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.
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 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).
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.
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.
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.
Feldspar melts by itself to be a glaze? Hold on!
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.
Substituting alumina in a clay body dramatically lowers 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, 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%!
When glazes are highly fluid they can...
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.
Crazing is a RED LIGHT. Pay attention! Even if it is a well known recipe.
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.
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.
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! 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.
No crazing out of the kiln. So it is good. Right? Wrong!
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 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.
Do your functional glazes do this? Fix them. Now.
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.
What would happen if you made a body from 50:50 kaolin:ball clay?
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.
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).
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 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.
If you think one slip fits any body, think again
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.
Same body, glaze, thickness, firing. Same thermal shock. Only the tile crazed?
Why did the glaze on the tile craze? The tile is double the thickness of the walls of the mug. Thus, when quenched in ice water, a greater gradient occurs between the hot interior of the clay and the cooling surface.
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.
A salt glazed mug fired at cone 10 in the kiln at the Medalta artist in residence program
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.
Pure Custer Feldspar and Nepheline Syenite on cone 10R porcelain bodies
These were applied to the bisque as a slurry (suspended by gelling with 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.
An extreme extremely runny glaze at cone 6. Is there a 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.
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