All Glossary

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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Details

Crazing refers to small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very small, ceramics are brittle so if a glaze is "stretched on" the ware, it will likely crack to relieve the stress. Crazing appears when ceramic is cooled and the glaze shrinks more than the clay to which it is rigidly attached. Shivering, on the other hand, usually first appears when ware is suddenly heated.

There are many treat-the-symptoms approaches to crazing but the bottom line is: If there is a thermal expansion mismatch between body and glaze it will reveal itself sooner or later no matter how you adjust firing or glaze thickness to hide the problem. If crazing is visible, it is already an indication of a significant difference in thermal expansion between glaze and body.

Crazing is often not easily visible when a glaze is colored or variegated, thus it is wise to take measures to be sure. To reveal it, just paint an area of the surface with a heavy black marker pen and then clean it off with methyl hydrate or other solvent. The craze lines will be revealed by black ink that absorbs into the cracks. This test has the additional benefit of revealing surfaces that stain (and are thus difficult to clean).

Glaze thermal expansion is a product of its chemistry (provided it is completely melted). By far the most effective method to adjust expansion is to reduce the amount of high-expansion oxides (like sodium, potassium) and replace them with similar function oxides of lower thermal expansion (using glaze chemistry software like Digitalfire Insight). There are a number of treat-the-symptoms approaches. For example, simply adding silica to try to dilute the high expansion oxides will obviously affect its gloss and melting temperature (because so much silica is needed). Adding silica to the body will make it more refractory and susceptible to dunting. Substituting a frit for one of lower expansion will change the overall chemistry of the glaze and so a host of other fired properties could be affected (like color, hardness, melting temperature).

Testing: Crazing is often delayed, ware that appears to be OK out-of-the-kiln later crazes. This happens because of repeated exposures to sudden cooling. When this happens the glaze is suddenly stretched (because of the thermal contraction it experiences when being cooled). Each time this happens the bond with the body can be affected. Ceramics do not perform well under tension. The situation is aggravated when ware is thick, this is because the underlying body will adjust to the temperature change much more slowly than the glaze (which is being forced into thermal contraction by its direct contact with the cold liquid). It is thus important to stress-test your ware. We do this using the IWCT test (heating it to 300F in a kiln or oven, being sure to hold it there for a while to make sure the heat penetrates, then plunging the piece into ice water). We then put the ware in a freezer and leave it for at least an hour, then plunge it into boiling water. Any functional ware, from terra cotta ware to translucent porcelain, should survive this without crazing or shivering.

Crazing impacts functionality (safety and strength). The cracks and their access to a possibly porous body below can provide a home for bacteria growth, this necessitates extra care in cleaning and sterilizing ware. Crazing profoundly affects ware strength. Ceramics are brittle, when cracks start they propagate, especially in vitreous ware. Crazed ware, out of the kiln, is thus pre-cracked and ready to fail. This is an inherent weakness because the cracks provide failure initiation sites for cracks, when ware is subjected to stress the jump into action. The author has measured 300-400% reductions in the strength of freshly fired ware with crazed glazes. A cone 01 piece having a well-fitted glaze can actually be stronger than a cone 10 one with a poorly fitted glaze! Consider also that the chemistry profile that creates the most crazing can also be inherently more leachable (e.g. the high KNaO and very low Al₂O₃ common in crackle glazes make them susceptible to leaching of added metallic colorants).

Crazing can also occur in glazes on low fire ware when the body absorbs water and expands. To avoid this it is best to leave a minimum of unglazed body surface and plug that with silicone sealant. Body formulations also often include calcium carbonate which is said to help prevent this phenomenon.

Not all people agree that crazing is bad. Here is an example of a post on a popular social site: "Crazing does NOT render a pot useless for domestic ware, glazed ware through the centuries has been more likely to be crazed than not. Humanity survived.". This should be balanced with the cautions (similar to the above) found in the documentation of crackle glazes offered for sale by the prepared glaze manufacturers.

Related Information

ChatGPT is completely wrong about the cause of glaze crazing!

