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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 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).

At first it might seem logical to send your glazes for testing at a lab. However is this really necessary? Suppose you are using a liner glaze (without colorants) and that contains no materials that could release barium, lead or lithium (obviously you need the recipe to know this). And, suppose it is melting well, not crazing and wears well. Why have it tested for leaching? There is an implication here: Start using a liner glaze whose recipe you know to contain nothing dangerous and stop worrying.

What if you are using a commercial transparent or white liner glaze, one with an unknown recipe? The changes are very good it too is safe. And expensive. But what if it is a commercial brightly colored glaze? They bottle says it is safe, but you know it contains heavy metal colors. How confident can you be it is really safe? Do the simple leaching tests mentioned on this page. However, notwithstanding that, should one really be using a brightly colored glaze on a good surface? Why not just send it to a lab? But they are going to want to know what elements to test it for. How will you know? They are not going to tell you if the glaze is safe, they are just going to generate a report full of numbers. Maybe the small number for one leaching element actually indicates more danger than a big number for another. What will you really do with those numbers?

What if you really need to use a colored glaze on a food surface and want to use one you make and know the recipe for? There are simple, practical tests referred to on this page to check for leaching. In this case you could send it to a lab because you would know what elements to have them test for. But, you would still be faced with trying to make sense of the numbers on the report.

Chemistry, a Way to Get More Insight

Glaze chemistry enables you to look at a glaze as a collection of oxides rather than just a recipe of materials (Insight-live, for example, automatically calculates the oxide formula of glaze recipes you enter and change). Fired glazes are what they are because of the chemistry more than anything else. Imbalances in the oxide formula can help explain a tendency to leach. Likewise a balanced oxide formula will suggest stability and likely resistance to leaching. The term 'balance' often simply refers to a glaze having oxide quantities in keeping with typical target (or limit) formulas for the temperature range. Limit formulas are about what mixes of oxides melt well and form a good glass (not whether they are food safe). That being said, if a glaze melts well and forms a good glass it is less likely to be leachable.

As noted, glaze bases can be inherently safe simply because they are made from nothing but non-hazardous materials (so it does not matter if they leach or not). But if heavy metal colorants are incorporated into the recipe these could be leached out of that glaze. Glaze bases can be inherently leachable (most often because of inadequate SiO2 or Al2O3 in combination with the presence of obviously toxic compounds like lead, barium, cobalt, chrome, etc.). Or stable glaze bases can be made unstable by additions of excessive percentages of colorant or conditioner, or by improper firing (e.g. not fired hot enough, over fired to the point of extreme melt fluidity). Likewise, glazes that might appear to be likely candidates for leaching because they contain significant lead or barium may, in fact, have very low leaching because of a carefully engineered oxide balance. Another factor is speed of kiln cooling: Certain glazes crystallize if cooled slowly (especially if a colorant is super-saturated in the melt), those crystals can be leachable.

The best prevention for leaching is simple leach testing (with acids and bases), enough awareness of chemistry and materials to know what to try to change to improve the situation and a willingness to do the testing and adjustment.

Copper can destabilize a glaze and make it soluble

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.

Glaze is coffee-staining and leaching after two years. Is it toxic?

Glaze is coffee-staining and leaching after two years. Is it toxic?

This is a cone 04 terra cotta piece. The coffee stain cannot be removed because the coffee has also leached off the surface gloss. Glazes are glass. Glass is leachable if the chemistry is out-of-balance. So is this glaze poisoning the user? No, it has an insurance policy. It is transparent, it is made from a mix of two frits (Ferro 3124, 3134) plus kaolin and silica. The recipe contains no heavy metal colorants or pigments and no toxic fluxes like lithium or barium. But the body is red, how can the glaze be white? A white porcelain-like engobe was applied at the leather hard stage and it was clear-glazed after bisque. The fix: The predominant frit, 3134, has almost no Al2O3. So I increased it (doing the chemistry in my Insight-live.com account) and began firing at cone 03.

This leaching mug needs a liner glaze. Seriously!

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

Commercial glazes on decorative surfaces, your own on food surfaces

These cone 6 porcelain mugs are hybrid. Three coats of a commercial glaze painted on outside (Amaco PC-30) and my own liner glaze poured in and out on the inside (G2926B). When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company), without crazing or shivering, it is purely an accident! So use them on the outside. But for inside food surfaces make or mix your own. When you know the recipe you can tune the thermal expansion. And the degree of melt. And the application properties. And you can use quality materials to source a balanced chemistry. The place to start understanding your glazes, organize testing and development and document everything is an account at Insight-live.com.

Two bases, 2% copper additions. Which is the better transparent?

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?

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?

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

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?

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?

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?

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

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.

Out Bound Links

In Bound Links

  • (Glossary) Water

    There is a need to discuss water in ceramic production as it related to a number of natural phenomena and production processes: Plasticity: Clays are plastic because water glues and lubricates the particles. The micro-dynamics of this are complex. Rheology: Suspensions (solids:water systems) e...

  • (Glossary) 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 ...


By Tony Hansen




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