One of the most common ways of measuring and specifying particle size of ceramic materials is by the use of sieves, often called meshes or screens.
How they work?
Particle sizes are separated industrially or in the laboratory by passing material over sieves that have opening of certain specified sizes. These sieves can range from extremely coarse devices that look like sewer grates for large size separations [range of inches] to sieves that look very much like window screens for middle separations [millimeter], to very fine woven screens for fine separations [~50 micrometers]. Separation can be either dry or wet. Wet screening is more efficient, but drying of the product adds cost.
What do the numbers mean?
Each sieve has a number, such as 20 mesh, 50 mesh, 200 mesh, 325 mesh. This number relates to the number of openings in the screen per inch, but the actual opening size is hard to estimate since the thickness of the wire varies (finer screens are made from finer wire). Modern practice is to refer to the screen by the size of the opening in microns.
Click here for a cross reference of screens and their opening dimensions: Sieves
Representing Screen analysis results
Screen results are represented in several ways, most commonly as weight percent of material passing [represented by a negative sign: -] or retained on [represented by a positive sign: +] a given screen. Thus, material specifications that indicate -30 mesh +80 mesh, call for a material with particles smaller than 30 mesh [590 µm] and larger than 80 mesh [177 µm]. The retained (+) and passing (-) nomenclature is also used in representing particle size analyses made with a series of screens. Consider the following table of results:
The third column gives directly the results of a laboratory screen analysis, i.e. the weights of the materials taken from each screen. The pan collects any material passing the final mesh and in this case represents material finer than 44 µm. However, these results are not so informative since we do not know the exact size of the material sitting on each screen. All we know about the 20% retained on the 200 mesh screen is that it is finer than 88 µm and coarser than 74 µm.
A better way to present the data is by cumulative finer or cumulative coarser tables and graphs, as shown in the following table.
These finer than and coarser than data are unequivocal and precise. Exactly 26% of the material is finer than 74 µm and 74% is coarser.
Graphs illustrate the differences in nomenclature. Click to view each form of representation.
Histogram
Cumulative Percent Finer
Cumulative Percent Coarser