General Design and Functions
Much of the mug design aims at thermal insulation: the thick walls of a mug, as compared to the thinner walls of teacups, insulate the beverage to prevent it from cooling or warming quickly. The mug bottom is often not flat, but either concave or has an extra rim, to reduce the thermal contact with the surface on which a mug is placed. These features often leave a characteristic O-shaped stain on the surface. Finally, the handle of a mug keeps the hand away from the hot sides of a mug. The small cross section of the handle reduces heat flow between the liquid and the hand. For the same reason of thermal insulation, mugs are usually made of materials with low thermal conductivity, such as earthenware, bone china, porcelain or glass.
A travel mug (introduced in the 1980s) generally employs thermal insulation properties for transporting hot or cold liquids. Similar to a vacuum flask, a travel mug is usually well-insulated and completely enclosed to prevent spillage, but will generally have an opening in the cover through which the contents can be consumed during transportation without spillage. Mugs with inner and outer wall but not vacuum treated are generally called double wall mugs. Usually stainless steel will be used for the inner wall while outer wall can be stainless steel, plastic or even embed with other materials.
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