Today, ChatGPT is parroting common wrong suggestions about the cause and solution of the serious issue of crazing (January 13, 2023). 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 ChatGPT's 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 in accordance with 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 thus improve the fit. And melt fluidity has nothing to do with crazing. Furthermore, if a glaze does not run off the ware, it is not overfired.

This feldspar melts by itself to be a glaze, but crazes badly

Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K₂O and Na₂O (KNaO), these two oxides have the highest thermal expansion of any oxide. By far! Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.

A down side of high feldspar glazes: Crazing!

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K₂O and Na₂O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could likely tolerate a 20% addition of that quite easily. That would reduce running, improve fit and increase durability. If the crazing does not stop the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some chemistry in your insight-live.com account.

Craze city: Feldspar and Nepheline Syenite on cone 10R porcelain bodies

These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved Epsom salts). On the left is Custer feldspar, the right is Covia Nepheline Syenite. Notice the crazing (feldspars, and nepheline syenite, always craze because they are high in K₂O and Na₂O, these oxides have by far the highest thermal expansions).

Crazing due to moisture expansion in a porous low fire body

The clear glaze on this cone 03 mug survived a 300F-to-ice-water thermal shock without crazing (IWCT test). However, in the process, water was absorbed by the bare foot ring and dispersed into the porous matrix of the lower part of the mug. Moisture expansion occurred as a result and produced the crazing. Over continued use (and rewetting of the base) the entire piece would eventually craze. Calcium carbonate is often added to low fire bodies to prevent this expansion.

Substituting alumina in a clay body dramatically lowered thermal expansion

These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!

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.

The same liner glaze crazes on the porcelain but not the stoneware

The stoneware has a higher silica content and is not vitreous. This means there are more quartz particles to impose their high expansion because fewer are taken into solution by the feldspar.

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.

Same body, glaze, thickness, firing. Same thermal shock. Only the tile crazed?

Why did the glaze on the tile craze? It is double the thickness of the walls of the mug. Thus, when quenched in ice water (BWIW test), a greater gradient occurs between the hot interior of the clay and the rapidly cooling surface.

Why did this piece come out of a decal firing crazed?

This Cone 10 matte mug has been refired to attach decals. The fired matrix of the body is now brittle and dense and contains millions of quartz grains of many sizes. During the refire up through quartz and cristobalite inversions each of them experiences sudden volume increases. This does not happen in the glaze because its quartz particles were dissolved in the melt and converted to silicates during the previous glaze firing. The suddenness of the expansion depends on the rate of temperature increase and its extent depends on the size of the quartz particles. The body's passage through these two zones stretched the glaze and cracked it. Had the glaze fit been better (under some compression) it would likely have been able to survive.

Really bad crazing! Which is really not good!

These two glazes look the same, they are both cone 6 satin mattes. On the same porcelain. But the matteness "mechanism" of the one on the left is a low Si:Al ratio melted by zinc and sodium. The mechanism of the one on the right, G2934, is high MgO melted by boron (with the same Si:Al ratio). The "baggage" of the mechanism on the left is high thermal expansion and crazing (drastically reducing strength and providing a bacteria opportunity). The glaze is "stretched" on the clay (because it has a higher thermal contraction). When the lines are close together like this it is more serious (they have been highlighted with dye). If the effect is intended, it is called "crackle" (but no one would intend this on functional ware). The glaze on the left calculates to a high thermal expansion so the crazing is not a surprise.

You absolutely must fix crazing on functional ware you sell!

These are commercial bottled glazes, a transparent and a decorative. On a porcelain at cone 6. They are not compatible with the body (its thermal expansion is too low or they are too high). This could create a germ zoo but also severely weakens the piece. I tapped this lightly with a spoon about 2 cm below the rim. It sounded like I was hitting a wet sack of rocks! And the handle fell off! Another few taps at it broke like auto-glass. This type of functional ware is entirely unacceptable. If you use commercial glazes test their compatibility your clay bodies. If you make your glazes, adjust their recipes to reduce the thermal expansion until no crazing occurs using a thermal stress test (e.g. the IWCT test, which anyone can do).

Why are these crazing lines dark like this?

This is an example of serious crazing in a glaze. The lines have gotten darker with use of the bowl! That means the color is organic, from food. This cannot be healthy.

Adding silica will fix crazing, right? Not here.

G2926B (center and right) is a clear cone 6 glaze created by simply adding 10% silica to Perkins Studio clear (a glaze that had a slight tendency delay-craze on common porcelains we use). Amazingly that glaze tolerated the silica addition very well, continuing to fire to an ultra gloss crystal clear. That change eliminated the crazing issues on most of our bodies. The cup on the right is one of them, that body is vitreous, near-zero-porosity, and fits most glazes. Why? Because it has 24% silica in the recipe. The center porcelain is also dense and vitreous, but it only has 17% silica, that is why it is crazing this glaze. Then I added 5% more silica to the glaze, it continued to produce an ultra smooth glossy, and applied it to the 17% body on the left. Why did not fix the crazing? That silica addition to the glaze only reduces the calculated expansion from 6.0 to 5.9, clearly not enough to fix the problem. So, the obvious solution seems to be use the porcelain on the right. Are you wondering why adding silica to a body raises its thermal expansion, and adding it to a glaze lowers it? Mineralogy is the reason.

Match calculated COE to dilatometer-measured body COE? No!

Why? Firing temperature, schedule and atmosphere affect the result. Dilatometers are only useful when manufacturers monitor bodies AND glazes over time and in the same firing conditions. Calculated values for glazes are only relative (not absolute). The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (your account at Insight-live has the tools and guides to do this). Then fire a glazed piece and thermal stress it (300F-to-ice-water IWCT test). If it still crazes, move it further. If you have a base glossy glaze that fits (and made of the same materials), try comparing its calculated expansion as a guide. Can you calculate body expansion from oxide chemistry? Definitely not, because bodies do not melt.

You cannot fix this crazing with a process or firing change

This is severe crazing (at cone 10R). It is happening because of the chemistry of the glaze, not the firing. The first option to check when fixing crazing is: Can the glaze accept an addition of SiO₂? This glaze is an excellent candidate for that because the melt is highly fluid, it will surely be able to dissolve extra SiO₂. But it could also accept Al₂O₃ because it is highly glossy (a little extra Al₂O₃ will not matte it and would also reduce expansion and increase fired hardness and durability). What to do then? I would start with a 10% addition of a mix of two parts silica to one part kaolin (this mix has a 10:1 SiO₂:Al₂O₃ ratio, about the same as most glossy glazes).

An extremely runny glaze at cone 6. Crazing is the cost!

This recipe melts to such a fluid glass because of its high sodium and lithium content coupled with low silica levels. Reactive glazes like this produce interesting visuals but these come at a cost that is more than just the difficulty in firing. Recipes like this often calculate to an extremely high thermal expansion. That means that not only will this form a lake in the bottom of ware when used on the inside, but the food surfaces will craze badly. The low silica will also contribute to leaching of the lithium and any colorants present.

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 than 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.

Salt glazed pieces almost always craze

Crazing in glazes is common in this type of ware but since the body is fired well into vitrification this is not considered a problem (the unique aesthetics of this type of ware trump such issues). Salt glazes, by their very nature, are high in sodium. Since Na₂O has such high thermal expansion pieces are almost guaranteed to craze. This was from kiln at the Medalta artist in residence program

This pitcher is oozing a black goo after water sat in it overnight

Even after two weeks it is still sticky. This was purchased at an import store. What could this black goo be? It is likely a sealer that they use to make the porous clay water tight, perhaps an organic sugar. The clay is porous (and thus also weak) because they want to save energy by firing their kilns as low as possible. A water soluble sealer can be OK if the vessel is not used for storage. But it is not OK because there is another problem: The glaze is crazed. That is what is permitting the water to be absorbed into the body. That water is dissolving the sealer and bringing it out. There is yet another issue: The glaze could very well contain lead. Lead makes glazes melt low, so it is a great for saving energy. But not so great for producing safe ware.

Inbound Photo Links

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