COPPER 3 -Bronze alloys

Post 488  –by Gautam Shah

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The alloy of copper with arsenic or tin is called bronze. First metal of human history was copper, to be developed about 5000 BC. First copper was produced from naturally available nodules, which were of pure copper. Search for ores that had properties like copper led to Copper-Arsenic mixed ores. At nearly same time Copper mixed with zinc yielded Brass. Copper ores were invariably with Arsenic contents, and produced a bronze alloy of Copper-Arsenic-Tin mix. Due to problems of distinctive classifications of prehistoric objects, scholars use an all-inclusive term ‘copper-based alloys’.

Chinese bells (476–221 BC) > from Wikipedia Uploaded by Spiritia

Copper-based alloys had many local variants, in terms of constituents and smelting-processing. But the achievements across many regions of the world were important enough to be a marked era as Bronze age. This period saw production of harder and more durable tools than any thing available in preceding stone and copper (Chalcolithic) ages.

Bronze flag, Shahdad Kerman, Iran 3rd millennium BC

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Bronze may be perceived as an improvised material than a new discovery. It was a material, which could be cast, formed by beating, and treated with heat to change the properties. Bronze-making, soon outpaced the copper production. Bronze alloys were produced from direct refining of arsenic and tin containing ore materials. Tin bronze was superior to Arsenic bronze in that the alloying process was controlled, and the resulting alloy was stronger and easier to cast. But Arsenic copper was easily and widely available, compared to Tin. During the Bronze age period bronze or brass were not neatly delineated terms.

Bronze cuirass 7th-6th C. BC> Pic from Wikipedia by PHGCOM

There are more than 400 types of copper-based alloys. Many of these are in use since prehistoric period, but with only realization at that point in history that some are different from others. There was no clear awareness as to what is a pure copper, tin or Zinc. It was believed to be another variety or version of the metal. The qualitative peculiarities were attributed to the raw material sources and methods of processing. Both of these attributes were however, exploited to achieve distinctive results. Copper form alloys more freely than most other metals, and mixes with wide range of alloying elements (metals and metalloids) to produce many alloys.

Ancient Greek Bronze Helmet, mid 4th-3rd C. B.C. >originally posted to Flickr as Bronze Helmet Author Claire H.

Modern day Bronze is an alloy consisting primarily of copper with about 12% tin. Some of the bronze alloys often contain other metals such as aluminium, manganese, nickel or zinc, and nonmetal or metalloids such as arsenic, phosphorus or silicon. Bronzes make a wide range alloy metals with peculiar properties like stiffness, wearing, ductility or machine-ability.

China cooking tripod 1046-771 BC. > Wikipedia pic by “Editor at Large”

Copper-based alloys are of many types. The chief categories include Brass and Bronze alloys. Other are Arsenic copper, Beryllium copper, Brass, Dutch metal, Monel, Muntz metal (zinc), Bronze, Arsenic bronze, Aluminum bronze, Bell metal, Gunmetal, Phosphor bronze.

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COPPER -1

Post 477 –by Gautam Shah

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Pure copper nodule 40 mm across

Copper resources of the world are estimated at nearly 5.8 trillion pounds. Of these only about 0.7 trillion pounds have been mined to date. The recycling and recovery rates of copper are so high that of nearly all of the copper mined throughout history, is still in circulation today. This means nearly 80 % of all copper ever mined is still in use today.

Malachite, Zaire> Uploaded by JJ Harrison

Copper was discovered by prehistoric man, in search of shiny stones that when beaten did not break down but rather flattened out. This was sensational discovery leading to search of shiny nodules across lands. Pure Gold and Copper nodules were forged into items of adornments and tools. It was known that several nodules of such pure metals could be forged to form a larger piece. Such nodule findings were rare. But two forms of copper carbonates greenish malachite and bluish azurite were easy to identify and collect from the grounds. Malachite was also used as a gemstone. Similarly many other bright minerals were identified.

