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Surface Area


Surface area (or SSA - Specific Surface Area) is a physical property you will see listed on the data sheets and certificates of analysis of many materials. Some materials (e.g. sedimentary clays) can contain particles that have a wide range of sizes, shapes, densities, surface texture, reactivities and unique chemistries and mineralogies. Other materials (e.g. kaolins) contain mostly one particle shape with the only significant difference being particle size. More than any other material, the physical properties of plastic clays and clay-containing slurries are directly tied to surface area. In clays, the total surface area of all particles in a sample help explain many properties, (e.g. plasticity, drying shrinkage, dry strength, melting behaviour).

In clays, more surface means a greater ability of the clay to exhibit plasticity. Water acts as a glue, holding all the particles together (because the surface chemistry of clay particles has an electrolytic affinity for water). The total forces by which they attract to it increase exponentially as surface area does.

Kaolins have comparatively large ultimate particles and thus have less surface area than ball clays (one popular kaolin has an SSA of 20-30 square meters per gram). Ball clays, having particle sizes up to ten times smaller, should have dramatically more surface area. But, their SSAs can sometimes be in the same range as kaolins, this is counterintuitive since they are much more plastic. Bentonites have particle sizes up to ten times smaller than ball clays, only a gram can have hundreds of square meters of surface area! Notwithstanding this, some manufacturers quote SSA values in the range of ball clays (or even kaolins)! It can thus be a little confusing, perhaps we can take from this that SSA is not much of an absolute indicator of the magnitude of any property (e.g. plasticity). Remember, surface area is just one part of understanding the physical properties of a material, especially clays (the reactivity and topography of that surface will contribute to properties also).

Surface area values are thus best suited as quality control indicators for a specific material. When the SSA value changes it is a heads-up to possible changes in process or materials that have occurred.

Example of a certificate of analysis for a kaolin

Example of a certificate of analysis for a kaolin

When companies ship materials they often include these with the shipment. The information reported is often very basic and properties important to ceramics are often not found.

One gram of processed hectorite has a surface area of 750 square meters!

One gram of processed hectorite has a surface area of 750 square meters!

A combination of surface area, surface topography, surface chemistry and surface electrolytics determines how plastic a clay is. This material is super plastic.

Out Bound Links

  • (Glossary) Particle orientation

    Clay particles are flat and prefer to orient or arrange in a lattice-like pattern during mixing and forming operations. In bodies where they mix with other non-clay ingredients, they continue to arrange to the extent to which they can tolerate the disruptions of the others. The smaller the clay part...

  • (Glossary) Ultimate Particles

    Processed ceramic materials are typically ground to 200 mesh and feel very fine to the touch. With some you can detect some particle grains between your fingers. The amount of these "physical particles" can be measured by washing or shaking the ceramic powder through a sieve. Using water washing and...

  • (Glossary) Particle Size Distribution

    When minerals and mixtures of minerals are ground into a powder a particle size distribution is produced, that is, populations of particles of various sizes. The relative sizes of these populations can be measured to rationalize the behavior of the powder in the ceramic process. Wide distribution...

In Bound Links

  • (Glossary) Water

    There is a need to discuss water in ceramic production as it related to a number of natural phenomena and production processes: Plasticity: Clays are plastic because water glues and lubricates the particles. The micro-dynamics of this are complex. Rheology: Suspensions (solids:water systems) e...


By Tony Hansen




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