Post 627 –by Gautam Shah
A coating is a thin surfacing that is applied or attached to the surface with various degrees of ‘chemical’ integration or amalgamation. Metal coatings are of four types: ‘Organic’ coatings like paints, organosols or other polymeric compounds, Non-Organic coatings of metals, Coatings of metalloids reaching to ceramic states, and Gas reaction-deposition systems.
Metals need a coating to alter the physical and chemical properties of a surface, such as to endow desired quality of texture, colour, patterns, electrical properties, surface reactivity, strength properties, etc. Often a surface treatment is in preparation for another treatment, as a temporary or permanent application. Maintenance of specific surface properties during sub-processing, transit, fabrication, installation and repairs require special coating systems.
Metal items require some form of ‘surface-deep’ preparatory ‘work’ to receive the coating, but such ‘work’ for small-or-thin-body entities like plates, sheets, foils, wires, threads may involve entire mass of the body. Post such preparatory work, involving heat leaves some stresses in the item. Small or thin body items have uniform stresses. But heavy items and assembled work can have differential stresses, which affects the final coating.
One of the first Metal coating realized by Man was Patina. Metal rusting is nominally a crust like degradation products, but some metals like Bronze, acquire a surface layer over a period of time. Verdigris is the natural patina. Metal artifacts exposed to different environments such air, sea-water, soils acquire patina, a layer consisting of oxides, carbonates, sulfides, or sulfates. Patinas are products of surface mass degradation, and so etch or reduce thickness of the surfaces. Some patinas, however, curtail further degradation of the surface, and so are encouraged. Patinas are often visually appealing and so desired. Effects similar to patina 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. Its surface may be smooth, glossy, or crusty. Newly made objects are deliberately patinated to simulate the antiquity in a process is often called distressing.
Patina over copper alloys, such as bronze, due to the chlorides leads to green, while sulfur compounds are brown. The basic palette for patinas on copper alloys is blue-black due to ammonium sulfide, brown-black with liver of sulfur, 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 the 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.
Metal to metal cladding systems were precursors to Plating technology. Such dual metal structures were created by beating, rolling, rivetting or co-forming. The chief purpose was to add strength to a weaker metal. Forging a metal over metal in some cases created partial homogeneity. It was easier to forge soft metals like tin, lead and copper over harder metals like Bronze, Iron etc. Tin and lead could be softened through heat and used for coating. Metal coating by cladding or sheathing, were useful for corrosion resistance, wear resistance, improved electrical/thermal conductivity and better handling (touch-feel).
Some of the simplest methods of sheathing used Gold, Silver and their amalgams with mercury. Gold, Silver, Tin and mercury based amalgams were used as liquidized coatings, where as Silver and Gold were fused as thin sheets or foils. Tin coatings were used for mirror making.