|Monthly Tech-Tip |
In ceramics, discussion about particle size most often focusses on clays and clay bodies. Fine particle sizes are associated with clean-burning and plastic clays, coarser ones with structural and sculpture bodies. Clays have already been processed by nature and have small particle sizes (those are agglomerated into the lumps we dig out of the ground), whereas other ceramic minerals have to be ground into a powder, often by massive machinery. Amazingly, the sizes, populations and relative populations in a ceramic powder can be measured physically using sieves and ultimately using lab equipment. Their shapes and surface topologies can also be characterized.
While a plastic clay might feel smooth to the touch it is actually millions of tiny rocks particles held together by the water. Clay particles have an affinity for water, they hold on to it like mini-magnets. And they are generally flat. The finer the particles the greater surface area they can expose to water and the greater will be the plasticity of the clay. When a piece of plastic clay is dried and put into water it will slake down, the individual particles are liberated. If propeller-mixed that slaked clay becomes a slurry and it can be poured through a screen (or sieve). Of course, any particles it contains will be filtered out by the sieve. That means we can weigh the oversize and compare that to the total amount to get a percentage.
Typical clays that might seem very fine can contain a surprising amount of larger particles. Sugar, for example, is around 40 mesh (that is a sieve with 40 wires-per-inch with openings measuring about 400 microns on-a-side). But many clays contain significant populations of particles that are double or triple that size. And they can contain particles that are incredibly small. 90%+ of some raw clays can be slaked and will pass a 325 mesh sieve (that is 325 wires-per-inch with openings of 45 microns, an average human hair is 70 microns). But those are physical particles, ones that we can measure on a sieve. But that 90% that passes the 325 sieve is ultimate particles, much finer, down to sub-micron sizes (a white blood cell is 25 microns)! Bentonite particles are the smallest (less than a micron), then ball clay and finally kaolin (up to 10 microns).
With glazes and enamels much effort goes into particle size reduction, this improves the working and fired properties. Typically there is less emphasis on measuring sizes than on production-testing to deduce the suitability (and increasing or decreasing grinding time, e.g. by ball milling, accordingly).
These are table salt crystals on a 60 mesh sieve. It has an opening of 250 micro meters (or microns). Half of the crystals passed this sieve, half are retained. Notice on the right, several crystals are in the openings, about to fall through. Imagine that an particle (or crystal) of bentonite or ball clay can be sub-micron in diameter, they can actually be 2500 times smaller on a side than these salt crystals! One-tenth-micron ultimate particles would thus fit 2500x2500 on the flat side of a salt crystal. And, since the clay crystal is much thinner than wide, perhaps ten could stack to the same dimension. That means theoretically 2500x2500x25000 (or 1 with eleven zeros) could pack into a grain of salt!
To measure particle size in a slurry or powder you need sieves. This is the most popular type used in labs. They are made from brass by a company named Tyler. The range of screen sizes for testing particle size is very wide (obvious here: the top screen has an opening of 56 mm, the bottom one 0.1 mm - the wires are almost too small to see). You can buy these on ebay for a lot less than new ones, search for "tyler sieve". The finer sieves (especially 200) are fragile and easily ripped. It is good to have a 50, 100 and 150.
A closeup of 35-48 mesh grog particles (courtesy of Plainsman Clays). Grogs are added to clay bodies to impart better drying properties. Grog particles perform their drying-shrinkage-reducing function (for plastic bodies) best when they have an angular rather than round shape.
We measure 50 or 100 grams of powder, mix it in water and then wash it through sieves of varying openings. The finer particles go through, we dry and weigh the coarser ones that do not.
Which one of these samples weighs more, the raw lump form of the clay or after it has been ground into a powder? Wrong. It is the lumps. Even though there is all that empty air space between those lumps, there is even more air spaces in the powder. The top one weighs 1662 grams (there is a 500g counter-weight barely visible), the bottom one is 1255. The finer I grind it the lighter it will be. If I were to fill in all the voids between the lumps on the top one with smaller sized lumps I could get alot more weight yet! It works the same on the ultimate particle level, when we combine powders of varying particle sizes we get a more dense and stronger dried product.
This is VeeGum T, a processed smectite clay (similar to bentonite, extremely small particle size). I have propeller-mixed enough powder into water that it has begun to gel. How long does it take for them to begin to settle? Never. This sat for a month with no visible change! That means it is colloidal.
Particle Size Distribution
Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.