A defect in glazed ware where the glaze is compressed too much by the body, the glaze actually peels off the ware on edges to relieve the stress. Shivering is thus the opposite of crazing and is also less common. This problem is potentially dangerous, since the tiny flakes of glaze having razor-sharp edges could be ingested (if they occur on functional ware). The solution is a glaze of higher thermal expansion or a body of lower expansion. Other solutions are sometimes recommended (like firing slower), but they do not fix this underlying problem.
Low expansion glazes craze less, but they can shiver
Example of serious glaze shivering using G1215U low expansion glaze on a high silica body at cone 6. Be careful to do a thermal stress test before using a transparent glaze on functional ware.
A classic Albany glaze that often shivers
These mugs have experienced very serious shivering. This is an Albany Slip glaze with 10% lithium carbonate, it is known to have a very low thermal expansion. This problem can be solved by reducing the amount of lithium or adding high-expansion sodium or potassium. However these fixes will likely affect the appearance.
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.
Shivering is not just a glaze problem with Terra Cotta
Low fire terra cotta mugs have cracked. Why? The white glaze is under compression, its thermal expansion is too low (that is why it is also shivering off the rim). As the piece is cooling the kiln the thick layer of white glaze first solidifies. As cooling proceeds the body shrinks (thermally) at a faster rate than the glaze. The puts the glaze under compression and stretches the body. As some point (e.g. last stages of kiln cooling, a thermal stress during use) the body cracks to relieve the stress (notice how the white glaze is pushing the cracks apart). Neither the body or glaze are at fault, in this case they are simply made by different manufacturers and are thermal expansion incompatible. One solution would be to mix it with a white glaze that is crazing (the opposite problem). Or you could add some nepheline syenite to the glaze to increase its thermal expansion (maybe 10% by dry weight).
Solving a difficult shivering problem
This demonstrates the difficulty you can encounter when trying to get an engobe working with a clay body. Here the slip/glaze is flaking off the rim of pieces at cone 04 (does not happen at 06). The front bi-clay bar demonstrates the white and red clays dry well together (the slight curve happened on the drying). They also fire well together (the curvature did not change on firing). The back two thin bars seem to demonstrate thermal expansion compatibility: a thick layer of glaze is not under enough compression to curve either bar during firing. While the white clay contains 15% frit and forms a good bond with the red body, that bond is not nearly as good as the one between the glaze and the white slip. Yet it is still flaking off the rim at the slip/body interface. Why? At first it seemed that failure was happening at quartz inversion (because the body had less quartz than the white slip). However now it appears that the combination of compressions of the slip and glaze are sufficient to break the slip-body bond on convex contours. The compression of the slip and glaze likely did not demonstrate well on the bars because at this low a temperature they are not vitreous enough to be easily curled.
Do you know the purpose of these common Ferro frits?
I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 has a very-low expansion, it is great for crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).
Shivering on a transparent over an engobe
Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body. But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerting added compression on the slip. This causes a failure at the slip-body interface.
The high thermal expansion of a low-fire talc body
Talc is employed in low fire bodies to raise their thermal expansion (to put the squeeze on glazes to prevent crazing). These dilatometer curves make it very clear just how effective that strategy is! The talc body was fired at cone 04, the stoneware at cone 6. The former is porous and completely non-vitreous, the latter is semi vitreous.
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.
What can you do using glaze chemistry?
There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.
An engobe is shivering off the rim of a low temperature mug
Classic terra cotta bodies are not vitreous, so engobes used on them need to have similar low fired shrinkage. But when terra cottas are fired above cone 04 they start to mature and fired shrinkage increases quickly, flaking off engobes that do not have sufficient added frit (to increase their shrinkage). Even if an under-compression engobe can hang on, the extra stresses of an overglaze of lower thermal expansion can compromise the engobe:body bond. That has happened on the center mug. That engobe has less frit (10% vs. 15% for the others). The clear glaze on the left has high thermal expansion and is crazing, while the engobe:body bond can tolerate that it, is obviously not desirable.
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