The fired strength of clays can be measured. The test is sometimes called M.O.R. or modulus of rupture, recognizing the fact that brittle ceramics fail suddenly (as opposed to others that fail after some plastic deformation). It is also known simply as tensile strength (because the point of failure is always where the sample is under most tension). Ceramics perform much better under compressive strength testing than they do when stressed flexurally (compressive testing is more common in the structural ceramics industry).
How much does clay shrink when bisque fired?
Not much. These mugs were exactly the same height before a bisque firing to cone 06. The clay is a porcelain made from kaolin, feldspar and silica.
Which is stronger: Cone 10R mug or cone 03 mug?
The mug on the left is high temperature Plainsman P700 (Grolleg porcelain). The other is Plainsman Zero3 fired at cone 03. Zero3 has a secret: Added frit which reduces the porosity of the terra cotta base (therefore increasing the density) dramatically. How? The frit melts easily at cone 03 and fills the interparticle space with glass, that glass bonds everything together securely as the piece cools. Although I do not have strength testing equipment right now, I would say that although the P700 mug likely has a harder surface, the Zero3 one is less brittle and more difficult to break.
Smash your ware to see if it is strong!
I use a nylon hammer, and glasses of course. I just filled two five-gallon pails and three boxes. Every type of clay and glaze I currently use. Every temperature. I started with a commercial Denby stoneware piece to get a feel for how quality ware should break. It becomes immediately evident which pieces are weak by the way they shatter. Breaks with knife-like edges indicate strong body/glaze combos. Strong ware breaks into fewer pieces. Crazed ware is weak. Low fire vitrified ware can be very strong. High-fire ware can be weak (e.g. iron stonewares having high porosities). Give attention to this, make quality ware.
Fired strength tester
Round or square fired bars are subject to a force that tests their tensile strength. The strength can be calculated from the force and the dimensions of the cross section where the break occurred.
The glaze broke the bottom off the pot!
A example of a highly fluid cone 6 glaze that has pooled in the bottom of a mug (and crystallized). Glazes normally need to be under some compression to avoid crazing (by having a lower-than-the-body thermal expansion), but if they are thick like this the body does not have the strength to resist the extra outward pressure the glaze can be exerting at the base from the inside. The result here is a separated base. Conversely, if the glaze is under tension (having too high an expansion), the cracks that develop within it to relieve the tension are deep and wider and thus more likely to propagate into the body below. The ultimate result: Poor ware strength.
The difference between vitrified and sintered
The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature (because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely). That is what vitrified means. But it has a high fired shrinkage, poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder (VeeGum), all that cements it together is sinter-bonds between closely packed particles (there is no glass development). Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, low thermal expansion and maintains its strength and hardness at very high temperatures.
A vessel being forced apart by the pressure of a low expansion glaze inside
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.
Vitrification can be obvious by simple visual inspection
The unglazed surface of the left piece has a sheen, it is a product of glass development during firing to cone 6. That body is a 50:50 mix of a cone 8 stoneware and a low fire earthenware red (a material that would normally be melted by this temperature). Together they produce this dense, almost zero-porosity ceramic. The unglazed surface on the right looks more like plaster, and it is absorbent, about 5% porosity. It is a mix of the same stoneware but with 50% ball clay. The refractory ball clay assures that the stoneware, which was already inadequately vitreous, is even more so. As you can imagine, the left piece is far stronger.
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