Glass is a kind of cheap artificial gem. It works well when it is used to imitate natural gems such as chalcedony, crystal, beryl (emeralds and aquamarine), jadeite, nephrite, turquoise and topaz, but it works poorly when it is used to imitate corundum, diamonds and other gems.
Glass can be regarded as a liquid, but it is a rigid liquid or a supercooled liquid with high viscosity, so it has solid properties under normal circumstances; On the other hand, glass can also be regarded as an amorphous solid substance. For glass, there is no clear boundary between its solid state and liquid state. The American Society for Testing Materials (ASTM) defines glass as an inorganic substance that cools from a molten state without crystallization.
Crystalline solids have a fixed melting point, while amorphous glass does not. When glass is heated, it first softens, gradually becomes a viscous melt, and finally becomes a real liquid.
In fact, the glass used in gemology to imitate gems is composed of silicon oxide (a timely component) and a small amount of alkali metal elements such as calcium, sodium, potassium or oxides of lead, boron, titanium, aluminum and barium. According to the properties of imitation gems, the proportion of components can be changed. The composition of glass varies greatly, from Shi Ying glass, which is almost composed of pure silica, to glass with less than 40% silica and more than 50% lead oxide. With the increase of lead oxide content, the refractive index, density and dispersion of glass increase.
(2) Types of glass
There are mainly two kinds of glasses as imitation gems: crown glass and flint glass.
The most commonly used components of crown glass are silicon, soda and lime, which are mainly used to make bottles, window glass and optical glass. Crown brand glass is also used to make imitation gem ornaments on fashion jewelry.
Flint glass contains SiO2 _ 2 and Na2O, PbO is crown glass, also called lead glass, which is used to replace CaO. Because Joseph Strass of Austria first discovered this kind of glass, it is also called Strass glass or Simplystrass. Because the existence of lead improves the refractive index and dispersion, imitation gems made of flint glass are often very realistic.
Other types of glass include silica glass and borosilicate glass. Silicone glass is almost made of pure time, and has been widely used in jewelry field in recent years. Borosilicate glass is mainly used for laboratory and kitchen utensils, and rarely used for imitation gems.
(III) Production of Gem Glass
The vast majority of glass imitation gem products are made by conventional melting process, which is less than the commercial Crown brand glass produced on a large scale. Glass melts in a ceramic crucible. Molding technology is usually used to obtain the required gem shape, which is easy to form smooth corners and concave facets, so sometimes polishing is needed to repair, but sometimes polishing may be limited to the table surface of the gem, sometimes limited to the crown surface, and sometimes all facets. Because tin oxide is often used as a polishing agent, many terms such as cutting tin, polishing tin, etc. Used in glass imitations.
In order to obtain various colors of glass, metal oxides are usually added to the glass as colorants. Commonly used color-developing elements and obtained glass colors are Mn (purple), Co (blue), Se (red), Fe (yellow-green), Cu (red-green-blue), Au (red), Cr (green) and U (yellow-green). The final color of glass also depends on the following factors, such as the type of glass, redox conditions in the manufacturing process, quenching after manufacturing, etc. When making colorless glass, the so-called "glass maker's soap" decoloring agent should be added to remove or reduce the green tone produced by iron impurities in glass.
Some colorless glass imitation gems are colored on the table by painting the pavilion with appropriate colors. The halo of some glass imitations, including very cheap opal imitations, is treated by vacuum coating technology similar to that on camera lenses. There are also many glass imitations, and a mirror-like "foil lining" is added to the exhibition hall to enhance the flash.
The transparency of glass can be controlled in the manufacturing process. If high transparency is to be obtained, high-purity additive components are needed; Tin oxide is added to translucent and opaque glass products.
(4) Properties of Gem-like Glass
1. Appearance characteristics
If the glass imitation is molded, it usually shows smooth small ridges (some natural gem materials with lower hardness may also have smooth small ridges) and concave surfaces. Concave surface is caused by cooling shrinkage of glass. Another characteristic of molded glass is that the surface is often pitted, which is the so-called "orange peel" effect. But some glass imitations are more difficult to identify after fine processing. Sometimes only the crown is polished and the pavilion is shaped.
