Turning delayed crazing into immediate crazing

This is a cone 04 clay (Plainsman Buffstone) with a transparent glaze (G1916Q which is 65% Frit 3195, 20% Frit 3110, 15% EPK). On coming out of the kiln, the glaze looked fine, crystal clear, no crazing. However, when heated to 300F and then immersed into ice water this happens. This is the IWCT test. At lower temperatures, where bodies are porous, water immediately penetrates the cracks and begins to waterlog the body below. Fixing the problem was easy: Substitute the low expansion Frit 3249 for high expansion Frit 3110.

Related Pictures

ChatGPT was completely wrong about the cause of glaze crazing in 2023!

A ChatGPT answer to a question about crazing

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ChatGPT was parroting common wrong suggestions about the cause and solution of the serious issue of crazing. Yet it trained on thousands of internet pages about the subject! Crazed functional ware is defective, and customers will return it. So fixing the problem is serious business, we need correct answers. Consider its suggestions: #1 is wrong. There is no such thing as an "incompatible mix" of ceramic materials. Crazing is an incompatibility in thermal expansions of glaze and body, almost always a result of excessive levels of high-expansion K₂O and Na₂O in the chemistry of the glaze. The solution is reducing them in favor of other fluxes (the amount per the degree of COE mismatch). #2 is wrong, firing changes don't fix the incompatibility of thermal expansions. #3 is wrong, refiring makes the crazing go away but not the stress of the mismatch, it will for sure return. #4 is completely wrong. Firing higher takes more quartz grains into solution in the melt and should reduce the COE (and mature the body more which often improves fit). And melt fluidity has nothing to do with crazing. Furthermore, if a glaze does not run off the ware, it is not overfired. Of course, this is the worst it will ever be, expect better in future.

The ultimate example of delayed crazing: 90 years!

Glaze chemistry is the key to understanding it

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A restoration project faced a tile-matching challenge. At installation in a bathroom 90 years ago, the tiles were not crazed. But between then and now it happened (shown inset upper right). Now, a restoration specialist is tasked with duplicating the aged effect (one unsuccessful attempt is shown here). The shade, opacity, degree of matteness, bubble-free matrix and surface character of the original are all real challenges. Duplicating the crazing is even more difficult. Why? Matching "time-crazing" with a crackle glaze pattern 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!

Other possibilities of making crazed matte:
-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. The downside is unpleasantness 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.
-Print the lines on the tile (using ceramic transfers) and use a translucent matte glaze (like G2934).

Videos

Links

Tests	Boiling Water:Ice Water Glaze Fit Test Ceramic glazes that do not fit the body often do not craze until later. This test stresses the fit, thus revealing if it is likely to craze later.
Tests	300F:Ice Water Crazing Test Ceramic glazes that do not fit the body often do not craze until later. This progressively stresses the fit until failure point, thus giving it a score
Recipes	G1916Q - Low Fire Highly-Expansion-Adjustable Transparent An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
Glossary	Glaze Crazing Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.

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