How are glass, ceramics and glass-ceramics defined?Frequently Asked QuestionsMost of us think of glass as the transparent materials found as windows in buildings and cars. However, strictly speaking the term glass describes a state of matter where the atoms/molecules are randomly arranged, as in the diagram below.

An ASTM definition of glass describes the inorganic, amorphous, product of a rapidly cooled melt. Using this definition, rapidly cooled metals can also be said to be glassy. Terms such as glassy, amorphous and vitreous all describe the same thing; a material with a randomly arranged atomic structure.

Contrary to popular belief, glasses cannot be thought of as very viscous liquids. Glasses exhibit a glass transition temperature, below which they are true solids and above which they flow albeit as a very viscous liquid.

The diagrams below demonstrate some of the structural differences between a glass and a ceramic on the atomic scale, even though they can have exactly the same composition. For example, silica glass has the same composition as quartz (crystallised silica). However, in glass, the building blocks (SiO4 tetrahedra) are arranged randomly; whereas silica crystals have a very ordered structure.

It is possible to turn a glass into a ceramic; by heating it up. This allows rearrangement from a random to an ordered structure and an ordered structure is more stable than a disordered one. Materials that are initially fabricated as glasses (and perhaps shaped using glass moulding techniques) and converted to a ceramic to enhance their properties are called glass-ceramics. A well known example of a glass-ceramic is the 'ceramic' cooker hob, shown in the photograph below, which has been developed to have a thermal expansion coefficient close to zero. This allows it to be rapidly heated and cooled without generating stresses in the hob material.

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Glass-ceramics are polycrystalline materials produced through controlled crystallization of base glass. Glass-ceramic materials share many properties with both glasses andceramics. Glass-

ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, which is usually not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass as well as special properties of ceramics. Glass-ceramics usually have between 30% [m/m] to 90% [m/m] crystallinity and yield an array of materials with interesting properties like zero porosity, high strength, toughness, translucency or opacity, pigmentation, opalescence, low or even negative thermal expansion, high temperature stability, fluorescence, machinability, ferromagnetism, resorbability or high chemical durability, biocompatibility, bio-activity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, high resistivity and break down voltage. These properties can be tailored by controlling the base glass composition and by controlled heat treatment/crystallization of base glass.

Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass manufacturing process. The glass is cooled down and is then reheated in a second step. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, glass-ceramics have, unlike sintered ceramics, no pores.

A wide variety of glass-ceramic systems exists, e.g., the Li2O x Al2O3 x nSiO2-System (LAS-System), the MgO x Al2O3 x nSiO2-System (MAS-System), the ZnO x Al2O3 x nSiO2-System (ZAS-System).

Contents  [hide] 

1   LAS System

2   Cooktops

3   Brands and manufacturers

4   Source

5   Literature

LAS System[edit]

The commercially most important system is the Li2O x Al2O3 x nSiO2-System (LAS-System). The LAS-system mainly refers to a mix of lithium-, silicon-, and aluminum-oxides with additional components e.g., glass-phase forming agents such as Na2O, K2O and CaO and refining agents. As nucleation agents most commonly zirconium(IV)-oxide in combination with titanium(IV)-oxide is used. This important system was studied first and intensively by Hummel,[1] and Smoke.[2]

After crystallization the dominant crystal-phase in this type of glass-ceramic is a high-quartz solid solution (HQ s.s.). If the glass-ceramic is subjected to a more intense heat treatment, this HQ s.s. transforms into a keatite-solid solution (K s.s., sometimes wrongly named as beta-spodumene). This transition is non-reversible and reconstructive, which means bonds in the crystal-lattice are broken and new arranged. However, these two crystal phases show a very similar structure as Li could show.[3]

The most interesting properties of these glass-ceramics are their thermomechanical properties. Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C. The dominant crystalline phase of the LAS-glass-ceramics, HQ s.s., has a strong negative coefficient of thermal expansion (CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s.. These negative CTE's of the crystal-phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTE's in the finished composite. Mostly for today's applications a low or even zero CTE is desired. Also a negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point, generally between 60% [m/m] and 80% [m/m] crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero. Also, when an interface between material will be subject to thermal fatigue, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to.

Originally developed for use in the mirrors and mirror mounts of astronomical telescopes, LAS-glass-ceramics have become known and entered the domestic market through its use in glass-ceramic cooktops, as well as cookware and bakeware or as high performance reflectors for digital projectors.

Cooktops[edit]

Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes. However, it is not totally unbreakable. Because it is still a brittle material as glass and ceramics are, it can be broken. There have been instances where users reported damage to their cooktops when the surface was struck with a hard or blunt object (such as a can falling from above or other heavy items).

At the same time, it has a very low heat conduction coefficient and can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the infrared wavelengths.

In the visible range glass-ceramics can be transparent, translucent or opaque and even colored by coloring agents.

A glass-ceramic cooktop

Today, there are two major types of electrical stoves with cooktops made of glass-ceramic:

A glass-ceramic stove uses radiant heating coils or infrared halogen lamps as the heating

elements. The surface of the glass-ceramic cooktop above the burner heats up, but the adjacent

surface remains cool because of the low heat conduction coefficient of the material.

An induction stove heats a metal pot's bottom directly through electromagnetic induction.

It is interesting to note that this technology is not entirely new, as glass-ceramic ranges were first introduced in the 1970s usingCorningware tops instead of the more durable material used today. These first generation smoothtops were problematic and could only be used with flat-bottomed cookware as the heating was primarily conductive rather than radiative.[4]

Compared to conventional kitchen stoves, glass-ceramic cooktops are relatively simple to clean, due to their flat surface. However, glass-ceramic cooktops can be scratched very easily, so care must be taken not to slide the cooking pans over the surface. Food with a high sugar content (such as jam) should never be allowed to dry on the surface if it spills, otherwise damage will occur[why?]. Cleaning is best carried out by using a soft cloth along with a special glass-ceramic cleaner that applies a thin protective film on the glass.[5]

For best results and maximum heat transfer, all cookware should be flat-bottomed and matched to the same size as the burner zone.

