By admin
http://www.cwhdallas.com/ussr-ceramic/
Ussr Ceramic
![Revolutionary Ceramics: Soviet Porcelain, 1917-27]() |
Revolutionary Ceramics: Soviet Porcelain, 1917-27
Sale Price: $40.00
|
|
|
This is a survey of the wide range of painted ceramics made in Leningrad (St. Petersburg) following the 1917 Revolution. Combining avant-garde decoration with revolutionary exhortations, the ceramics were intended to carry Bolshevik propaganda far and wide in a campaign to woo the masses. Today, these "agitceramics" are seen both as historical documents and collectible decorative pieces in their own right. With over 200 illustrations and complete with historical introduction, expert analysis and informative captions, this book is for collectors and non-collectors alike.
|
Understanding Synthetic And Imitation Gemstones
Laboratory-created stones, or synthetic gemstones, as the name implies, are made in a laboratory. Synthetic gemstones do not have the rarity of naturally colored stones and they are less expensive than naturally mined stones. Because of the way they are made, synthetic gems may show subtle differences in shape and colour that help lo distinguish them from their natural counterparts. A synthetic gemstone is identical to a natural gemstone in almost every way. This includes the same basic crystal structure, refractive index, specific gravity, chemical composition, colors, and other characteristics. Since the same gemological tests are used for stone identification on both natural and synthetic gems, it is sometimes even possible for a gemologist to be puzzled as to whether or not a stone is natural or synthetic. Such stones can be made colourless, or, by the use of metallic oxides, in many colours, and thus can be made to resemble many natural stones, including the Amethyst, diamond, spinel, emerald, Opal and corundum (ruby and sapphire). Alexandrite.
Lapis lazuli, turquoise, and coral produced by the French manufacturer, Gilson, are similar to their natural counterparts, but are not true synthetics because their optical and physical properties differ from the natural gems. Gilson lapis lazuli, for example, is more porous and has a lower specific gravity.
Methods of making Synthetic gemstones
Growth from Melt:
* The Flame Fusion or Verneuil Process
Originally developed (1902) by a French chemist, Auguste Verneuil, the process produces a boule (a mass of alumina with the same physical and chemical characteristics as corundum) from finely ground alumina (Al2O3) by means of an inverted oxyhydrogen torch that opens into a ceramic muffle. With slight modifications, this method is used to produce spinel, rutile, and strontium titanate.
In Verneuil method for making rubies, a rod with a “seed crystal” is lowered into melted minerals and then brought back up. Repeating this process over and over grows a large crystal on the end of a rod from the melted minerals. The ruby can then be detached and cut and polished.
* Pulling or Czochralski's Technique
A method of growing synthetic crystal by high-melting point devised by Polish scientist Jan Czochralski, who discovered the method in 1916 while investigating the crystallization rates of metals. Czochralski and is named as Czochralski pulling technique. Where a seed crystal is gently lowered until it is in contrast with pure melt in the crucible and it is then pulled slowly upwards. The product shows rod-like single crystals. Used to make rare-earth garnets, lithium niobate, synthetic scheelite, and synthetic alexandrite.
* Brigman-Stockbarger Technique
The method involves heating polycrystalline material in a container above its melting point and slowly cooling it from one end where a seed crystal is located. Single crystal material is progressively formed along the length of the container. The process can be carried out in a horizontal or vertical geometry.
Growth from Solution:
* Hydrothermal Method
Aquamarine and Quartz crystals are grown in a solution in autoclaves, where temperature and pressure are controlled to create the feed material called lascas in the hotter portion. Seed crystals are in the cooler portion upon which the lascas redeposits, forming synthetic quartz. This process can take 30 to 60 days, and is also used to grow amethyst, citrine, or rock crystal.
* Flux Melt Method
Pioneered by the French chemist Edmond Fremy, the flux-melt technique is still used to make emeralds. The powdered ingredients are melted and fused in a solvent (flux) in a crucible. The material must be kept at a very high temperature for months, before being left to cool very slowly.
* High Temperature/Pressure Method:
One type of solution growth from water at high temperatures and pressure is known as the hydrothermal technique. Growth from solution is achieved by an increase in saturation. Like adding increasing amounts of sugar to water, the sugar will dissolve until the solution is saturated and can absorb no more and then it will begin to re-crystallize. With cooling, crystallization will increase and with heating it will decrease. A seed crystal is often employed to initiate the crystallization and to provide a point for the deposition to begin. Crystal growth rates are a function of time, temperature, and concentration. Hydrothermal techniques are used to make emeralds, quartz, rock crystal and amethyst.
* Skull Melting Process
This process was perfected in USSR specifically for crystallizing synthetic cubic zirconia, for uses in optical, electronic and laser equipment. Cubic zirconium oxide has a very high melting point and is a very reactive material. No container can hold this melt since cubic zirconia has a melting point of 2750°C, and hence a cold crucible or skull is used.
Imitation stones can be made up of any substance, like glass, paste or strass, faience, porcelain, acrylic, and plastics. Ceramics are the most common and least expensive simulants and they are used as substitutes for many kinds of popular gem materials like turquoise, coral, jade, pearls. Imitation stones are made to simulate a particular gemstone, but they do not have the same chemical, physical, and optical properties of the natural gemstone like the synthetic gemstone and hence can be easily distinguished from synthetic and natural gemstones. Volcanic glass, a man-made substance from obsidian and volcanic ash, is a possible candidate for imitation aquamarine and pale sapphire. Some examples of imitation stones are cubic zirconia, Synthetic Moissanite (Diamond) , and yttrium aluminum garnet which are diamond simulants. Since diamonds are so valuable and popular, the market for diamond impostors is huge, and many additional simulants like cubic zirconia, GGG, YAG, strontium titanate, synthetic rutile, and moissanite have been created over the last 50 years. Other imitation stones are Emerald Doublet, Foil Glass (Opal), Imitation Lapis Lazuli, Synthetic Forsterite (Tanzanite), Synthetic Forsterite (Peridot).
read more about Natural, Organic, Synthetic and Imitation gemstones
About the Author
__________________________
Iris Dcosta
www.valuablestones.com
Professors bring unique experiences to the classroom
By: Elijah McKenzie At a college as diverse as the University of Louisville, students are fortunate to have an eclectic mix of backgrounds, experiences, talents and traditions in their professors. Additionally, many professors hold other jobs as well, allowing students to se...
Thanks for visiting!
