The composition or structure of a ceramic (for example, nearly all elements, almost all types of bonds, and all crystal surfaces) makes the subject of ceramics very broad, and it is difficult to define identifiable properties (such as hardness, stiffness, electrical conductivity, etc.) for the entire group. Generally, ceramics have properties such as high melting temperature, high hardness, poor conductivity, high elastic modulus, high chemical resistance, and low ductility. However, there are exceptions (for example, piezoelectric ceramics, glass transition temperature, superconducting ceramics, etc.). Many composites like fiberglass and carbon fibers, although containing ceramic materials, are not considered part of the ceramic family. The first ceramics made by humans were pottery objects (such as pots or vessels) or sculptures made from clay, which were either used alone or mixed with other materials such as silica, then fired and hardened by heat. Later, ceramics were glazed and fired to create smooth and colored surfaces, and porosity was reduced by using glassy and amorphous ceramic coatings on crystalline ceramic substrates. Today, ceramics include household, industrial, and construction products, and there is also a wide range of ceramic arts. In the twentieth century, new ceramic materials were developed for use in advanced ceramic engineering, such as semiconductors. The word "ceramic" comes from the Greek word κεραμικός (keramikos), meaning "pottery" or "for pottery," which itself comes from κέραμος (keramos), meaning "potter, tile, pottery." The earliest known reference to the root "ceram-" relates to the Mycenaean Greek word ke-ra-me-we, meaning "ceramic workers," written in the syllabic script known as Linear B. The word "ceramic" may be used as an adjective to describe a material, product, or process, or it may be used as a noun, either singular or more commonly as a plural noun.