Neoclassical vase in malachite in the Hermitage Museum, St Petersburg Uploaded by Dezidor

Azurite 80 x 60 mm Azuritechessy.jpg Uploaded by Archaeodontosaurus

Copper was known around 10000 BC or even earlier. Earliest copper object, a pendant dating 8700 BC, was found Iraq. Copper, in natural nodules form, remained a prized material for a while, but soon became a metal of utility. Unalloyed copper is soft for tools and weapons, but fairly suitable for shaping of utensils. Mesopotamia and Egypt exploited copper for creating tools used in farming, wood working, cooking, etc. These were hoes, adzes, saws, combs , pots, dishes, chisels, harpoons, cloak-pins etc. By 6000 BC it was realized that, although copper was not amenable to casting, it could be worked by hammering, chasing, engraving, and cold-rolling. Sumerians used copper sheets to form sculptures over wooden forms and fastened on walls with copper nails or wires set in bitumen.

Imdugud (also Zu or Anzu), the lion-headed eagle; Sumerian metalwork (sheets of copper), Temple of Ninhursag at Tell al-‘Ubaid; ca. 2500 BC

The Roman supply of copper almost entirely came from Cyprus, and so was known as metal of Cyprus, shortened to cyprium, later corrupted to cuprum. Copper is found at many locations as a primary mineral in basaltic lavas and also as reduced form of copper compounds. It occurs in combination with many minerals, such as chalcocite, chalcopyrite, bornite, cuprite, malachite, and azurite. It is an extremely ductile and malleable metal with high tensile strength. It is an excellent conductor of heat and electric. It is a very noble metal and by acting as a cathode can corrode other contagious metals except stainless steel. It is resistant to most acids and sea water.

Bronze decoration

A Copper alloy with tin is known as bronze. Bronze is a stronger alloy, and harder than both the pure metals. Bronze can be cast. A copper with zinc is called Brass. It was also known that by hammering the copper became hardened, ideal for creation hard edged tools. Copper and its alloys, bronze and brass, mark the first science revolution of man. However, relative scarcity of tin in many regions of the world did not allow use Bronze equally everywhere. Tin-based bronzes were preferred due to the hazard of arsenic poisoning from fumes produced by the oxidation of arsenic-containing minerals. Copper-arsenic alloys, of superior properties to copper in both cast and wrought form, were produced in many regions. Arsenic contents varied from 1 to 7 percent, with up to 3 percent tin. In many civilizations the production of pure copper, arsenical copper, and tin bronze continued together for some time.

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PATINA

Post 358 – by Gautam Shah

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India_statue_of_nataraja

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.

Patinated and ormolu Empire timepiece representing Mars and Venus, an allegory of the wedding of Napoleon I and Archduchess Marie Louise of Austria in 1810.

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Metal artifacts 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.

Bronze_head_(Glyptothek_Munich_457)_with_and_without_patina_Bunte_Götter_exhibition

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 casehardened layer, called cortex, or corticated layer on the surface of Flint tools or a chert nodule.

Desert Varnish stones

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 incrusted 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.

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Firearms nominally carry Parkerizing, a bluing finish, (bonderizing, phosphating or 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.

Statue of Liberty NY USA -Patina

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.

Chinese Wok

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.

464px-WLA_cma_Young_Woman_Nursing_Her_Child_bronze_with_brown_patina

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 vinegar (acetic acid).

Auguste Rodin Balzac Bust 1892

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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.

Kresge Auditorium MIT

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.

Copper Roof Dresden

TIN

Post 352 –  by Gautam Shah 

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Tin was one of the earliest metals to be known, and used in the form of an alloying material with copper to form the bronze. Pure copper is occasionally available as nodules, but its ore is fairly common. Tin is rare, and not found together with copper.

