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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. The subject of leaching and glaze safety can be complex, but it can also be easy to evaluate and test glazes using simple logic and tests (see links on this page). It is best to do these before sending your glazes for testing at a lab (where the results may be very difficult to interpret and make practical use of). 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, that have cultivated a deliberate lack of awareness on this subject.

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.

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.

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. 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?

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.

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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