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Fired Strength

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.

Key phrases linking here: fired strength - Learn more

Details

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 a lot 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 than you might expect. In fact, 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), 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 a number of new factors must be considered. Glaze fit is very important. Crazing (thermal expansion too high) is a defect that produces micro-cracks that provide convenient sites for failure when stresses occur. Glazes under excessive compression (thermal expansion too low) shiver and can fail in a spectacular, and somewhat hazardous way. 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. A simple way to test glaze fit is to age pieces for a few months and then break them. Ones with good fit, even if fired at low temperatures, will be much more difficult to break.

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, when their particle size distributions are tuned to produce maximum density and surface contact their higher firing temperatures 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).

Related Information

How much does clay shrink when bisque fired?

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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?

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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!

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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.

Vitrification can be obvious by simple visual inspection

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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.

To gauge strength: Break pieces often. Pay attention to what happens.

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A broken slip cast cone 6 mug

This is M340 casting, L3798H. The handle was just glued on with slip. I had questions about this new body. Are my glazes under compression? How well is the handle stuck on? Because I have broken so many pieces I know how they break. It is a good sign when there are no fractures along the handle joins. And when the handle is the last to go. And when the item breaks into large pieces rather than shattering into small ones. Try doing this on some of your pieces and you might be surprised. Either by how strong they are. Or by how easily they break.

Low fire mug survives two-foot drop on to cement

In slow motion. This is Plainsman Snow clay, a super white burning dolomite body. Although low-temperature ware is not as strong as stoneware it has more elasticity. Other than a tiny chip out of the base and handle, this mug still has the ring of strength. Where does this piece get all this strength? 1. It is glazed inside and out. 2. Glazes adhered well to this porous body, forming a deep interface. 3. The compression under which the glazes exist not only resists crazing but adds a lot of strength to the piece. Notice how many times it bounces on the handle!

Inbound Photo Links



Fired strength tester


The glaze broke the bottom off the pot!


The difference between vitrified and sintered


A vessel being forced apart by the pressure of a low expansion glaze inside

Links

Glossary Mullite Crystals
Glossary Sintering
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.
Glossary Vitrification
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.
Glossary Restaurant Ware
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 Fired Strength Round Bars
Materials Wollastonite
URLs http://en.m.wikipedia.org/wiki/Flexural_strength
Flexural Strength at Wikipedia
URLs https://www.eieinstruments.com/tiles_&_ceramics_testing_instruments/bending_strength_test/mor-testing-machine
MOR Testing Machine
URLs https://phys.org/news/2023-10-electron-rich-metals-ceramics-tough.html
Electron-rich metals can be added to ceramic to make it tough to crack
By Tony Hansen
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