These are all test I have done over the last few weeks. There are three cycles here. One was testing for a black stain for brushwork. The second was testing a few glazes to substitute for one I use now, and the third, mostly done today is to find a new black. I have a decent black now, but it's not quite what I am looking for. I want something more like skin rather than a piece of heavy clothing. Something that allows the form to shine. I would like a matte to satin-matte but that is not easy. Since I will use this on the inside of pots as well as the outside it needs to be food-safe.
Unfortunately many mattes are actually under-fired or improperly formulated gloss glazes. Often they don't have enough Silica in them (the glass-former) and so if an acidic food is in the pot it is possible for chemicals to leach from the glaze into the food. Most of the ingredients I and most other potters use are fine but there are a few nasties which is fine but they need to be in a properly formulated glaze at a safe level. There are some like lead, barium, cadmium and some others which are just plain dangerous and aren't used much in functional (for food use) pottery any more. Cobalt, manganese and chrome are commonly used but you don't want them leaching into your food (or breathing the dust while using or fumes while firing). There were a few recipes I opted not to test because at 5% of the recipe the silica was just too low. It depends on the recipe what the proper % would be, but generally you will see it at 20-25% so you can see why 5% is just not going to cut it.
To reduce any paranoia that might be building in anyone, in an almost properly formulated glaze it would take a lot of leaching from an average cup or bowl to be dangerous. I have a bowl with a barium glaze as the liner that I don't hesitate to use sometimes. Would I want all of my dishware to be lined with this glaze? Ummm no. That would be dumb. I would also be dumb after a while. (I have a couple of recipes that I tested today that called for barium and I chose to substitute strontium carbonate instead which is commonly done, but it will affect the glaze so we'll see how they come out). There are also safe-ish levels of dangerous oxides. Properly formulated 3% copper carbonate should be fine. However I probably would not test a recipe for food pottery that called for 15% copper. That's really high and will almost definetely leach. But I'm not a scientist, I just play one in my studio. Some of this is just conjecture.
Unfortunately the end result of all this testing is a lot of unused glaze samples and that is mostly what is filling 1/2 of this 30 gallon bucket.
Some people write in to pottery forums asking questions. A free exchange of ideas. Free for them, costly for others. One I have seen more than once goes something like this:
"Hi, I am a newbie and I'm looking for a reliable yellow (blue, black, green, whatever . . .) glaze but materials are so expensive that I hate to waste them testing unless I know it's a good glaze so does anyone have a recipe they could share? Thanks a lot! I just love clay and you guys are great!"
These are not my favorite people.