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This is not actually the original glaze. At installation in a bathroom 90 years ago, the tiles were not crazed. But over time it happened (inset upper right). Now, a restoration specialist is tasked with duplicating it. The shade, opacity, degree of matteness, bubble-free matrix and surface character are all real challenges. But duplicating the crazing is even more difficult. Why? Matching "time-crazing" with a crackle glaze pattern will be almost impossible. And, it will be temporary (it will craze much more after installation).
The reason why functional mattes seldom craze can be seen in the chemistry. This chart compares the thermal expansions of the oxides that combine to form the fired glaze matrix. ~80%+ of the makeup of almost all common base glazes (without colorants, opacifiers) is SiO2 and Al2O3 (orange bars). Mattes almost always need a low Si:Al ratio (e.g. below 6:1). The rest is fluxing oxides to melt them (the blue bars + B2O3). Here is the problem with making a crazing matte: Almost all crazing is caused by high levels of K2O and/or Na2O (the top two bars on the graph). But they produce high gloss (as can be seen in this test tile). The main matting fluxes and agents are MgO, CaO, SrO, BaO; they have a low COE (and don't craze glazes). Further, both zircon and tin oxide, the opacifiers needed, also have low thermal expansions!
There are some other possibilities:
-A matte glaze can have a high SiO2:Al2O3 ratio and craze if it is very melt fluid (containing lots of KNaO) and cooled slowly so that micro-crystals cover the surface and make it unpleasant to the touch.
-Glossy glazes can be matted by the addition of micron-fine alumina (e.g. 800 mesh, this is done in the tile industry).
-A low expansion body with no ball clay or silica (e.g. just kaolin and feldspar with enough bentonite to get the needed plasticity) will craze most glazes. Adding pyrophyllite will further lower its COE.
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