|Monthly Tech-Tip |
Sieves are important in ceramics for removing particulates and agglomerates from glaze, engobe and body slurries.
35 mesh: 500 microns 50 mesh: 297 80 mesh: 177 100 mesh: 149 140 mesh: 105 200 mesh: 74 325 mesh: 44
A 325 screen has 325 wires-per-inch (the finer of the two screen closeups shown here). Those are grains of salt on it (45 micron openings, a typical human hair is 60 microns wide). A 40 mesh screen is much coarser, it has 425 micron openings (that is a particle of quartz trapped in an opening). A minus 45 mesh powder will be too fluffy to drop through a 325 screen. But particles smaller than 45 microns in a slurry will pass. To get a slurry through a screen this fine one needs to take special measures. I needs to have a high water content so it is fluid. Using a soft brush definitely helps. And a source of vibration. And it is necessary to clean the screen often to remove trapped oversize material. 325 mesh screen fabric is fragile and a sieve like this needs to be treated with care. These cost hundreds of dollars.
This is a cone 6 transparent base glaze. It contains frit, silica, kaolin, wollastonite. Almost all glazes have materials that are slightly soluble and over time these can form scale on the sides of the bucket or even precipitate particles into the slurry. The defects here are those scales. Before dipping a production piece in any glaze that has been in storage it is a good idea to assess it first to see if it needs to be sieved.
This glaze has just been applied to a bisqued tile. It contains wollastonite, which can agglomerate in storage. It was propeller-mixed at high speed, but that was not enough to break down the white lumps (agglomerates). But they can be broken down by sieving the slurry through 80 mesh or finer. Many other materials behave in a similar manner (e.g. barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon, titanium dioxide).
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.
The coarsest screen is at the top, the finest on the bottom. The opening for each is shown on the label. They are chosen such that each successive screen going down has an opening that is about half the area of the one above it. Using this series you can produce a practical measurement of the distribution of particle sizes in ceramic materials and bodies used in traditional ceramics (structural products industries, like brick, measure coarser particles than this, starting at perhaps 10 mesh and ending at 70). The 325 screen on the bottom is only used sometimes, it is difficult to finer-that-325 particles to pass through it because it blinds. It is not possible to shake powder through sieves that are this fine, samples must be washed through. We use the SIEV test to log results.
Do you need to rescreen a glaze slurry. Using a brush like this you will be able to get it through the screen much faster. This is because the rubber edge forces particles into the screen openings, plugging them. The brush is gentler, the oversize material just rolls around on top. If you are screening a glaze for the first time, however, the spatula is better if there are agglomerated particles that need to be broken up (e.g. wollastonite, cornwall stone). When rescreening, any oversize particles (e.g. precipitates) should be discarded.
Potters often store glazes for long periods so tiny spherical precipitate particles can form. These were found in a months-old bucket (about 2 gallons). These can appear over time, depending on factors like temperature, electrolytes in your water or solubility in the materials (likely, the frit is slightly soluble). The glaze slurry should be screened periodically (or immediately if you note the particles when glazing a piece). This is an 80 mesh sieve. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula. The loss of material on the screen is tiny and inconsequential to the glaze.
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!
US Sieve conversion chart (mesh, microns)
ShowerShelf.com home-made vibratory screen can handle 1.8SG slurry
|Tests||Wet Sieve Residue|
|Tests||Sieve Analysis 35-325 Wet|
|Tests||Sieve Analysis Wet|
|Tests||Sieve Analysis Dry|