|Monthly Tech-Tip |
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.
The relationship between the thermal expansion of body and glaze is called the "glaze fit". Glazes fit the clay body they are on when they do not form the familiar crack pattern of crazing when the ware is suddenly cooled or flake off on contours when ware is suddenly heated. Thermal expansion happens in thousandths of an inch as ware is heated during dish washing, in the oven, on the burner, etc. This sounds like a small amount of change, but ceramics are brittle and if the glaze is expanding or contracting more than the body to which it is attached, something has to give way. Thermal expansion and contraction are not to be confused with fired shrinkage that happens as a body densifies during heatup in the kiln.
Failure due to a poor fit glaze does not always happen right away, sometimes the glaze has enough elasticity to resist cracking or peeling, but repeated heatings and cooling will eventually reveal a poorly fit glaze. Ideally a glaze should have an expansion that is slightly lower than the body so that contraction during cooling puts the glaze under compression. This prevents crazing, the most common misfit.
Each clay:glaze combination needs to be considered on its own, tested as a unique instance. The 300F:Ice water test is a way to reveal the tendency of a glaze to craze. When people or factories make their own glazes they are in a position to adjust the chemistry to move the expansion up or down to deal with any misfit (lots of references are available on this site on how to do this). A less ideal solution is to change the recipe of the clay body, but few people are in a situation to do this. People who use commercial glazes are the most vulnerable, when these glazes fit their clay bodies it is actually just an accident.
Many suggestions can be found in books and on the internet on how to fix poor glaze fit. These often recommend firing or glaze thickness changes and use fancy words like "cristobalite inversion". But do not fall for band-aid fixes, if a glaze does not have a thermal expansion that is compatible with your clay body then the best approach is to change the expansion of one or the other. You can reason this out as follows: If a glaze is crazing it is stretched on the ware and has cracked to relieve the stress. The stretching happened because during cooling in the kiln the glaze contracted more than the body. So to fix the problem the glaze needs to have a lower contraction (and thus lower expansion). Or, the body needs to have a higher contraction (or higher expansion).
Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.
This mug is pinging loudly and literally self-destructing in front of my eyes! Why? The glaze is under so much compression (the inside is pushing outward, the outside inward). Spiral cracks are developing all the way up the side. Small razor-sharp flakes are shivering off convex contours. Why? I accidentally fired a low-temperate talc body at cone 6 (the glaze is the Alberta Slip base cone 6 glossy). The clay body is not overly mature, but it just has an extremely high thermal expansion (talc is added to increase the expansion to fit low fire commercial glazes, they would craze without it). Shivering is serious, it is a mismatch of thermal expansion between body and glaze. It can happen at any temperature.
Many people would find the fired appearance of this cone 10 reduction red fireclay (Plainsman FireRed) compelling. But it is not at all suitable for functional ware. This crack grew wider over a period of a week (after firing) because the inside glaze is exerting forces it cannot resist. Notice that where there is a glaze cover on the upper outside section there is no cracking. But the stresses within are still there, waiting for an opportunity for release. It is inherently risky to glaze ware only on the inside if you are not able to determine the fit. This is especially so if the body is not vitreous and does not have the strength to resist the outward pressure. But even if it is vitreous, the internal stresses will weaken its ability to withstand bumps.
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.
Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.
This is a slip-cast vase. The body is a typical talc:ball clay blend. Only the outside of the vessel is glazed. Why did it just burst apart like this on its own after firing. Without the almost 50% talc these bodies have, all but very specially formulated glazes will craze severely. The talc increases the thermal expansion so that during cooling in the kiln the body contracts more than the glaze, putting it under compression (and thereby preventing crazing). But, these bodies have no flux, they typically have 10% or more porosity, and often are not strong enough to resist for long the tensive forces the glaze can put them under. This is especially so when walls are thin and only the inside or outside is glazed (or when a glaze is under compression on one side of the wall and tension on the other). This issue is more important at low fire but potters do something very unfortunate: they use glazes from one manufacturer and trust they fit bodies made by another without do any fit-testing.
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 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.
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.
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
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.
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.
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.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
If you are a potter and want to make restaurant ware, read this. Many of the things you already think you know will mislead you in this type of venture.
|Tests||300F:Ice Water Crazing Test|
Insight-live share showing how to fix crazing with cone 6 Leach's Clear