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Glaze Durability

Ceramic glazes vary widely in their resistance to wear (cutlery marking, scratching) and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature. In industry technicians are accustomed to evaluating glazes by looking at their oxide chemistry and rationalizing the relationship between it and fired durability. Glazes having plenty of Al2O3 and SiO2, for example, are more durable (when they are melted properly). By contrast, potters tend to focus on the recipe. This is a problem and it is the reason that potters can be found using very non-functional glazes (often flux saturated and therefore lacking in Al2O3 and SiO2) or firing them in a non-functional way (e.g. under firing).

Misconceptions are common in this area. Many feel that just because a glaze looks melted it is durable. Another common belief is that firing temperature is an indicator of durability, the higher it is fired the more durable it will be. These beliefs, when coupled with the active traffic in glaze recipes online, greatly increase the chance that non-durable ware will be made. Sadly, a large proportion of online glazes are not durable and possibly not safe, even though they are published in fancy formats from apparently reputable sources. It is thus important to have a critical eye when looking at new recipes.

Another factor in why many people are using glazes of poor durability is the lack of appreciation of the basic science. In excess of 99% of glaze recipes available are for special purpose colors and surfaces. But very few quality transparent recipes are available. Ones that are expansion adjustable, fire clear and transparent, work with stains, have the right melt fluidity, melt well yet do not blister or pinhole, are easy to use, etc. And people do not test these with their bodies and adapt them (e.g. thermal stress testing for shivering, crazing). But functional surfaces must be based on these. Further, most other glazes are simply transparents with added colors, opacifiers and variegators.

With some experience it is possible to quickly judge durability issues when looking at a new glaze recipe. For example, at low temperatures (cone 06-04) boron is essential, so we expect to see a significant frit presence (or a natural boron source like Gerstley Borate), up to 80% is not uncommon. At high temperature we expect to see no boron materials (feldspars, calcium carbonate, dolomite, wollastonite, strontium carbonate are active melters there). At middle temperature these do not melt well so they need help. That help is almost always boron, so you will see 10-40% frits or Gerstley Borate. Some middle temperature glazes use zinc and/or lithium in addition to or instead or boron (these are power melters). But it is important that melters not be in excess, this will make the glaze leachable (since the SiO2 and Al2O3 percentages are pushed down).

At all temperatures, you should see clay in the recipe. 15-30% is typical. At all temperatures you should see silica, from 5-40%. If there is no clay or silica that is a red light if it is supposed to be a durable functional glaze.

Testing durability is common sense. Expose it to an acid and to abrasion and scratching by hard materials.

Please see the Limit Formula glossary topic for more information on how to look at a formula and judge its balance.

Glossy blacks are best made adding a black stain to a quality base transparent

Glossy blacks are best made adding a black stain to a quality base transparent

The glaze on the left is called Tenmoku Cone 6 (a popular, and old, CM recipe). It is 20% calcium carbonate, 35% Custer feldspar, 15% OM4 Ball Clay and 30% silica, 10% iron oxide. If you have any experience with glaze you will note two things that a fishy here: There is no boron, lithia or zinc sourcing material. How can this melt enough at cone 6? It looks melted, but the ease of scratching it shows it is not. So, it appears that if we saturate an incompletely melted glaze with a lot of refractory brown colorant on a dark body the effect can be black. A better idea is the glaze on the right. We start with a stable, reliable base transparent, G2926B. Then we add 5% Mason 6666 black stain (stains are smelted at high temperatures, quenched and ground, they are inert and relatively safe). A bonus is we end up with a slurry that is not nearly as messy to use and does not turn into a bucket of jelly.

Can a cone 6 functional glaze having only whiting and feldspar melt enough?

Can a cone 6 functional glaze having only whiting and feldspar melt enough?

This flow test compares the base and base-plus-iron version of a popular CM recipe called "Tenmoku Cone 6" (20% whiting, 35% Custer feldspar, 15% Ball Clay and 30% silica, 10% iron oxide). Although iron is not a flux in oxidation, it appears to be doing exactly that here (that flow is just bubbling its way down the runway, the white one also fires to a glassy surface on ware). It looks melted in the tray on the right but notice how easily it is scratching on the tile (lower left). This demonstrates that looks can be deceiving. Cone 6 functional glazes always have some percentage of a power flux (like boron, lithia, zinc), otherwise they just do not melt into a hard glass. Maybe a glaze looks melted, but it has poor durability.

3% and 2% zircopax added to G2926B cone 6 clear. Why?

3% and 2% zircopax added to G2926B cone 6 clear. Why?

The outside glaze is a copper blue, but that is not the one we are interested in here. It is the clear glaze on the insides of these two identical cone 6 porcelain mugs. Why add such a small amount of zircon to it? It is not being added to opacify, it is being added to toughen the surface and reduce the thermal expansion.The presence of the 2% zircon has not affected the gloss or transparency of the glaze on the right. However the 3% on the left has opacified it just slightly and made the surface a little silky. So that is too much for this glaze (although it might be OK if the melt fluidity was higher). So, if you are interested in the most functional possible surface, consider a 2% zircon addition to your transparent.

Out Bound Links

  • (Glossary) Dishwasher Safe

    Dishwasher safe ceramic ware is generally ware that is not overly absorbent and therefore will not water log in use over time (and therefore weaken and become a microwave oven hazard). Dishwasher safety also refers to the ability of ware to handle the changes in temperature it will be subjected to ...

  • (Glossary) Food Safe

    In recent years potters and small manufacturers have become aware (or have been forced to become aware) that ceramics and pottery are not as inert as they once thought. A variety of potential health impacts exist to users of ware they make. These include flaking off of glaze chips (that could be ing...

  • (Tests) GLLE - Glaze Leaching Test
  • (Glossary) Cutlery Marking

    In glazes with this fault, rubbing a metal knife or spoon on the surface will leave black marks that cannot be completely rubbed off. This is a common fault in glazes, especially mattes. Even commercial tableware often exhibits this problem. It usually happens because the micro-surface of the glaze ...

  • (Glossary) Leaching

    Glazes are not as inert and stable as many people think. All are slightly soluble and will thus leach to some extent, even if minute, into liquids they come into contact with. However some glazes are dramatically more soluble than others. It is common for leaching glazes to suffer from more than one...

  • (Glossary) Target Formula, Limit Formula

    The term 'limit formula' historically has typically referred to efforts to establish absolute ranges for mixtures of oxides that melt well at an intended temperature and are not in sufficient excess to cause defects. These formulas typically show ranges for each oxide commonly used in a specific gla...

  • (Glossary) Transparent Glazes

    A fully transparent glaze is simply one that does not have opacity. But there are degrees of transparency. For example, if a glaze is matte it will show the color of underlying body and decoration, but these will be muted (so it is actually translucent). Completely transparent glazes look like a gla...

By Tony Hansen

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