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The fired glaze exhibits a network of fine cracks. These may be plainly visible after firing or may need enhancement with ink. Crazing may also appear after a period of time or after ware has been exposed to thermal shock. Fired strength (an thus functional ware quality) are directly related to crazing since ware strength is enhanced by having the glaze under slight compression whereas it is severely reduced (up to four times less) when the glaze is under tension. If the underlying clay matrix is porous and soaks up water then safety could be a concern with crazed ware since the cracks could be wide enough to provide a friendly breeding ground for colonies of bacteria. Containers used to store food are a special concern since a small colony in a crack can become a large culture in the food. If you have any doubt whether this is an important issue ask a commercial food service inspector about the subject.
Is the crazing a result of mistreatment of ware during use?
Is crazing a result of inappropriate choice of manufacturing method or materials?
Is crazing due to a simple thermal expansion mismatch between body and glaze?
Could the Coloring Oxides in the Glaze be Involved?
Is the crazing a result an under fired body?
Is the crazing a result of sloppy manufacture?
Is ware crazing days or even months after firing?
Special Note: Solving crazing and shivering problems while retaining the visual character of a glaze is a classic problem for the application of ceramic chemistry calculations. There is a chapter in the lesson section of the INSIGHT manual on how to deal with this problem, it is a very practical approach.
Crazing in cone 10 reduction celadon glazes, especially on porcelain, is common because they are high in K2O/Na2O. However this problem can be solved by increasing the SiO2 and substituting some of the KNaO for lower expansion fluxes like CaO.
The unexpected reason for this crazing can be seen in the chemistry
This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion that CaO does.
Use a low silica porcelain to craze test your glazes
Cone 6 transparent glaze testing to fit Plainsman M370: Left and right: Perkins Studio Clear. The far left one is a very thick application. Center: Kittens Clear. The porcelain for all is Plainsman P300. Why? Because P300 is much more likely to craze the glaze because it has a lower silica content (about 17% and only kaolin whereas M370 has 24% silica plus the free quartz that comes with the 20% ball clay it also contains). If a thick layer works on P300 it is a shoe-in to fit M370. If it also passes the oven:icewater test.
Small recipe change, big improvement in craze resistance
This cone 04 mug has survived a 300F to Ice-water thermal shock test, one which a similar recipe failed badly. This is G1916T glaze on Plainsman Buffstone fired to cone 04 (the failing recipe was G1916Q). The difference? This one switches the Frit 3110 for Frit 3249.
Overly compressed glaze can severely weaken ware
This shivered mug has shattered on its own because the glaze is under so much compression on the inside. Spiral cracks have developed all the way up the side. Small flakes of razor sharp glaze are popping off, I cannot even leave this on a table so I have put it into a pail. The mug is pinging loudly and will likely completely self destruct in a day or two more. Why? I accidentally fired a low-fire talc body at cone 6 (with a cone 6 glaze). The clay body is not overly melted, it just has an extremely high thermal expansion (talc is added to increase the expansion to fit low fire commercial glazes (they would all craze without it). Shivering is serious.
Crazing after a year of use. What is the problem?
A cone 10R grey stoneware mug that has begun to craze on the inside. The greyer coloration around the craze lines indicates that water is soaking into the slightly porous body. This mug has lost the ring it had when it was new (it is only about a year old). It could be refired to be as good as new but would soon return to this condition. The only real solution is to reformulate this glaze to reduce its thermal expansion.
Why did this piece came out of a decal firing crazed?
This Cone 10 matte mug has been refired to attach decals. During the refire the Quartz-containing body passed up through quartz and cristobalite inversions while the glaze did not (all of its quartz was converted to silicates during the previous glaze firing). The sudden expansion in these two zones stretched the glaze and cracked it. Had that glaze been better fitted (under some compression) it would have been able to survive.
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