Bronze

First Bronze was produced by smelting copper and arsenic. This was a very toxic process. The arsenic was soon replaced with tin. Tin bronze was superior to arsenic bronze, as the former alloying process was controllable. Tin-based bronze was nontoxic and produced an alloy that was stronger and easy to cast. Tin was not available at places where copper ore was available, so tin had to be bought from elsewhere. Tin resources and trade routes of ancient times had a great bearing on development of bronze cultures. The earliest tin-alloyed bronzes date to 4500 BC, but pure tin was produced after 600 BC.

Early Ewer Iran

Tin is a silvery malleable metal, a chemical element with symbol Sn from Latin =stannum. It does not easily get oxidized in the air. Tin has many applications other than as an alloying material. Tin, with and without lead, is used for soldering metal joints, plating electrical and other wires, steel plates and components, and for forming pewter metal items.

Pewter Tankard

Pewter metal Medal 1863

Pewter German Beer mug

PEWTER is a tin alloy. The earliest piece of pewter was found in Egyptian tombs from 1450 BC. It was extensively used by the Romans for household vessels and ornamental use. The word pewter is possibly a variation of word spelter, a term commonly used for zinc alloys, with similar grey a shiny surface. Pewter is a malleable metal alloy, traditionally 85–99% tin, with the rest formed of copper, antimony, silver, bismuth and sometimes, less common today, the lead. Copper and antimony acts as hardeners, whereas lead is used in decorative (non-food) utilities. Lead containing pewters have a bluish tint. Pewter due to its low melting point (170–230 °C) is easy to cast and work with techniques like chasing, engraving, etc. Pewter was used for making flatware, tableware and church prayer items till early part of 20th C. The early pewters, due to the lead content were hazardous for health. The constituents of pewter were first controlled in the 12th C. by town guilds in France, and later elsewhere. By 16th C. three grades of pewter were common. The first type, the fine metal’, was used for tableware. The second grade known as trifling metal containing up to 4% lead was meant for statuettes and holloware. The third type of pewter, known as ‘lay or ley’ metal, with 15% lead, was used for non-food utilities and for items not used by direct contact. Pewter remained in use till porcelain and glass tableware became common.

Food pack Tinned

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Tinning of Brass cooking vessels Traditional manner India

Handi-and-karahi

TIN PLATING is metal coating of usually pure tin, on variety of metal surfaces, such as mild steel, copper, brass, etc. Cooking utensils were tin plated to prevent copper oxide and sulphate contamination and weather tarnishing of the surface. Romans were using tinned copper vessels for cooking. A tinned iron sheet or tin-plates were developed in Europe in the 14th -15th C. AD. Food canning industry of 1800s would not have existed without the tin plate packing. Tin-plated sheets can be drawn and worked further without any flaking damage to the coating as tin itself acts as a lubricant. Until the middle of the 20th C. tin-plates were manufactured in by immersing individual sheets in a bath of molten tin, whereas now this is done in a continuous electroplating process. Both bright and matte-finish tin coatings can be produced electroplating.

Babbitt metal bearing

BABBITT METAL is a tin-based alloy. It was originally a casting material for bearings. However, now it is used as a thin surface layer as metal-matrix composite. It is used as a bearing metal for engines and constant-turning tools such as saw-blades.

De-soldering of Tin

SOLDERING is a process of joining metal components by a tin metal or its alloys. Tin is melted as a wire or its powder. Familiar electronic solder contains silver, lead and tin. Metals that are to be soldered should be clean and free of oxide film. A flux of pine rosin, hydrochloric acid or zinc chloride is usually applied, which cleans the surface and seals it from the tarnishing effect of the atmosphere.

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TIN ALLOY COATINGS of tin-zinc, tin-nickel, tin-copper, and tin-lead are used. These are used as both protective and decorative finishes. Tin-zinc coatings, are used in automotive industry. Tin-nickel is highly resistant to corrosion and tarnish, finds special use in electrical equipment and scientific instruments. The tin-copper coatings show colour range from bronze to white. Tin-lead coatings are applied to create solder-able surfaces (known as terneplate) on other metals for outdoor corrosion protection and also for gasoline tanks.

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