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URL: ASTM D-4236 - Standard Practice for Labeling Art Materials for Chronic Health Hazards - It is not what you think!

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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.

Make and sell your own jars of glaze at your teaching studio

Your studio or workshop can save a lot of money making and even selling your own glazes to students (at least for common colors and types). The supplies shown here enable printing quality labels on ordinary paper and applying them to jars that cost about $2. What about the glazes themselves? Just do what glaze companies do: Add stains to a base transparent glossy or matte glaze (for example G1916Q for cone 04 or G2926B for cone 6). Each 16oz jar needs 350g powder, 450g water, 5g CMC gum and 5g Veegum (leave out the Veegum and use less water for thicker coats). Stains are expensive (5-20 cents/gm here in 2024), at 7.5% (25g) that is $1-5 per jar. The materials for the G1916Q base cost $10/kg, and for G2926B $4/kg. Thus each jar costs $3.33 and $1.33 respectively. Making recipes of 5000g at a time would enable making 15 jars. A best-case is thus about $5/jar, worst case about $10/jar (compare that with commercials that are about $15-70). Of course, you can save money by looking for deals on materials and stains and recycling jars. Consider other advantages of making your own: You can tune the rheology, you know what is in it, you can adjust recipes to fit your clay bodies (to prevent crazing or shivering).

Are you worried about labelled, testing and SDSs? Take a look at ASTM D-4236 and see if you still need to worry.

Metal saturated glaze after lemon test

The manufacturer warns that these cone 6 metallic glazes are not food-safe. That being said, many potters use them on food surfaces anyway. This can be deliberate or just because the warning label gets ignored, the product has no webpage or the QRCode on the bottle is not clear enough to scan. To demonstrate I did an overnight lemon juice immersion on the bottom end of this tile. Then I washed and thoroughly dried it. The amount of dissolution is amazing! Metallics are made using manganese, nickel, cobalt, chrome and iron - which of these would you like in your diet? To get a good effect 20-40% metal oxide is required.

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.

The color? Good. The liner glaze? Safer. The crazing? Weakens the piece.

Color like this, from commercial brushing glazes, has become so trendy that multiple problems associated with it are being ignored by potters and hobbyists at cone 6. First, crazing (this network of fine cracks): When people use dense-burning bodies, ware doesn't leak, so it is deemed to be OK. When ware is made using stoneware clays having higher porosities, and it leaks, the clay bodies are blamed. And the poor strength resulting from the crazing is also blamed on the clay. However, this potter has done two right things:
1. Using an iron-stained honey glaze on the inside (e.g. GA6-B). It does not, cannot, leach heavy metals. Many are misinterpreting the ASTM D-4236 label on glaze jars and using intense heavy metal colored glazes on food surfaces!
2. The honey glaze inside does not craze so the mug does not leak even though the body has a higher porosity than the supposed vitrification magic number of <0.5%.
The bottom line: Use glazes that don't craze, DIY ones if possible or necessary, don't use really bright colors on food surfaces.

Dec 6: I have been waiting since Dec 1 for someone to notice this was AI-generated! That happened today. I used an AI image for an obvious reason: A real piece would offend the maker. AI produced this on first try as representative of what's on social. Yes, AI photos are less authentic than DIY, so are pieces made in isolation of awareness of the critical design and safety flaws outlined here. This page gives part of the solution and links to full solutions.

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