What is Matt Glaze? See Appendix A below
One or all of the symptoms below may occur with matt or crazed glazes. What is acceptable to you is an individual decision. Testing to discover which of the three will be presented in your matt or crazed glaze is advisable.
1) Food Staining (e.g., coffee, tomato sauce or other acidic foods)
2) Food particles might not wash off thoroughly because of the porous/ unsmooth surface and can result in bacteria growth
3) Matt or crazed glazes may be more likely to leach hazardous glaze materials into the food while a glossy surface acts as a tight seal
Testing items 1 and 2 above is not difficult and will determine if your matt or crazed glaze stains or holds food particles.
The difficult issue is item number 3 (hazardous materials leaching into food). We have done our own research and spoken with Jeff Zamek who wrote the book, Safety in the Ceramics Studio, and have concluded that for legal reasons, not actual problems with the glaze, it is best to not claim matt glazes to be food safe. There have been no reported cases of people eating or drinking from matt glazes having a poisoning problem. It’s just not an issue on that level with over 60 years of matt glaze being used by potters. Also, Zamek’s research found that the National Health records did not indicate health issues from matt glazes. However, especially in the past few years, people have become very litigious and even if the potter can prove no cause-and-effect, it takes time and money to defend.
Given the above, we, at Lakeside Pottery, have decided it is better to be on the conservative side. We instruct our students and studio users to not use matt glazes for food ware. We do show our students that with some shapes, pitchers for example, a glossy glaze can be applied to the inside while the outside can be matt.
Note: We have found that some glazes are impacted by acidic foods (e.g., lemon, vinegar) where the glaze color changes. One of our glazes that change color with acidic foods is Water Color Green
Conclusion: Don’t take anybody’s word on the safety of a particular recipe. Even if it was safe at one point, small change in materials, firing, and clay could result in an unstable glaze. It can be harder to recognize an unstable matte glaze because a heavily leaching surface will not evidence itself with loss of gloss as it would in a glossy glaze. The only way to be sure is to have them lab tested.
See our matt glazes recipes
Appendix A: What is Matt Glaze
(source: Ceramic Art Daily)
To understand matte, we need to understand gloss. Glossy glazes are those that reflect light in a “specular,” coherent, mirror-like fashion so that you can see reflected images. Glossy glazes are very smooth, smooth on the scale of the wavelength of visible light (390nm750nm, where a nm is ~1/400,000,000 of an inch). Thus any bumps, pits, or undulations on the glaze surface are smaller than approximately 390nm or 1.5/100,000 inch, so as far as the light is concerned the surface is perfectly smooth. If the light’s wavelength is larger than the bump, then you won’t be able to see the bump. Conversely, the protruding crystals in matte glazes are larger than this and therefore scatter the light.
By far the most common origin of matte glazes is devitrification, which is the formation of crystals within the glaze during the cooling phase after firing. This type of matte glaze relies on having higher concentrations of certain oxides within the glaze (see below), and also may require a particular (slower) cooling schedule after the firing. The dependence on cooling rate is why some glazes will end up matte when fired in one kiln and shiny in a different, faster cooling kiln. It is also why some matte glazes come out of some firings feeling smooth, but then come out of other, slower-cooled firings with a rough surface due to larger crystals on the surface of the glaze.
Oxides commonly employed to create matte glazes are MgO, CaO, SrO, BaO, Al2O3, TiO2, ZnO, and MnO. Silica is often a component of the crystals, but since it is in all glazes in large amounts, it is not listed. The required amount of any oxide depends on the identity and quantity of other oxides present, the firing temperature, and many other variables such as what clay it is applied to. Matting oxides are often used together. Some crystals grow on the surface of the glaze, and others start at the clay/glaze interface and grow up through the entire thickness of the glaze. The growing crystals have a different oxide composition than the remaining glaze, and the surrounding glaze is now somewhat depleted of the crystal oxides, resulting in two different materials that usually have very different properties. This is why most matte glazes are opaque, and why they are often a very different color than very similarly composed shiny glazes.
You might be tempted to modify your favorite glossy glaze to turn it into a matte, but remember that any change to a glaze’s recipe will affect all of its properties, not just the one you were trying to change. You are probably better off finding a recipe for a glaze that is already matte. Millions of glaze tests have already been run, so you can probably find a recipe that is very close to what you want. Decide what you want in texture and color, then find a recipe that has the appropriate dominant fluxing oxide(s) and appropriate colorant(s). Some testing and alteration may be necessary to adjust the glaze to your exact tastes and your particular combination of clays, firing, and application methods.