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Watch Glass - it is from the existing article

Types
There are two types, Glass watch glasses and Plastic watch glasses used in the laboratory. These watch-glasses are available in various sized and widths. Watch-glass are usually thicker then any other glass or plastic lab-ware.


 * Glass watch glasses - This can be reused after it is sterilized in an autoclave or an laboratory oven. The glasses made provide high resistance to Thermal Shock, chemical resistance and withstand mechanical durability.


 * Plastic watch glasses - These are disposable watch-glass used in laboratory work to avoid any cross-contamination while preparing the samples. They are very good at implementing in low-temperatures and has a operating span of -70 degree Fahrenheit to 275 degree Fahrenheit (-56.6667 to 135 degree Celsius), it can also resist from UV light degradation. These plastic watch glasses are less expensive and light in weight.

Common Glasses
3 typical materials used to make watch-glass, as these materials are the transparent covering over the face:


 * 1) Acrylic Crystal - It is the same material as a plastic and it is transparent, strong and versatile. These are not very scratch resistant as it produces good results without costing a lot of money, but these scratches can be removed under certain processes.
 * 2) Mineral Crystal - Ordinary glass are treated by using heat or chemicals to make it more resistant to scratches. Moreover, If there is too many scratches then it has to be replaced as this could result in the breakage of the glass.
 * 3) Sapphire Crystal - There are two types of sapphire crystals that most watch-glass are made up of, either the original sapphire or the synthetic sapphire. It cost more then the other two crystals, but it is long-lasting and does not shatter or gets scratched easily.