|Monthly Tech-Tip |
Ceramic glazes vary widely in their resistance to wear and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature.
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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.
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.
Look at how fluid G3879 is at cone 06 even though it has the Al2O3 and SiO2 of a cone 6 (or even cone 10 glaze)! It have found that glazes with lots of boron can tolerate amazingly high levels of Al2O3 and SiO2 and still melt very well. And they create many options to lower thermal expansion that would not otherwise be available. The G3806N recipe has the amazing ability to tolerate large additions of kaolin. Each addition sacrifices some melt fluidity but the glaze stays glossy and gets more durable (because of the increased Al2O3 and SiO2). And the thermal expansion drops even more. A highly melt fluid, super gloss with low thermal expansion is super difficult at cone 6, but here it is. The secret is high boron. From frits.
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.
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.
Dishwasher safety is a concern in ceramic table ware, especially if the ware has been imported or made by a small company or potter.
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
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.
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.
Glaze Leaching Test
Simple tests to evaluate the stability of a ceramic or pottery glaze against leaching metals in food or drink.
|By Tony Hansen|
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