Glass is a non-metallic inorganic material, typically classified as an inorganic thermoplastic polymer. It becomes malleable when heated above 650°C and solidifies into a transparent, durable form upon cooling. Once cooled, glass exhibits excellent properties such as transparency, corrosion resistance, wear resistance, and pressure resistance. Compared to steel, it has a lower coefficient of thermal expansion and is a poor conductor of both electricity and heat. Glass is considered an amorphous material, meaning it lacks a crystalline structure.
The specific gravity of glass ranges from 2.46 to 2.5, with a linear expansion coefficient of 9–10×10â»â¶/°C (up to 350°C). Its specific heat is approximately 0.2 between 0°C and 50°C, and its practical surface tensile strength reaches about 500 kgf/cm². The term "glass" originated from the Tang Dynasty, where materials containing lead oxide were referred to as "crystal glass." The amount of lead oxide added varies by country and tradition. Generally, when lead oxide content exceeds 24%, the glass becomes more transparent and has a higher refractive index, making it heavier and softer. This type of glass is ideal for creating intricate glass artworks. In Europe and the U.S., crystal wares are often made from this type of glass, and they are relatively affordable compared to regular glass. Some man-made glasses, known as "glazed," contain lead, which gives them a crystal-clear appearance, and some people refer to them as "crystal." From an artistic perspective, these items can hold collectible value. However, natural crystals or naturally formed glass differ significantly in their growth processes and energy properties, which should be understood when evaluating their worth.
In countries like Austria and Italy, high-end crystal glass wine bottles, ornate glass decorations, cute animal-shaped pieces, Buddhist lotus glazes, and crystal chandeliers are all made from crystal glass. These items are believed to carry unique magnetic field energy. Crystal glass is essentially made from natural crystal, commonly referred to as lead glass. Adding lead increases the refractive index and reduces hardness, making it easier to shape. Many popular crystal cups on the market are also made from lead glass.
Crystal and glass may look similar at first glance, but they are fundamentally different materials. Here are the main differences:
First, the composition varies: Crystal is a crystalline form of silica, while glass is a molten mixture that contains silica but lacks a crystalline structure.
Second, their effects differ: Glass is primarily decorative, while crystal has additional properties, such as piezoelectricity and potential health benefits.
Third, price differs significantly: Crystal is generally much more expensive than regular glass.
Fourth, physical properties vary: Crystal has a higher hardness (Mohs scale 7), compared to glass (Mohs scale 5.5). This means crystal can scratch glass, but not vice versa. Crystal conducts heat better than glass, so it feels colder to the touch. When viewed through a polarizer, only crystal allows light to pass through.
Fifth, processing techniques differ: Glass can be easily cast when hot, reducing material and labor costs. Crystal, however, cannot be re-melted once it's formed, so it must be shaped through cold methods like cutting or grinding, which are more labor-intensive and costly.
Crystal is harder and more resistant to wear, while glass is softer and more prone to scratches. Crystal maintains its color over time, whereas glass tends to yellow with age.
In terms of historical distinction, ancient texts like the Diamond Sutra mention "glaze" as one of the seven treasures of Buddhism, indicating that it was valued differently from crystal and glass. The chemical compositions of natural crystal, glass, and glaze are all based on silica, but there are notable differences. According to authoritative works like *Chinese Ancient Glass Research*, ancient Egyptian "Féon" contained up to 99% silica, while Chinese Zhou Dynasty glass had slightly more than 90%. This 9% difference highlights the fundamental distinction between glass and crystal.
The ancient technique of making glazed items involved adding colored crystal materials called "glazed stone" to the mixture. These materials changed the structure and properties of the final product, resulting in variations in shape, color, and transparency. The Ming Dynasty’s medicinal jade, for example, had a different glaze formula, leading to less transparency and muted colors. The quality of glaze depends on the raw materials and preparation methods, which were traditionally kept secret. This explains why ancient Chinese glass and Western crystal glass, such as "Faience," have a 9% compositional difference.
In summary, glass and crystal are distinct materials with different origins, properties, and uses. Understanding these differences helps in appreciating their unique qualities and values.
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