Brands and manufacturers[edit]

Some well-known brands of glass-ceramics are Ceran (cooktops), Eurokera (cooktop, stoves and fireplaces), Zerodur (telescope mirrors), and Macor. German manufacturerSchott introduced Zerodur in 1968, Ceran followed in 1971. Nippon Electric Glass of Japan is another worldwide manufacturer of glass ceramics, whose related products in this area include Firelite and Neoceram fire-rated glass. Keralite, manufactured by Vetrotech Saint-Gobain, is a specialty glass-ceramic fire and impact safety rated material for use in fire-rated applications. Glass-ceramics manufactured in the Soviet Union/Russia are known under the name Sitall.

The same class of material was also used, until the late 1990s, in Corningware dishes, which could be taken from the freezer directly to the oven with no risk of thermal shock.

http://en.wikipedia.org/wiki/Glass-ceramic

In the twentieth century, heat-resistant glass and glass-ceramic materials were developed. Like ceramic materials, they meet the need for attractive ware used for mixing, cooking, serving and storing. Major features are attractiveness, one-dish convenience, and inert, non-porous surfaces that won't absorb food odors and flavors.

While most are very rugged, they can break under impact. However, some glass, ceramic and glassceramic cookware manufacturers warranty their products against thermal breakage, and offer free replacement should the ware break in normal use within the warranty conditions.

Heat-resistant glass cookware may be made of clear or tinted transparent material or opaque white (commonly called "opal" glass). Glass-ceramic cookware may be white or transparent and tinted. Ceramic cookware is available in white or a variety of colors.

Properties of Glass, Ceramic and Glass-ceramic

Heat-resistant glass can be used for storing, cooking and serving. Some pieces can be used on the rangetop, while others are suitable only for the oven. Those designed for baking can be taken from the refrigerator and put into preheated ovens after the utensil reaches room temperature. As a rule, they should not be used on the stove top or under the broiler. Heat-resistant glass stove top products should usually be used with a wire grid on an electric range but should never be taken from the refrigerator or freezer and placed directly on a hot stove's element. Similarly, sudden cooling may be harmful to glass cookware. Hot glass cookware should not be allowed to come into contact with wet countertops, nor should they be placed in water while they are still hot.

Some ceramic cookware is made of heat-resistant materials that can go from the freezer to a hot oven or microwave. None is suitable for top-of-range or broiler use. Like glass cookware, ceramic cookware holds heat for a long time while providing the additional benefit or an attractive serving dish. Ceramic cookware is available in a wide variety of shapes, colors and designs.

Among the most thermally shock-resistant materials ever developed by man, glass-ceramic is a true spaceage material. It was first used in rocket nosecones because the glass-ceramic material could take the extreme temperature changes encountered in their supersonic flight from the earth's surface into outer space and back. Glass-ceramic cookware offers wide food preparation versatility. It can be used for stove top cooking and is excellent for roasting, broiling or baking---in the conventional or microwave oven. It can go directly from the freezer to the stove top, broiler or hot oven. Glass-ceramic cookware can be immersed, hot off the stove, into sudsy dishwater for easy cleanup.

Manufacturing

Glass is a non-crystalline material manufactured by melting a combination of raw materials including sand, soda ash, limestone, feldspar and borax. The glass used in cookware is normally melted in a large refractory furnace or tank at temperatures exceeding 2000° F. A small portion of the molten glass is drawn out of the tank and is blown or pressed into a mold. The mold essentially cools the glass, causing it to solidify. Following forming, the glass article is cooked to room temperature on a schedule specifically designed to insure the development of the desired heat-resistant characteristics.

Glass-ceramic is a special glass composition that is melted and formed like heat-resistant glass. Following forming, the articles are subjected to a special heat-treating schedule resulting in the development of a fine crystalline structure throughout the article. It is this crystalline structure

(which may be transparent or opaque) that gives the glass-ceramic its unique performance characteristics.

Ceramic cookware is manufactured from a mixture of water, clays, fluxing minerals (often feldspar) and finely ground sand. The particular forming methods depends largely on the water content of the mixture. A high water content (relatively liquid solution) permits casting of the ware in a mold. Lower water content results in a plastic mass that can be forced into the desired shape by a variety of methods.

After forming, the ware is dried and fired (subjected to temperatures in excess of 2000°F) in a ceramic kiln to bond the components of the "body" together. Following this initial firing, the surface of the ware is coated with a glaze that, upon firing in a second ceramic kiln, develops a smooth nonporous surface much like glass. For glass and ceramic cookware with nonstick interiors three layers of nonstick coating are applied to specially prepared interior surfaces and then cured at approximately 800°F.

Use and Care

Ceramic, glass and glass-ceramic materials are excellent retainers of heat. Baking dishes and casseroles made of these materials hold the food's heat long after it is removed from the oven. It is usually recommended to use these items at slightly lower oven temperatures for a shorter length of time because the covered cookware continues to cook foods even after it's been removed from the oven. A rule of thumb is to reduce the recommended oven temperature about 25°F (14°C).

Check the manufacturer's recommended care and use instructions before using any glass, ceramic and glass-ceramic bakeware. These items are usually cleaned with hot sudsy water and soaked if food has been burned on the item. Avoid knives, sharp kitchen tools, scouring pads and abrasive cleaners so that surfaces retain their original smooth finish. This is especially important for ovenware with nonstick interiors. Nylon and plastic scrubbers are acceptable for stubborn sticking problems.

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