate and 30 g of citric acid. It can be used on copper and its alloys, silver, nickel, iron, 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-grey, blue, and gray-black to iron or carbon steel.[32] Variant for tin and pewter: 250 g sodium thiosulphate, 60 g copper acetate, 25 mL acetone, 1 L water, 45-85°C, 1–20 minutes, gold -pink -blue - green.[33] Variant for stainless steel: 100 g sodium thiosulphate, 10 g lead acetate, 12 g potassium sodium tartrate, 12 g copper sulfate, 1 lit water, 18-22°C temperature of solution, 5–50 minutes, yellow, brown, red, green, blue, violet, object must be in contact with piece of copper 300 times smaller surface than surface of treated object.[34]
Different colors on titanium
As a simple electrolyte, it is possible to use a 3% solution of trisodium phosphate, a cathode of stainless steel, object as anode. The color depends on voltage. Many other electrolytes can be used—even Coca-Cola. Straw yellow / 10V – violet / 29 V – blue / 30 V – blue green 45 V – light green / 55 V – purple-red / 75 V – grey / 110 V. It is mandatory that this process must be performed wearing rubber gloves (potentially dangerous voltage!).[35]
Various colors on stainless steel 18 Cr / 8 Ni
7.5 g of sodium dichromate, 1000 mL of sulfuric acid (1.24 g/cm3), lead cathodes, object as an anode, 70-90°C temp, 0,06 A/dm2, voltage 1.3 V – colors depend on the duration of the procedure (5-50 min.), brown, blue, reddish brown, yellow, green. According to Russian literature after processing items should be soaked in a solution of potassium bichromate (5-10%), 5–15 minutes, 70-90°C temperature of the solution.[36] According to one Chinese patent, treated objects can be then immersed in a hot diluted sodium silicate solution (1-5%,95-100 C,3-10 min.).[37] Hexavalent chromates are carcinogenic and toxic, molybdate-based solutions are now being proposed as a substitute (for example molybdate 30-100g/boric acid 10-18 g/manganese sulfate 0.5-5 g /1 litre water. 0.1 -20 A/dm2, 0.1–15 minutes).[38][39][40]
Related Research Articles
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity."
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated, and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.
Patina is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and metal alloys, or certain stones and wooden furniture, or any similar acquired change of a surface through age and exposure.
Electropolishing, also known as electrochemical polishing, anodic polishing, or electrolytic polishing, is an electrochemical process that removes material from a metallic workpiece, reducing the surface roughness by levelling micro-peaks and valleys, improving the surface finish. Electropolishing is often compared to, but distinctly different from, electrochemical machining. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating. It may be used in lieu of abrasive fine polishing in microstructural preparation.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The chloralkali process is an industrial process for the electrolysis of sodium chloride (NaCl) solutions. It is the technology used to produce chlorine and sodium hydroxide, which are commodity chemicals required by industry. Thirty five million tons of chlorine were prepared by this process in 1987. The chlorine and sodium hydroxide produced in this process are widely used in the chemical industry.
Plating is a finishing process in which a metal is deposited on a surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.
Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating. It is named after the blue-black appearance of the resulting protective finish. Bluing involves an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite, the black oxide of iron. In comparison, rust, the red oxide of iron, undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off, causing the typical reddish rusting away of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. In colloquial use, thin coatings of black oxide are often termed 'gun bluing', while heavier coatings are termed 'black oxide'. Both refer to the same chemical process for providing true gun bluing.
Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance, and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.
Bronze disease is an irreversible and nearly inexorable corrosion process that occurs when chlorides come into contact with bronze or other copper-bearing alloys. It can occur as both a dark green coating, or as a much lighter whitish fuzzy or furry green coating. It is not a bacterial infection, but the result of a chemical reaction with the chlorides that usually occurs due to contamination of the bronze object by saltwater or from burial in specific types of soil where chloride salts are present. If not treated, complete destruction of the affected artifact is possible. Treatment is very difficult, costly and not always effective. Transfer of chlorides from the contaminated artefact to other artefacts can spread the condition.
Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged workpiece. Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding is similar to electrochemical machining but uses a wheel instead of a tool shaped like the contour of the workpiece.
Copper has earned a respected place in the related fields of architecture, building construction, and interior design. From cathedrals to castles and from homes to offices, copper is used for a variety of architectural elements, including roofs, flashings, gutters, downspouts, domes, spires, vaults, wall cladding, and building expansion joints.
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices. This phenomenon is named after Italian physician Luigi Galvani (1737–1798).
Electrochemical coloring of metals is a process in which the surface color of metal is changed by electrochemical techniques, ie cathodic or anodic polarization. The first method of electrochemical coloring of metals are certainly Nobili's colored rings, discovered by Leopoldo Nobili, an Italian physicist in 1826. In addition to the multicolored coatings mentioned, he has also been able to obtain monochrome coatings, and he called that technique metallocromia. Electrochemical coloring of metals based processes are black, green and blue nickel plating, black chromium plating, black rhodium plating and black ruthenium plating. Anodic oxidation of aluminum, titanium, niobium, tantalum and stainless steel are also electrochemical colouring processes.
Niiro, also known as niiro-eki (煮色液), niiro-chakushoku (煮色着色), nikomi-chakushoku (煮込み着色) or niage (煮上げ), is an historically Japanese patination process, responsible for the colouration of copper and certain of its alloys, resulting in the irogane class of craft metals, including shakudo, shibuichi and kuromido. It is now practiced in a number of countries, primarily for the making of jewellery and decorative sword fittings, but also for material for hollowware and sculpture. Importantly, the same process operates differently on different metals so that a piece with multiple components can be treated in one patination session, developing a range of colours.
References
↑ Fishlock, David: Metal Colouring, Teddington 1962., p.8
↑ LaNiece, Susan; Craddock, Paul: Metal Plating and Patination: Cultural,Technical and Historical Developments,Boston 1993.,p.6
↑ Hughes,R.;Rowe,M. The Colouring,Bronzing and Patination of Metals, London ,1982, page 10
↑ 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
↑ Fishlock, David: Metal Colouring,Teddington 1962., page 126
↑ 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
↑ Fishlock, David: Metal Colouring,Teddington 1962.
1 2 3 Deutsches Kupfer Institut: Chemische Färbungen von Kupfer und Kupferlegierungen, Berlin 1974
↑ Angier, R.H.: Firearm Blueing and Browning, Onslow County 1936.
↑ Angier, R.H.: Firearm Blueing and Browning, Onslow County 1936.
1 2 Buchner,G.: Die Metallfärbung und deren Ausführung, Berlin 1891.
↑ Untracht, O.: Jewelry concepts and technology, New York 1980.
↑ Bobrikova, I.G.; Selivanov, V.N.: Tehnologii elektrohimicheskoi i himichskoi hudozhestvenno dekorativnoi obrabotki metalov i ih splavov, Novocherkask 2009., p. 87
Fishlock, D. (1962). Metal Colouring. Teddington: R. Draper. OCLC3982659.
Hughes, R.; Rowe, M. (1991). The Colouring, Bronzing and Patination of Metals (3rded.). London: Thames and Hudson. ISBN9780500015018. OCLC24734412.
LaNiece, S.; Craddock, P. (1993). Metal Plating and Patination: Cultural, Technical and Historical Developments. Oxford: Butterworth-Heinemann. ISBN9780750616119. OCLC27336439.
Runfola, M. (2014). Patina: 300+ Coloration Effects for Jewelers & Metalsmiths. Loveland: Interveave Press. ISBN9781620331392. OCLC871436497.
External links
Lewton Brain, C.: Patinas for small studios (pdf-file), 1985
Budija, G.: Collection of formulas for the chemical,electrochemical and heat colouring of metals, the cyanide free immersion plating and electroplating (pdf-file), Zagreb 2024
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