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 the 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. Read and watch our liner glazing step-by-step and liner glazing video for details on how to make glazes meet at the rim like this.

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). All the samples were fired using the C6DHSC firing schedule. 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.

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

What if 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.

Meet two glazes at the rim using wax emulsion. Why? How?

Reactive glazes (melt-mobile, crystallizing or heavily pigmented) are the least suitable for food surfaces because they have the potential to leach metals. Liner glazing ware is an excellent way to deal with this problem. Not only does this approach improve functionality but it can be aesthetically pleasing and practical in production. This liner is GA6-B, a pottery glaze recipe we promote with confidence. Not only is it less likely to be leaching metals but also less likely to craze - this assures water tightness on non-vitreous bodies and eliminates any potential for bacteria growth in the cracks (especially if the body has porosity). Unfortunately, glazes that leach are also likely to stain and cutlery mark - these add more reasons why they are most often unsuitable for food surfaces. The straightness of the dividing line is affected by both the application technique and the degree to which the two glazes bleed into each other and run. Read and watch our liner glazing step-by-step and liner glazing video for details on how to do this - it is practical for any potter or hobbyist (or even in production). And tap/click the picture above for other examples of this.

Links

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 Be skeptical of claims of food safety from potters who cannot explain or demonstrate why. Investigate the basis of manufacturer claims and labelling and the actual use to which their products are put.
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	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.
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.
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.
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.
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.
URLs	https://www.astm.org/d4236-94r21.html ASTM D-4236 - Standard Practice for Labeling Art Materials for Chronic Health Hazards - It is not what you think! A standard that "applies exclusively to art materials packaged in sizes intended for individual users of any age or those participating in a small group". It "concerns those chronic health hazards known to be associated with a product or product component(s) when it is present in a physical form, volume, or concentration that in the opinion of a toxicologist has the potential to produce a chronic adverse health effect". The word "toxicity" is not mentioned on the page nor any methods for determining such. Furthermore, the labelling refers to hazards to which the potter is exposed in applying the glaze to the ware, long term, in small hobby quantities. IT DOES NOT ADDRESS leaching hazards the ware presents to users of the pottery. Even then, the standard states that "it is the RESPONSIBILITY OF THE USER ... to establish appropriate safety, health, and environmental practices ... based upon knowledge that exists in the scientific and medical communities". It also admits that "since knowledge about chronic health hazards is incomplete and warnings cannot cover all uses of any product, it is not possible for precautionary labelling to ensure completely safe use of an art product." It is interesting that one manufacturer displays this warning on pages relating to dipping glazes and accessory products (which are used by manufacturers): "Safety Warning: Tableware producers must test all finished ware to establish dinnerware status, due to possible variations in firing temperature and contamination." This warning does not appear on brushing glazes, even the reactive metal-saturated ones that potters and hobbyists use!
Media	Liner Glazing a Stoneware Mug I will show you how to glaze a mug with a liner glaze inside and a colored one outside so that they meet in a perfect line at the rim.

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