An oxide (/ˈɒksaɪd/) is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O ion with oxygen in the oxidation state of −2. Most of the Earth's crust consists of oxides. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further oxidation.
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👉 Oxides in the context of Underglaze
Underglaze is a method of decorating pottery in which painted decoration is applied to the surface before it is covered with a transparent ceramic glaze and fired in a kiln. Because the glaze subsequently covers it, such decoration is completely durable, and it also allows the production of pottery with a surface that has a uniform sheen. Underglaze decoration uses pigments derived from oxides which fuse with the glaze when the piece is fired in a kiln. It is also a cheaper method, as only a single firing is needed, whereas overglaze decoration requires a second firing at a lower temperature.
Many historical styles, for example Persian mina'i ware, Japanese Imari ware, Chinese doucai and wucai, combine the two types of decoration. In such cases the first firing for the body, underglaze decoration and glaze is followed by the second firing after the overglaze enamels have been applied. However, because the main or glost firing is at a higher temperature than used in overglaze decoration, the range of colours available in underglaze is more limited, and was especially so for porcelain in historical times, as the firing temperature required for the porcelain body is especially high. Early porcelain was largely restricted to underglaze blue, and a range of browns and reds. Other colours turned black in a high-temperature firing.
In this Dossier
Oxides in the context of Pit fired pottery
Pit firing is the oldest known method for the firing of pottery. Examples have been dated as early as 29,000–25,000 BCE, while the earliest known kiln dates to around 6000 BCE, and was found at the Yarim Tepe site in modern Iraq. Kilns allow higher temperatures to be reached, use fuel more efficiently, and have long replaced pit firing as the most widespread method of firing pottery, although the technique still finds limited use amongst certain studio potters and in Africa.
Unfired pots are nestled together in a pit in the ground and are surrounded by combustible materials such as wood, shavings, dried manure, leaves, and sometimes metal oxides and salts to affect the surface of the pots. The top of the pit may be protected with moist clay, shards, larger pieces of wood, or metal baffles. The filled pit is then set on fire and carefully tended until most of the inner fuel has been consumed. At around 1,100 °C (2,010 °F) the maximum temperatures are moderate compared to other techniques used for pottery, and the pottery produced counts as earthenware. After cooling, pots are removed and cleaned; there may be patterns and colours left by ash and salt deposits. Pots may then be waxed and buffed to create a smooth glossy finish.
Oxides in the context of Flux (metallurgy)
In metallurgy, a flux is a chemical reducing agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining. They are named for the ability to make molten metals easier to flow during smelting.
Some of the earliest known fluxes were sodium carbonate, potash, charcoal, coke, borax, lime, lead sulfide and certain minerals containing phosphorus. Iron ore was also used as a flux in the smelting of copper. These agents served various functions, the simplest being a reducing agent, which prevented oxides from forming on the surface of the molten metal, while others absorbed impurities into slag, which could be scraped off molten metal.
Oxides in the context of Firescale
Firestain is a layer of oxides that is visible on the surface of objects made of metal alloys containing copper when the object is heated, as by a jeweler heating a ring to apply solder during a repair. On copper-containing alloys of gold or of silver (such as sterling silver), it presents as a red or purple stain. This is because at high temperatures, oxygen mixes with the copper to form cuprous oxide and then cupric oxide, both of which disrupt the bright polished surface of the finished piece. There are various methods used to either prevent or repair firestain.
Oxides in the context of Carbothermic reaction
Carbothermic reactions involve the reduction of substances, often metal oxides (O), using carbon (C) as the reducing agent. The reduction is usually conducted in the electric arc furnace or reverberatory furnace, depending on the metal ore. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are applied for production of the elemental forms of many elements. The ability of metals to participate in carbothermic reactions can be predicted from Ellingham diagrams.
Carbothermal reactions produce carbon monoxide (CO) and sometimes carbon dioxide (CO2). The facility of these conversions is attributable to the entropy of reaction: two solids, the metal oxide (and flux) and carbon, are converted to a new solid (metal) and a gas (COx), the latter having high entropy.