Ceramic Glaze

Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.

Details

A glaze is a glass made of powdered minerals and man made materials whose recipe has been tuned to melt to the desired degree. Glazes are different than window or bottle glass, they need to be thermal expansion compatible with the clay body or porcelain to which they are attached (soda-lime glass, for example, would craze badly).

Glazes melt and all the particles go into solution whereas clay bodies vitrify or sinter (many of the particles retaining their crystal or amorphous identity. At higher temperatures, like cone 10, a wide range of cetamic powders will melt on their own to form a glaze, others that do not will react with melters. As temperatures go down, fewer and fewer particles melt enough and more and more of the fluxing burden rests with the flux particles (e.g. frits, feldspar). Glazes can also tolerate non-melting particle additions to achieve the color, opacity and visual character.

Glaze recipes need to contain materials that contribute both to the desired fired chemistry of the glass as well as to the physical working properties in production. Glazes are normally suspended in water (specific materials in the recipe, like clays, keep the particle mix in suspension and also harden it during drying). Glazes are applied to ware by painting, dipping, or spraying.

Glazes are classified in many ways (e.g. unleaded, raw, fritted, slip). The chemistry of glazes is the aspect most related to the way they fire. People who learn to understand the relationship between the chemistry and the fired properties can exercise a lot more control. Many physical factors also play in the way a specific glaze fires (thickness of application, body it is applied to, firing schedule and temperature and atmosphere, method of application, etc). The physical presence of the glaze slurry in production is related to the physics of the materials, those who understand the interplay of these as well as the chemistry can create glazes that have good working properties and fire the way they want.

Related Information

Links

Glossary	Glass vs. Crystalline In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
Glossary	Crackle glaze Crackle glazes have a crack pattern that is a product of thermal expansion mismatch between body and glaze. They are not suitable on functional ware.
Glossary	Crystalline glazes A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling
Glossary	Glaze fit In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Glossary	Glossy Glaze
Glossary	Mocha glazes A technique used by ceramic artists to decorate pottery. It happens when bleeding occurs at the edges of a thin colored acidic mixture painted over a still-wet slip.
Glossary	Once fire glazing Refers to the practice of firing ceramics in one firing (rather than two) to produce a fully glazed product. This practice requires more technical expertise.
Glossary	Shino Glazes Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
Glossary	Reactive Glazes In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Glossary	Celadon Glaze A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.
Glossary	Wood Ash Glaze Common washed wood ash can supply important ceramic oxides when melted, so it can comprise significant percentages in a recipe. Plus it can produce unique visual effects.
Glossary	High Temperature Glaze
Glossary	Medium Temperature Glaze These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Low Temperature Glaze In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).
Glossary	Matte Glaze Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
Glossary	Transparent Glazes Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Glossary	Fast Fire Glazes Industrial ceramics are fired very quickly and require minimal micro bubbles and zero pinholes and blisters. Fast fire late melting glazes accomplish that.
Glossary	Feldspar Glazes Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
Glossary	Rutile Blue Glazes A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)
Glossary	Bamboo Glaze A high temperature reduction glaze made by adding a small amount of iron oxide to a magnesia matte base glaze
Glossary	Metallic Glazes Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
Glossary	Glass-Ceramic Glazes
Glossary	Liner Glaze Liner-glazing is a very good way to assure that your ware has a durable and leach resistant surface. It also signals customers that you care about this.
Glossary	Gunmetal glaze A type of ceramic glaze: Pebbly matte black. For functional surfaces, it has enough glossy to not cutlery mark and be easy to clean.
Glossary	Iron Red Glaze A type of ceramic glaze, typically fired around 2200F, where iron oxide in the cooling glass precipitates out to form a striking red crystalline mesh on the surface.
Glossary	Oil-spot glaze A mottled glaze created by double-layering glazes of different melt fluidity and color
Glossary	Carbon trap glazes A type of ceramc glaze.
Glossary	Fat Glaze
Typecodes	Additives for Ceramic Glazes Materials that are added to glazes to impart physical working properties and usually burn away during firing. In industry all glazes, inks and engobes have additives, they are considered essential to control of cohesion, adhesion, suspension, dry hardness, surface leveling, rheology, speed-of-drying, etc. Among potters, it is common for glazes to have zero additives.

By Tony Hansen
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