•The secret to cool 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 test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
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
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?
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.
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By Tony Hansen