All Glossary

Glaze shivering

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.

Key phrases linking here: glaze shivering, shivering, shiver - Learn more

Details

A defect in glazed ware where the glaze is compressed too much by the body, the glaze peels off the ware on edges of contours to relieve the stress. Shivering is thus the opposite of crazing. It 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.

The principal home of information on this issue is in the troubleshooting section in the Shivering topic. Here we will concentrate on recognizing it and avoiding bad advice on dealing with it.

Bad advice on reducing shivering in stoneware glazes: These are from two popular pages (links below). ChatBots regurgitate this same wrong information to avoid simple glaze chemistry, so it must appear internet-wide.
-Shivering can be corrected by adding high expansion materials to the glaze, such as alkaline frit, nepheline syenite or feldspar, which contain sodium and potassium. Frits and feldspar bring much more Al₂O₃ and SiO₂ than Na₂O, does the glaze need that?
-Decrease the silica in either the body or the glaze. Excessive silica is not likely the problem, it's the glass-former and most important oxide. Decreasing this is the wrong way to increase the needed Na₂O.
These have in common approaching this problem on the material level, which is mostly wrong, it is a chemistry-level issue.

This problem is more likely to occur when the clay-glaze interface is not as well developed, such as with earthenware. At low temperatures, one must play more of a balancing act since the glaze cannot be under as much compression before this issue (or the opposite one of crazing) occurs. Many low-temperature clay bodies (white and red) have talc added to increase their thermal expansion, this is done to reduce the likelihood of crazing with popular commercial glazes. But this also increases the chance of shivering, especially for people who mix their own glazes. It is important to have an adjustment in your base glazes to tune the fit, for example, a higher and lower expansion frit in the recipe. If shivering is observed, the proportion of higher expansion frit can be increased. Clay body manufacturers must pay special attention when they need to change talc suppliers, adjusting the body recipe to compensate for any differences in the expansion the new talc imparts (for example substituting a little of the talc for pyrophyllite to lower the overall expansion).

High temperature reduction fired brown burning stonewares can be an issue. The warm variegated colors of these bodies depend on their not being vitrified. This less developed surface does not develop as good of an interface with the glaze as buff stonewares or porcelains. This means that glazes under excessive compression will shiver off more readily. One needs to have realistic expectations about the thermal expansion resistance of these bodies.

Shivering also occurs in stoneware bodies that have high quartz content, such as those made with high percentages of fireclay and ball clay (and even silica additions on top of that). Increasing the KNaO content of the glaze will fix the problem (feldspar is the major contributor). By the same token, high feldspar glazes are the most likely to work.

Normally clay bodies should have a higher thermal expansion (and thus higher thermal contraction) than the glaze. When this is so the glaze is put under compression as the piece cools in the kiln. This situation not only strengthens the ware but prevents the glaze from crazing when the surface is cooled suddenly (and contracts). The higher the temperature and the better the body-glaze interface, the greater the compression that can be tolerated and the greater the potential is to increase strength and resist future crazing. Of course, the question is, how much higher should the body expansion be? It is generally best to do the testing for this rather than rely on calculated numbers. Make ware and subject it to the sudden heating of ice water to boiling water. If crazing or shivering occurs, do the calculations to move the glaze expansion in the right direction and retest.

Related Information

Shivering on terra cotta is a red light not to ignore

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).

What can you do using glaze chemistry? More than you think!

There is a direct relationship between the way ceramic glazes fire and their chemistry. These green panels in my Insight-live account compare two glaze recipes: A glossy and matte. Grasping their simple chemistry mechanisms is a first step to getting control of your glazes. To fixing problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. To substituting frits or incorporating available, better or cheaper materials while maintaining the same chemistry. To adjusting melting temperature, gloss, surface character, color. And identifying weaknesses in glazes to avoid problems. And to creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. And even to creating glazes from scratch and using your own native materials in the highest possible percentage.

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.

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.

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.

Glaze under excessive compression can flake off the engobe

The amber glaze on the outside of the left mug contains 20% super-low thermal expansion Ferro Frit 3249 as the melter. With no underlying engobe it can form enough of a bond with the body that it does not flake off at the rim (even though it is under excessive compression because its low thermal expansion). This flaking is called "shivering". The engobe, which does not melt like a glaze, has a more fragile bond with the body (and the glaze is pushing enough to make that bond fail). The mug on the right employs 20% Frit 3195 melter instead, producing a glaze that fits better. I hammered both of these rims repeatedly with a metal object to stress them, that one on the right definitely fits better.

High thermal expansion talc body cannot be COE-calculated

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 and the stoneware at cone 6. The former is porous and completely non-vitreous and the latter is semi-vitreous. This demonstrates something else interesting: The impracticality of calculating the thermal expansion of clay bodies based on their oxide chemistry. Talc sources MgO and low fire bodies containing it would calculate to a low thermal expansion. But the opposite happens. Why? Because these bodies are composed of mineral particles loosely sintered together. A few melt somewhat, some change their mineral form, many remain unchanged. The body's COE is the additive sum of the proportionate populations of all the particles. Good luck calculating that!

Links

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk