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.
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 or 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 by heating it to 300F in a kiln or oven (being sure to hold it there for a while to make sure the heat penetrates) and 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.
Safety issues: While there are popular forms of ceramics which craze by nature (e.g. shino, crystalline glaze) crazing impacts functionality (safety and strength). First, it provides a home for bacterial growth that necessitates extra care in cleaning and sterilizing ware, especially if the body is absorbent and harbors moisture. Also, when glazes do not fit ware strength is affected. Ceramics are brittle, when cracks start they want to propagate (especially in vitreous ware). The author has measured 300-400% reductions in ware strength for crazed glazes vs glazes under some compression (which actually strengthens ware). Thus, a cone 01 piece with a well fitted glaze can be stronger that a cone 10 one with a poor fitted glaze! A third issue is that the chemistry profile that creates the most crazing is inherently more leachable. Crackle glazes have high KNaO (up to triple the recommended maximum in normal glazes, this pushes downward or eliminates other other fluxes) and they often have very low Al2O3; these factors are recipes for leaching of any added metallic colorants. Popular ones that have up to 90% Ferro Frit 3110 are good examples of this.
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 the absolute minimum of unglazed body. 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.
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.
This flake shivered off the rim of a low fire terra cotta mug. It is Fishsauce slip. It is about 2 inches long and has razor sharp edges. This is not the sort of thing you would want to be falling into your coffee or food and then eating! This flake did give evidence that it was loosening so there was little danger of me consuming it, but smaller flakes can go unnoticed. Slips (or engobes) must be drying compatible, have the same firing shrinkage, the same thermal expansion and be quartz inversion compatible with the body. It is easy to ignore all that and pretend that it works, but the bond between engobe and body is fragile at low fire and easily compromised by the above incompatibilities. Slips must be fitted to the specific body, glaze and temperature; that involves a testing program and often a little chemistry. I have documented online how to I adapted this slip to Plainsman Terrastone 2 using my account at insight-live.com.
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.
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 it tolerated that silica addition very well and continued to fire to an ultra gloss crystal clear. That change eliminated the crazing issues. The cup on the right is a typical porcelain that fits most glazes (because it has 24% silica and near-zero porosity). The center one only has 17% silica and zero porosity (that is why it is crazing this glaze). I added 5% more silica to the glaze, it took that in stride, continuing to produce an ultra smooth glossy. It is on cup on the left. But it is still crazing just as much! That silica addition only reduces the calculated expansion from 6.0 to 5.9, clearly not enough for this more severe thermal expansion mismatch. Substituting low expansion MgO for other fluxes will compromise the gloss, so clearly the solution is to use the porcelain on the right.
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).
It would craze glazes! This is fired at cone 6 and the crazing was like this out of the kiln. This is about as bad as I have ever seen. One might think that there is adequate quartz in this high of a percentage of ball clay to at least minimize crazing, but no so. This demonstrates the need for adequate pure silica powder in stoneware bodies to give them high enough thermal expansion to squeeze glaze on cooling to prevent crazing like this. This is also not proving to be quite as refractory as I thought, it looks like it will have about 3% porosity at cone 10.
These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.
Crazing in glazes is common in this type of ware but since the body is fired well into vitrification this is not considered an problem (the unique aesthetics of this type of ware trump such issues). Salt glazes, by their very nature, are high in sodium. And it has a high thermal expansion.
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.
The piece on the left is 65% ball clay and 35% nepheline syenite. The one on the right is 65% kaolin and 35% nepheline syenite. Both fire vitreous at cone 6. But the glaze is crazing on the kaolin and not on the ball clay. This is because the ball clay contains significant quartz, that raises the thermal expansion and that puts the squeeze on the glaze and prevents the crazing.
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 not only creates a germ zoo but severely weakens the piece. I tapped this lightly with a spoon about 1-2 cm below the rim. It sounded like I was hitting a wet sack of rocks! And after the first tap the handle fell off! With each tap the pitch dropped. The piece breaks apart the same way auto-glass does in a collision. This type of functional ware is entirely unacceptable. If you use commercial glazes test their compatibility with the bodies you use. 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). If buy ware like this return it for a refund.
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.
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 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 (which drastically reduces strength and provides a haven for bacteria). The glaze is "stretched" on the clay (because it has a higher thermal contraction). When the lines are close together like this it indicates a more serious issue (I have highlighted them 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.
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.
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.
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.
These were applied to the bisque as a slurry (suspended by gelling with powdered or dissolved epsom salts). The nepheline is thicker. Notice the crazing. This is what feldspars do. Why? Because they are high in K2O and Na2O, these oxides have by far the highest thermal expansions. So if a glaze is high in feldspar it should be no surprise that it is going to craze also.
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 (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 the Frit 3110.
The side of this white porcelain test mug is glazed with varying thicknesses of VC71 (a popular silky matte), then fired to cone 6. Out of the kiln there was no crazing, and it felt silky and wonderful. But a 300F/icewater IWCT test was done and then it was felt-pen marked and cleaned with acetone. This is what happened! This level of crazing is bad, the dense pattern indicates a very poor fit. Then why was it not crazed coming out of the kiln? The glaze is apparently elastic enough to handle the gradual cooling in the kiln. But what the kiln did not do, time certainly will. This recipe has 40% feldspar (a big high-expansion KNaO contributor), that much in a cone 6 glaze it a red flag to crazing. Coupled with that was low Al2O3 and SiO2, another tip-off.
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 fix: A transparent base that fits your ware. Add colorants and opacifiers to that. Another fix: substitute some of the KNaO for a lower expansion flux (like MgO, SrO, CaO, Li2O) and add as much SiO2 and Al2O3 as the glaze will take (using glaze chemistry software).
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.
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.
Crazing is a disaster for a production potter. Consider what one said: "I have just recently been contacted by a customer due to small lines in her bowl. I am now terrified residual crazing could be happening to lots of functional pieces I have sold! Nightmare! I have a terrible feeling in my stomach. Could anyone tell me if it is the glaze and if there is anything I can do to alter the recipe?" Yet this is easy to fix.
An example of a highly fluid glaze melt that has pooled in the bottom of a bowl. The fluidity is partly a product of high KNaO, thus it is also crazed (because KNaO has a very high thermal expansion). While it may to decorative, this effect comes at a cost. The crazing weakens the piece, much more than you might think (200%+). Those cracks in that thick layer at the bottom are deep, they want to continue down into the body and will do so at the first opportunity (e.g. sudden temperature change, bump). Also, fluid glazes like these are more likely to leach.
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, alumina has the lowest. 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%!
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 other oxide. 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 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.
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.
|Media||Analysing a Crazing, Cutlery-marking Glaze Using Insight-Live|
|Media||Desktop Insight 3 - Dealing With Crazing|
|Media||How I Improved a Popular Cone 6 Clear Glaze Using Insight-Live|
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Dishwasher safety is a concern in ceramic table ware, especially if the ware has been imported or made by a small company or potter.
Gas fired rustic ceramic ware is cooled from red-hot in a closed container with organic material. The zero-oxygen atmosphere produced reduces carbonate metal decoration to its metallic form.
|Glossary||Salt, soda firing
Salt firing is a process where unglazed ware is fired to high temperatures and salt is introduced to produce a vapor that glazes the ware.
Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.
Crackle glazes are used on decorative ceramic ware. They have a crack pattern that is a product of thermal expansion mismatch between body and glaze.
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.
|Glossary||High Temperature Glaze
Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Examples of what people say about crazing ceramic glazes
INSIGHT Glaze Chemstry Software
ASTM C424 - 93(2012) Standard Test Method for Crazing Resistance
ASTM C554 - Thermal Shock Test Method for Crazing Resistance
|Tests||Boiling Water:Ice Water Glaze Fit Test|
|Tests||300F:Ice Water Crazing Test|
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
|Oxides||Na2O - Sodium Oxide, Soda|
|Articles||Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
|Articles||Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
|Articles||Crazing and Bacteria: Is There a Hazard?
A post to a discussion on the clayart group by Gavin Stairs regarding the food safety of crazed ware.