Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
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).
Common sense suggests that the more vitrified a clay is, the stronger it will be. Likewise, we assume that higher temperatures produce stronger ware. The growth of mullite crystals in porcelain at high temperatures can contribute alot to strength. However other factors also contribute to fired strength (particle packing, vitrified vs. sintered, shape and surface properties, the presence of a glaze and its fit) and products fired at lower temperature can rival the strength of high fire.
For glazeless vitrified ceramics, maturity is a key factor in achieving optimal fired strength. Testing is required since optimal strength may produce a body with more fired warping than desired. Strength may also drop off less than expected at lower levels of vitrification. Bodies that have been vitrified too much and have become glassy lose strength and become brittle. One reason is that over maturity can detrimentally affect the development of mullite crystals (pyrophyllite is often added to porcelains to encourage better development of a mesh of long mullite crystals within the matrix). Lower temperature clay bodies can develop considerable strength at much higher porosities that you might expect. Infact, one of the strongest bodies we have ever tested was fired at cone 1 with around 3-4% porosity (more than 10,000 psi). However, in industry, good strength is achieved at much higher porosities than this, especially when body materials are very fine and the process densifies the matrix well. Wollastonite suppliers claim that additions of their material can greatly improve the fired strength of non-vitreous bodies. Thus, the optimal fired strength of a body is a product of a number of compromises involved with firing, forming, materials, glazing and the needed thermal expansion.
Ceramic is brittle, so any surface discontinuities (e.g. micro-tears made during forming from poor plasticity), large cavities or pores (e.g. from material burned away during firing) or aggregate particles (coarse grog particles are often surrounded by micro-cracks as a product of drying and firing) provide places for cracks to propagate from. A body matrix can have coarser particles, but these must be complemented by a range of sizes that produce an overall matrix that has densified well during drying and firing.
When ceramics are glazed and number of new factors must be considered. Glaze fit is very important. Crazing is a defect that produces micro-cracks that provide convenient sites for failure when stresses occur. We have measured a 300% difference in fired strength between a poorly fitted glaze and a well fitted one. A white stoneware, for example, measured about 2500 psi with a crazing glaze, while a well fitted one measured 8000 psi. Care must be exercised not to have glaze under too much compression as this could produce shivering and contribute to spectacular failures for certain types of ware.
People accustomed to working only with vitrified bodies are often surprised at how strong sintered ones can be. Even though the latter lack the glass to cement particles together and do not develop crystalline mesh matrices their particle size distributions, density and the much higher temperatures to which they are fired produce surprising strength. Low fired ware can also be very strong if a low melting glass (like a frit) is incorporated into the recipe (e.g. Zero3 stoneware and porcelain).
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.
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 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.
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.
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.
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.
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.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
Flexural Strength at Wikipedia
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