2. Gloss and fracture
Most of the glass has conchoidal fracture after breaking and presents glass luster on the fracture surface, and sometimes some glasses with high lead content have sub-rhombic luster. Goldstar-like glass containing copper crystal inclusions may form hackly fracture due to inclusions. Except chalcedony and high-quality turquoise, most of the gemstones have no conchoidal fracture, which can be distinguished from glass imitations.
3. Micro-characteristics
Typical glass imitation gem products contain bubbles, most of which are spherical, but they can also be oval, slender or even tubular. On the surface of the cut gem, a hemispherical cavity left by bubbles can be seen. Glass imitations may form streamline structures or irregular staggered belts. This is one of the characteristics of viscous liquid after cooling.
4. Difficulties
The hardness (HM) of glass is 5 ~ 6, which is lower than most natural gems they imitate.
5. Refractive index
The refractive index of glass is generally between 1.47 ~ 1.70, and the highest is 1.95. The difference of refractive index mainly comes from the difference of chemical composition, as shown in Table 4- 1- 10. The main components and some physical properties of common glass types are listed in the table. Among them, glasses with refractive index higher than 1.70 usually have lower hardness.
6. Optical characteristics
Glass is amorphous and isotropic (single refraction). However, due to the stress, when some glasses rotate under the crossed polarizer, they often appear serpentine crossed or non-crossed extinction bands, showing abnormal birefringence. Some devitrified glasses (such as emeraldlike imorite or Metajade) will partially recrystallize and show full brightness under cross polarizer.
7. Density
The density varies greatly with chemical composition, ranging from 2.20g/cm3 to 6.30g/cm3, as shown in Table 4- 1- 10.
Table 4- 1- 10 Composition and physical properties of common glass types
8. Thermal conductivity
Glass has a low thermal conductivity, so it usually feels warm when worn on your hands. However, natural or synthetic crystalline materials usually feel cold because they have better thermal conductivity.
9. Special optical effects
Visible cat's eye effect, starlight effect, placer gold effect, color change effect, luster effect, halo effect and color change effect.
(5) Common glass imitation gem products and their identification.
1. Glass imitation of transparent gemstone
Although glass can be used to imitate any transparent gem, in fact, beryl (emerald and aquamarine), timely (crystal, topaz and amethyst) and topaz are the most commonly used imitations. Be very careful when identifying the imitations of beryl and topaz, because the refractive index and density of glass imitations are within the range of these two gems. At the same time, it should be noted that some glass imitations of emeralds may contain groups of bubbles, forming "fingerprint-like" inclusions similar to gas-liquid two phases (interestingly, this inclusion is somewhat similar to the secondary flux inclusions in synthetic emeralds). But glass is isotropic after all, while beryl and topaz are anisotropic, which can be identified accordingly.
2. Glass imitation of opaque gemstones
Glass is often used to imitate many translucent to opaque gems, especially the common chalcedony (or agate) varieties on the market. Relief glass imitations are also often used to imitate agate relief, that is, glass of one color is used as the backing and glass of another color (usually white) is used to make relief patterns.
Translucent glass imitations such as opal, pearl and moonstone are made by adding oxides, phosphates and other ingredients to the glass containing lime to form insoluble calcium compounds in the glass, and making the glass translucent or opaque according to different contents.
Other non-transparent glass imitations include turquoise and lapis lazuli, and organic gem glass imitations include ivory, coral and shells. There are also glass imitations used to imitate nephrite and jadeite, which have been analyzed. Crystallized glass was first made by Imori laboratory in Tokyo, Japan, and can be made into various colors with different crystallization degrees. This kind of glass imitation can see fernlike structure under the microscope.
3. Gemstone glass imitation with special optical effect.
(1) "Discolored" glass
Many kinds of glass imitations are used to imitate opals. The most economical method is to add metal foil to the molten glass, or to mix the crumbs of pearl oyster into the glass.
