Post 358 – by Gautam Shah
Patina is a thin crust like surface layer forming on Bronze. Similar surface layers formed on other metals, stone, pottery, wood, etc. are also called Patina. Metal patina is a coating formed due to environmental exposure consisting of oxides, carbonates, sulfides, or sulfates. Patinas are formed by degradation of the surface mass and so etch or reduce its thickness. Some patinas, however, curtail further degradation of the surface and so are encouraged. Patinas are visually appealing and so desired.
Metal artefacts exposed to different environments such air, sea-water, soils acquire patina. Similar effects can be achieved by designed exposure and by treating with various chemicals. Patinas are commonly green, but may vary in colour such as of red, brown, black, blue, or gray colours. Its surface may be smooth, glossy, or crusty. Patina also refers to accumulated changes on surface texture and colour, due to long term use of an object like coins or items of wood furniture.
Desert patina or Varnish is a thin, dark red to black mineral coating (of iron and manganese oxides and silica) seen on exposed pebbles and rocks in desert terrains. This is deposition of moisture dissolved minerals drawn to the surface by capillary action of evaporation. Wind abrasion removes the softer salts, and polishes the surface to a glossy finish. In geology and geomorphology, the stone patina also refers to a case-hardened layer, called cortex, or corticated layer on the surface of Flint tools or a chert nodule.
PATINA has probable origin from a Latin word for shallow dish, or ‘patere’, =to lie open. By extension, the word is applied to the discoloured or in-crusted surface of marble, flint, etc. The chemical process, by which patina forms, is called patination, and an artefact coated by patina is said to be patinated. Newly made objects are deliberately patinated to simulate the antiquity. The process is often called distressing.
Firearms nominally carry Parkerizing, a bluing finish, (bonderizing, phosphatizing) a conversion coating treatment for corrosion and wear resistance to a steel surface. These objects develop patina after the bluing finish gets worn. Firearms with such patina finish are highly valuable antiques.
Patinas are created over frying-cooking vessels such as Kadhai and Tava (frying pan and Roti baking pan, India), Woks for Chinese preparations, and other metal baking dishes, by seasoning them with oils and salts, when used for the first time. The patina prevents rusting and food sticking to the vessels. To protect the patina, such vessels are scrubbed lightly, and washed gently with cold water.
Verdigris is the natural patina formed over copper, brass or bronze, when exposed to the air or seawater, over a period of time. It is usually a basic copper carbonate, but near the sea could be a basic copper chloride. The name verdigris comes from the Middle English vertegrez, from the old French verte grez, an alteration of vert-de-Grèce =green of Greece or vert-de-gris =green of grey.
Patina over copper alloys, such as bronze, due to the chlorides leads to green, while sulphur compounds are brown. The basic palette for patinas on copper alloys is blue-black due to ammonium sulphide, brown-black with liver of sulphur, blue-green for cupric nitrate, and yellow-brown due to ferric nitrate. For new artefacts accelerated patination carried out by applying chemicals with heat. Colours range from matte sandstone yellow to deep blues, greens, whites, reds and various blacks. Some patina colours are achieved by the mix of pigments and chemicals. The surface is enhanced by waxing, oiling, or other types of lacquers or clear-coats. French sculptor Auguste Rodin used to instruct assistants to urinate over bronzes stored or buried in the yard. A temporary-washable patina is produced on copper by vinegar (acetic acid).
In architecture, metals, like copper, bronze, etc. have been used for a very long time, for wall cladding, door panelling, ceiling tiles, and roof covering. Copper provides excellent corrosion resistance. Copper surfaces form tough oxide-sulfate patina coating that protects underlying copper mass and resists further corrosion. Copper corrosion products are less toxic. Copper sheets have been used in many building to cover rounded domes, and articulated roof surfaces. Architectural copper is, though susceptible to oxidizing acids, heavy-metal salts, alkali, sulfur and nitrogen oxides, ammonia, sulfur and ammonium compounds. Brass, an alloy of copper and zinc, has good resistance to atmospheric corrosion, alkali, and organic acids.
One of the most common surface degradation products is rust on steels Rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper or bronze surfaces. Rust is permeable to air and water, therefore the interior metallic iron beneath a rust layer continues to corrode. Rust prevention thus requires coatings that preclude rust formation.