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, G2926B, poured in and out on the inside. When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company) it is by accident, neither company designed for the other! For inside food surfaces make or mix a liner glaze already proven to fit your clay body, one that sanity-checks well (as a dipping glaze or a brushing glaze). In your own recipes you can use quality materials that you know deliver no toxic compounds to the glass and that are proportioned to deliver a balanced chemistry.

Related Pictures

Commercial supposedly safe glazes leaching. A liner glaze is needed.

Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice overnight. The white areas indicate leaching has occurred! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such normally tend to leach metals, supposedly the manufacturers were able to tune the chemistry enough to pass tests. But the overlaps interact, like drug interactions they are new chemistries. Cobalt is clearly leaching. What else? We do not know, these recipes are secret. It is better to make your own transparent or white liner glaze (either as a dipping glaze or brushing glaze). Better to know the recipe to have assurance of adherence to basic recipe limits.

Are commercial glazes really guaranteed food safe? When manufacturers claim adherence to standards like ASTM D-4236 what are they saying? The small amounts of liquid glazes are safe for the artist to brush on. They are not making claims about leaching on finish ware.

Two black cone 6 glazes recipes. One leaches metals. Why?

Black-glazed sample tiles showing leaching

We tested susceptibility to leaching of a stained glaze (top two) and another colored with raw metal oxides (bottom two) using the GLLE test (they were left overnight in vinegar and lemon juice). The top two are G3914A (GA6-B Alberta Slip base with an addition of only 4% Mason 6600 black stain). There is no sign of leaching (Drano and bleach likewise did not affect it). The bottom two are G2926B transparent with 10% manganese dioxide and 5% copper oxide added (a common way to get black). The G2926B transparent base is proven, its surface shows no signs of gloss-loss in any testing we have done. But with manganese and copper added it is a very different story. Not only was this not nearly as black, but the vinegar and lemon juice matted and blued the top sections. Obviously, a ceramic stain added to the Alberta Slip GA6-B base is a far better way to make a leach-resistant black. Is it better than adding the black stain to G2926B? Yes, because far less is needed since the GA6-B base already has plenty of pigmentation.

It is possible to spot a leaching glaze just by looking at it!

A matte black coffee mug with leaching glaze

This glaze looks too matte, too metallic, too crystalline. This picture was sent to me by a worried person who had bought it and noticed it discolouring on the inside. The potter may very well have considered this safe just because it was fired to cone 10. It is common among potters to overload glazes with raw metal oxide blends, often 15% or more (e.g. manganese, copper, iron, nickel, cobalt). These percentages cannot be held in solution in the melt as it cools and solidifies, so they precipitate out and crystallize, especially if the glaze is not melting well or has insufficient SiO₂. The crystalline forms of these metals might look nice to some people, but the glaze is likely to leach them. It is better to use a ceramic stain to create a black like this, adding it to a stable matte base glaze (one that melts well and has sufficient SiO₂ and Al₂O₃ to create a durable glass). The concept of a limit recipe is helpful in eyeballing recipes for their likelihood of leaching.

Metal leaching from ceramic glazes: Lab report example

This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and wastewater analysis. They also provide pottery glaze leaching analyses (an acid solution is kept in contact with the glaze and then analysed for trace levels of specific metals). Each suspected metal to be tested for entails a separate charge ($30-60 in this case, could be less for you). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images" to find charts with lots of data. Searching pages for this term will find books having detailed sections on each of the metals. Typically you are only interested in one metal in a specific glaze (often cobalt or manganese). There are ways to sleep better (about the likelihood your glazes are leaching metals) if you cannot do this: Do a simple GLLE test. And avoid online trafficking in hazardous recipes. Better to find a quality base glaze (matte and transparent) that works well on your clay body. Then add colorants, opacifiers and variegators; but doing so in a conservative manner.

Imagine realizing that pottery you're selling is all going to craze!

Every single piece you have been making is all eventually going to craze! Do you refund all the customers? All because you trusted a liner glaze recipe found online. At a reputable site. This recipe is called "Classic White". And it has "classic crazing". On going back to the source web page you find there is no mention of even the possibility of crazing. Then you type the recipe into your account at Insight-live.com and are horrified to see how high the thermal expansion calculates to (see red circles on picture). Then you realize that, based on simple recipe limits, you could have steered clear of this just by looking at it. Then you find there are other well-documented base glossy and matte recipes that do not craze (like G2934 and G2926B). Even if they do they are adjustable to enable fixing the issue. And you can even blend them to create any degree of satin desired. And they use a mix of tin oxide and Zircopax to avoid crawling. And they are durable, don't cutlery mark, don't leach, don't run, don't settle in the bucket.

Links

Recipes	G2926B - Cone 6 Whiteware/Porcelain transparent glaze A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
Recipes	G2934 - Matte Glaze Base for Cone 6 A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
Glossary	Leaching 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
Glossary	Food Safe 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.
Glossary	Dipping Glaze In traditional ceramics and pottery dipping glazes can be of two main types: For single layer and for application of other layers overtop. Understanding the difference is important.
Glossary	Base Glaze Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.
Glossary	Commercial hobby brushing glazes These are an incredible benefit to pottery beginners and pure hobbyists. But they can also be an obstacle to progress and affordability as your skills improve.
Glossary	Pour Glazing A method of applying glaze evenly and quickly to pottery.
Articles	Are Your Glazes Food Safe or are They Leachable? Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Articles	Is Your Fired Ware Safe? Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Articles	Where do I start in understanding glazes? Break your addiction to online recipes that don't work or bottled expensive glazes. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and drying properties. Even some chemistry.
Articles	Glaze Recipes: Formulate and Make Your Own Instead The only way you will ever get the glaze you really need is to formulate your own. The longer you stay on the glaze recipe treadmill the more time you waste.

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