|Monthly Tech-Tip |
All glass leaches to some extent when it comes into contact with water (or acids and bases), especially if the contact occurs over a period of time or the liquid is hot. This is evident by a change in the gloss and/or texture of the glass surface over time. Glazes can pass a test of acid attack (e.g. lemon juice) yet fail the attack of bases (e.g. detergents). These simple tests can be done by anyone. After each, dry the specimens and compare the non-treated and treated surfaces for differences in surface color, texture, character or reflectivity.
Vinegar Test: Fill a glazed container half full of vinegar and leave it for several days. If the vinegar turns yellow, this is an indication of lead release.
Dishwasher Test: Take two identical items and put one in your cupboard and leave one in the dishwasher for two months.
Soda Ash Test: Mix 50 grams of soda ash (sodium carbonate) to one liter of water, bring to a boil in a stainless steel pan, simmer glazed specimens for 6 hrs checking every half hour to add more water.
Lemon Slice Test: Lemon juice is more acidic than vinegar. Keep a lemon slice against a glazed surface for several days. To make sure that it stays wet envelop the whole thing in plastic wrap. If needed, put a weight on top to keep the lemon in contact with the surface.
Drano, bleach: Use a similar procedure.
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.
This was left for 24 hours. Wrapped in stretch wrap. Then the surface of the glaze was inspected under a lamp to detect any differences between the lemoned and non-lemoned surfaces. Lemons are highly acidic. This glaze passed because the base recipe, G3806N, was methodically developed so that it has plenty of Al2O3 and SiO2 (in the fired chemistry) to build a stable glass.
There is a direct relationship between the way ceramic glazes fire and their chemistry. These green panels in my Insight-live account compare two glaze recipes: A glossy and matte. Grasping their simple chemistry mechanisms is a first step to getting control of your glazes. To fixing problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. To substituting frits or incorporating available, better or cheaper materials while maintaining the same chemistry. To adjusting melting temperature, gloss, surface character, color. And identifying weaknesses in glazes to avoid problems. And to creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. And even to creating glazes from scratch and using your own native materials in the highest possible percentage.
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 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.
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 control assurance of adherence to basic recipe limits.
Note any differences surface color, texture or character.DWSH - Dishwasher (V)
Note any differences surface color, texture or character.SODA - Soda Ash (V)
Note any differences surface color, texture or character.LEMO - Lemon Slice (V)
Note any differences surface color, texture or character.
Glaze Melt Flow - Runway Test
A method of comparing the melt fluidity of two ceramic materials or glazes by racing them down an inclined runway.
Ceramic glazes vary widely in their resistance to wear and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature.
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.
|Oxides||CuO - Cupric Oxide|
Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
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.
Having Your Glaze Tested for Toxic Metal Release
Having Your Glaze Tested for Metal Release
Attack on Glass: Corrosion Attack Mechanisms
Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
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.
Tests conducted on glaze batches used in production (as opposed to tests conducted on the materials used to make those glazes).
|By Tony Hansen|
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