John Slocombe invented an opal glass imitation marked with Slocombe stone on the market. This glass imitation has various body colors, including white, green, black, nearly colorless and orange. Among them, orange is used to imitate the "fire" opal in Mexico. In reflected light, colored patches are similar to metal foils, but in transmitted light, they look like colored celluloid (cellophane). The refractive index of this material is 1.50 ~ 1.52, and the density is usually 2.4 1 ~ 2.50g/cm3. Like other glasses, this glass imitation also contains bubbles and flow structures.
(2) Imitation placer gold glass
"Jinxingshi" or "sandstone" in the commercial name is a colorless glass containing a large number of flaky copper crystals. Brown metallic copper crystal makes the whole material appear orange-brown, resulting in a flashing placer gold effect. This kind of glass imitation is similar to natural sunshine stone and has the effect of placer gold.
"Jinxingshi" is made by adding cuprous oxide to glass, and cuprous oxide is reduced to metallic copper during quenching. Copper powder exists in the form of small triangular and hexagonal crystals and can be identified under the microscope. This method can also be used to produce blue glass containing copper flakes and to imitate lapis lazuli containing pyrite.
(3) Glass imitation with cat's eye effect
There are many ways to produce glass imitations with cat's eye effect.
A kind of glass cat's eye called "fire eye" contains long and parallel bubbles, which are produced during the inflation process, similar to carbonated drinks.
A better method is to fuse the fine fiber bundles together by heating to form a "mosaic" structure. These "mosaics" are mainly composed of colorless glass fibers used in optics. The colored glass imitation made by this method can imitate all kinds of natural gems with cat's eye effect, including cat's eye. These glass fibers are stacked in quadrilateral or hexagonal arrangement, with about 65,438+050,000 fibers per square centimeter. The total length of the fibers in the oval gemstone exceeds1609m. ..
(4) glass imitation with starlight effect
Imitations of star ruby and star sapphire can be made of glass, but the effect is poor. Several groups of thin lines are carved on the bottom of oval-cut translucent glass, or bonded to the bottom of the gem with carved metal foil to produce starlight effect. A very beautiful starlight-like gem is made by molding glass into an oval shape, and at the same time, six raised lines are made on the surface to form a starlight shape, and then a layer of dark blue glaze is coated on the surface of the gem. Therefore, the star line looks like a natural starlight gem, located on the surface and below the surface of the gem. Of course, when turning this gem, the star line will not move, but this starlight effect is obvious even in very weak light conditions, which is usually not the case with natural or synthetic starlight gems.
(5) Glass imitation with color-changing effect
There are two kinds of glass imitations with obviously different body colors under different light sources. One of them is glass with rare earth elements added, and its commercial name is Alexandrium, which is pink under incandescent lamp and purple under fluorescent lamp (fluorescent lamp). Another kind of glass imitation, called Tourma-like (tourmaline-like), is light pink under incandescent lamp and unsaturated yellow-green under fluorescent lamp. Other gemological properties of these two glasses are basically the same as those of other glass imitations.
(6) Imitation pearl glass
Some beautiful imitation pearls are made of glass. More complex artificial pearls were made 300 years ago. The inner surface of hollow glass beads is lined with rainbow substance made of fish scales, and then the inside is filled with wax.
At present, the best imitation pearl glass is made by coating several layers of guanine on the surface of white translucent glass.
Usually glass imitates pearls, and teeth feel slippery, while natural pearls and cultured pearls feel sandy. The smooth surface of imitation glass can be seen by magnifying observation, and cracks can be seen on the coating, especially near the drilling hole. Bubbles can be seen in the coating, and the pearl-like fracture shows glass luster.
4. Other special glass imitations
The glass imitation of emerald can be obtained by melting and cooling, and the chemical component molecular formula is Be3Al2Si6O 18+Cr. It has various names, such as beryl glass and "scientific emerald". This kind of glass imitation has lower hardness and weaker luster than crystal emerald.