You'll see a lot of matte glazes out there in the world that are simply under-fired glossy glazes. It's not a good way to make a matte, because it means you are not getting good glass formation because it's not going hot enough. These types of matte glazes are not usually very durable, and can leach because the materials are not locked up in the glass.
The better way to make a matte glaze is through the use of materials that promote matteness, such as magnesium. The glaze can be fully matured (runny even), with good glass formation, and still be matte. This type of glaze is much more durable, and more likely to be food safe.
The big problem is that there are a lot of folks out there who don't care to do their homework before using a glaze. They assume that if someone else was using the glaze, then it's good to go. A good number of the glaze recipes I find online I would never use without doing some reformulation. It's easy to make a pretty glaze. It take a little more work to make a durable glaze.
- TJR likes this