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 risk factor. It is also common to find leaching glazes in use by practitioners, even professors, ones who have cultivated a deliberate lack of awareness on this subject (people like these make it harder for the rest of us to market functional ware to increasingly wary customers). The subject of leaching and glaze safety often involves glaze chemistry (more about that below). But it is also alot about common sense. Obviously, there is more concern if the glaze will be exposed to a hot acidic or caustic liquid for lengthy periods than if the vessel will simply be used for serve cold food. Or if it is not melted well. Of if it contains a high percentage of a heavy metal colorant. Simple tests and logic apply here (see links on this page).
Copper can destabilize a glaze and make it soluble
A closeup of a glossy Cone 6 glaze having 4% added copper carbonate. The bottom section has leached in lemon juice after 24 hours. This photo has been adjusted to spread the color gamut to highlight the difference. The leached section is now matte.
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.
Commercial glazes on decorative surfaces, your own on food surfaces
These cone 6 porcelain mugs are hybrid. A commercial glaze inside (Amaco PC-30) and my liner glaze the inside (G2926B, which is googleable). When commercial glazes fit a clay (without crazing) it is by accident. But when you make your own glazes, you can tune them to fit your clay. The inside needs to be food safe and craze free, so I need to know what is in it. Want to start fixing and fitting your glazes? Open an account at Insight-live.com, enter the recipes and upload good pictures and then contact me (I will give you suggestions).
Two bases, 2% copper additions. Which is the better transparent?
Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.
Does copper cause glazes to leach?
These are four cone 6 glazes of diverse chemistry. They have varying melt fluidities. They are soaked (half way up) in lemon juice over night. None show any evidence of surface changes. All contain 2% copper carbonate. If the copper was increased, especially to the point of going metallic or crystallizing, likely the leaching test would have different results. So, if you use copper sensibly (in moderate amounts), there is a good chance you can make a glaze that resists leaching.
How much rutile can a glaze take before it becomes unstable?
The 80:20 base Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left:6% rutile, 3% titanium; right:4% rutile, 2% titanium right). That oatmeal effect is actually the excess titanium crystallizing out of solution in the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is often susceptible to cutlery marking and leaching. This is because the crystals are not as stable or durable as the glass of the glaze.
Transparent inner glaze over an encapsulated stained engobe
Encapsulated stains can reach their limits in a glaze host at cone six and begin to dissolve and decompose. That is an obvious problem on a food surface. But in a less fluid underglaze they can survive longer. The bright orange color on the left was likely done this way. The transparent over glaze is isolating it from any contact with food or drink. However people are more wary of the risk of glazes leaching heavy metals and having bright colours on food surfaces may not send the right message.
An extreme extremely runny glaze at cone 6. Is there a cost?
This recipe melts to such a fluid glass because of its high sodium and lithium content coupled with low silica levels. Reactive glazes like this produce interesting visuals but these come at a cost that is more than just the difficulty in firing. Recipes like this often calculate to an extremely high thermal expansion. That means that not only will this form a lake in the bottom of ware when used on the inside, but the food surfaces will craze badly. The low silica will also contribute to leaching of the lithium and any colorants present.
What can you do using glaze chemistry?
There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.
What happens when glazes lack Al2O3?
This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.
A settling, running glaze recipe gets a makeover
The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay. Adjustment 1 (column 3) eliminates the feldspar and sources Al2O3 from kaolin and KNaO from Frit 3110. The chemistry of the new chemistry is very close. To make that happen the amounts of other materials had to be juggled (you can click on any material to see what oxides it contributes). But the fired test reveals that this one, although very similar, is melting more (because the frit releases its oxide more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition (to supply more SiO2). SiO2 is the glass former, the more a glaze will accept, the better. Silica is refractory so the glaze will run less. It will also fire more durable and be more resistant to leaching.
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