Digitalfire Ceramic Glossary

•The secret to cool bodies and glazes is alot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at
•The place to get the knowledge is

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A functional clay body is one that produces a ceramic that is durable. However there are a number of caveats with this. First, the item must maintain that strength and durability in service (degradation is common e.g. because of water logging). Of course, the body has to be fired sufficiently high to vitrify enough to have strength considered to be suitable for the application. Second, if glazed, it needs to fit the glaze (and/or engobe). If the glaze is under compression or tension this can greatly weaken the body both immediately after firing and progressively over time as micro-crack networks grow and water penetrates into available porosity.

Absorption of a fired clay is a key factor in functionality of ware made using it. There is no specific absorption rate that indicates functionality, however obviously more dense ware will be more durable. Absorption is a product of porosity in the body. Obviously a body having micro-cracks as porosity is not going to be as strong as one having normal inter-particle pore space. Porosity is typically a product of the nature of the pore space which in turn is a product of the nature of the matrix that hosts it. Pore shape and inter-connections and the micro-porosity of the ceramic matrix determine the degree to which water can penetrate to fill it. Thus a body may have a much higher porosity than a standard porosity test indicates if the pore-interconnects are lacking. Many potters, for example, assume that higher temperatures produce more functional ware. But this is not necessarily true. The body must fire vitreous for strength and durability and the degree to which it vitrifies is determined by the recipe, not the temperature. High temperature red burning iron ware bodies are an example of bodies producing ware of questionable functionality, they fire porous by necessity to achieve the desired variegated red/brown coloration.

One body having a higher porosity can also be stronger than another having a lower porosity, this can be the case for several reasons. First, the nature of the matrix that hosts the pore network can be a greater determiner of overall strength than the simple existence of pores. A matrix having large silica particles with cracks radiating outward (because of quartz inversion during firing) will obviously not be strong. Also, a well fitted, impermeable glaze can greatly strengthen a body. Many glazed ceramic tiles, for example, are remarkably strong, yet they can have a high porosity. Also, one body of higher porosity may have a matrix that better terminates the growth of micro-cracks than another, and thus maintain its strength over time better.

Functional glazes on food surfaces hold up over time, they do not scratch or stain or mark or fade or craze. This is because they are hard, smooth and resistant to leaching. Glazes on all surfaces are defect free and have a thermal expansion compatible with the body. This does not happen by accident. These glazes are balanced chemically, having enough SiO2 and Al2O3 to build a strong glass, they are not saturated with colorants (like metal oxides), crystallizers or variegators (like rutile, titanium). This site is dedicated to giving technicians the tools and knowledge they need to make functional ware.

An excellent test of functionality is to simply begin using a mug in your kitchen. Pour hot coffee in it every day, bang it around and put it in the dishwasher with the rest of the dishes. Look for crazing over time and changes in the ring it makes when tapped with a spoon (indicating water logging or micro-crack propagation) and changes in the glaze surface (glossy or marking or staining or crazing).


Water-logging happens when a clay is underfired

The cone 6 glaze is well developed, it is not crazed. But the clay underneath is not developed, not vitreous. This crack happened when the mug was bumped (because of poor strength). It is barely visible. When the mug is filled with water, this happens. How fast? This picture was taken about 5 seconds later. If this was crazing, and this piece was in actual use, the clay would gradually become completely water logged. Then one day someone would put it in the microwave! Boom.

Underfiring a clay is OK if the glaze fits? No it is not.

Left: Plainsman M340 fired to cone 6 where it achieves about 1.5% porosity, good density and strength. Right: H550, a Plainsman body intended to mature at cone 10, but fired to cone 6 using the same glaze. Although the glaze melts well and the mug appears OK, it is not. It is porous and weak. In fact, it has cracked during use (the crack runs diagonally down from the rim). It was then dipped into water for a few moments and immediately the water penetrated the crack and began to soak into the body (you can see it spreading out from the crack). If this glaze were to craze the entire thing would be waterlogged in minutes.

Bamboo glaze that is actually functional

The stunning cone 10R Ravenscrag bamboo glaze (GR10-J plus 0.5% iron oxide) on a Grolleg porcelain. Up close it can feel and look like a fine wood surface (when used on a porcelain). The cone 10 recipes page at has more info.

The best firing temperature for this body?

This cone 6 brown functional stoneware has been fired across a range of temperatures. Cone 4 is too porous. From cone 7 it is expanding and density is not improving, it will likely warp or bloat. Cone 7 is losing the red color, there is no room for over-firing (by accident). The porosity at cone 6 is so much better than cone 5 and color is still stable. Therefore, cone 6 is the one we want.

This leaching mug needs a liner glaze. Seriously!

Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice over night. The white areas on the blue and rust areas on the brown have leached! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such are normally subject to leaching, the manufacturers were able to tune the chemistry of each to make them resistant. But the overlaps mingle well (because of the fluidity), they are new chemistries, less stable ones. What is leaching? Cobalt! Not good. What else? We do not know, these recipes are secret. It is much better to make your own transparent or white liner glaze. Not only can you pour-apply it and get very even coverage, but you know the recipe, have control, can adjust to fit your body.

A matte that is matte because it is not melting completely

Left: This specimen of VC71 cone 6 matte glaze was felt-marked and cleaned with acetone. A closeup of the ink specks reveals they are held in micro-bubbles breaking at the surface. This specimen has also been thermally stressed in a 300F/icewater test (causing the crazing pattern, which curiously, only shows up on part of the surface). Right: An adjustment to VC71 that adds more boron and Al2O3/SiO2 (while preserving the Si:Al ratio). It is much glossier, confirming that, even though the VC71 matte surface feels functional to the touch, it is a product of improper melting.

Do your functional glazes do this? Fix them. Now.

These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.

What is the secret to cool and functional clay bodies and glazes?

The secret to cool and functional bodies and glazes is a lot of testing. The secret to know what to test is material and chemistry knowledge. The secret to learning from testing is documentation. The place to document is an account at

Looking for a non-crazed non-cutlery marking cone 10R dolomite matte?

This is G2571A cone 10R dolomite matte on an ironware body made from native North Carolina clays. Few glazes have the pleasant silky feel that this has yet are still functional. The feldspar content in the body has been tuned to establish a compromise between the warmer color low percentages have with the higher strength that higher percentages impart. Careful porosity tests were done and recorded in an account at The objective was to bring the body close to 3% absorption.

We fight the dragon that others do not even see

There are thousands of ceramic glaze recipes floating around the internet. People dream of finding that perfect one, but they often only think about the visual appearance, not of the usability, function, safety, cost or materials. That resistance to understanding your materials and glazes and learning to take control is what we personify as the dragon. Using the resources on this site you could be fixing, adjusting, testing, formulating your own glaze recipes. Start with your own account at

Out Bound Links

  • (Glossary) Vitrification

    Vitrification is the solidification of a melt into...

  • (Glossary) Maturity

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In Bound Links

  • (Glossary) Food Safe

    In recent years potters and small manufacturers ha...

  • (Glossary) Microwave Safe

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By Tony Hansen

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