Most people can easily distinguish a matte glaze from a gloss glaze, but the optical phenomenon, as well as the chemistry responsible for these glazes, is much more mysterious. A matte glaze is not simply an underfired glaze and, depending on the formula, the dry or non-gloss surface may actually be a slip or terra sigillata—not a glaze at all. If you’re not sure whether matte glazes are appropriate, there are some simple rules to help decide when a matte glaze may be right for your work.
First we’ll discuss the general physical/optical phenomenon that makes a glaze matte. 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 (390nm–750nm, 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.
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Dictionary definitions of matte often use the word “dull,” but that isn’t very informative for our purposes and it doesn’t hint at the many good reasons to use a matte glaze. Matte glazes can be just as “bright” as gloss glazes, if brightness is referring to the amount of light reflected. A more useful definition for us is that a matte glaze is one that isn’t glossy because it scatters reflected light in many or all directions. It scatters the light because it doesn’t have the super-smooth surface of a gloss glaze. However, since we’re talking about smooth on a microscopic scale, a matte glaze can still feel very smooth. In fact, some matte glazes feel smoother than any gloss glaze, because the reduced contact area caused by the tiny, regular bumps actually diminishes the friction-causing attraction forces between the glaze surface and your finger.
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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, 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.
Article resource: Ceramic Art Works Daily