Here at WoodStove-Fireplaceglass.com we use the term ceramic glass generically, meaning pyro-ceramic, neo-ceramic or robax glass. This glass has a slight "Amber" tint to it. It is referenced to as a glass, but in actuality it is a transparent ceramic material. This type of glass-ceramic should always be used in any wood stove. Never, ever use a tempered glass in an air tight wood stove.
There are several reasons to cause your wood stove glass break. If you have had your stove a while and the glass eventually breaks, it usually just means it is "wore out". By this I mean over the years do to expansion and contraction, chemicals from the wood and glass cleaners, your glass is continually breaking down. Eventually to a point where it fails and cracks.
The next most common reason is do to improper installation. The Ceramic glass should be installed what we call "finger tight". Meaning not loose, but surly not tight. The tendency is to install the glass tightly to get a good seal. But the glass is produced to handle the high temperatures. But by doing this the glass expands very slowly. The stove however is expanding at a much faster rate. Then the glass can get in a bind and increase the pressure on the glass and pop the glass.
There also should be a glass gasket installed around the glass edge to protect it. Without the gasket to cushion the glass a couple things can happen. The metal temperatures can spike sometimes above the 1400 degree threshold that the glass can withstand. Or, when installed the glass clips can be tightened up to much and when the metal expands it applies to much pressure to the glass and breaks it.
The glass can also break from force also. Ceramic glass is not any stronger than that of normal window glass. It is just designed to handle the high temperatures of the wood stove. So if it get hit by a log, fireplace tool or dropped on the floor, it will break easily.
The short answer is no. You should replace the glass as soon as possible. For a couple of reasons. For safety, if the crack expands and your not watching it, sparks, coals or even flames could escape and cause a fire. Also, the stoves are designed to be air tight, the cracked glass will cause air infiltration, pulling in air from your home making the wood burn up much faster than it should, not producing the amount of heat that it should and sucking warm air out of your home.
But you may not have any other choice for a short period of time. A couple of ways to get by. If a chunk of glass is missing, you can have a piece of steel usually cut fairly quickly and replace the glass with that. If its just a crack, high temperature furnace cement will work for a limited amount of time. it is like a putty and turns hard. It will dry out and become brittle and fall off. Like i said very temporary
Here is the technical definition from Wikipedia, the free encyclopedia:
This polycrystalline material is produced through controlled crystallization of base glass. These materials share many properties with both glasses and ceramics.
Ceramic glass has 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. This product has the fabrication advantage of glass as well as special properties of ceramics. They do not require brazing but can withstand brazing temperatures up to 700 °C. 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, bioactivity, 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. In manufacturing, ceramic glass is valued for having the strength of ceramic but the hermetic sealing properties of glass. It is 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 Ceramic glass. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, it has, unlike sintered ceramics, no pores.
Glass Ceramics are produced in a way that has qualities of glass, and of Ceramic. Transparent, smooth and formable like glass, but slower expansion rate, durability and heat resistance like Ceramics.
That's the gist of it!
how it started
An accidental overheating of a glass furnace led to the discovery of materials known as glass-ceramics. When the glass was overheated, small crystals formed in the misshaped material that prevented cracks from propagating through the glass.
The first step toward glass-ceramics involves conventional techniques for preparing a glass. The product is then heated to 750-1150C, until a portion of the structure is transformed into a fine-grained crystalline material. Ceramic glass is at least 50% crystalline after it have been heated. In some cases, the final product is more than 95% crystalline.
Because ceramic glass is more resistant to thermal shock, cookware made of this material can be transferred directly from a hot stove burner to the refrigerator without breaking. Because they are more crystalline glass-ceramics are also slightly better at conducting heat than conventional glasses. They are also stronger at high temperatures than glasses. Some are used to make electrical insulators that have to operate at high temperatures, such as spark plug insulators.
Ceramic glass is not always transparent. Products like wood stoves, fireplaces and blast furnaces, tend to use transparent ceramics like pyro-ceramic, neo-ceramic or robax glasses.
Spark plugs, electrical insulators, and brackets are common uses of non-transparent ceramic-glass.
It is possible to turn a glass into a ceramic by heating it up. This allows rearrangement from random to an orderly structure. A orderly structure is more stable than a disordered one. Materials that are initially fabricated as glasses (and perhaps shaped using glass molding techniques) and converted to a ceramic to enhance their properties are called glass-ceramics. A well-known example of a Ceramic glass 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.
In the twentieth century, heat-resistant glass and Ceramic glass 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 glass ceramic 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). In cookware it 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 range top, 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 space-age 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.
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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