the specific surface area
Specific surface area is how much exposed surface a powder hides inside each gram of it, measured in square metres per gram. It sounds abstract until you picture chopping. One sugar cube has a small surface, but crush it to powder and the same sugar suddenly presents a huge total surface, because every new fracture creates two fresh faces. Fine powders are all surface: a single gram of a 0.1 micron oxide can carry ten to twenty square metres of surface, an area larger than a big beach towel folded into a pinch of dust.
The reason it matters so much to a ceramist is that surface area is the very driving force of sintering. Atoms at a surface are unhappy, missing neighbours they would have deep inside a solid, so a surface stores extra energy, and nature always tries to lower it. When a powder sinters, particles weld and pores close precisely to shrink that total surface and shed its stored energy, which is why a high-surface-area powder sinters faster and denser at lower temperature. There is a clean relationship for smooth particles: specific surface area equals 6 divided by density times diameter, so surface area and particle size are two views of the same thing, and a rising surface area is a direct sign of finer particles. Measuring the surface therefore measures the fineness and the sintering fuel in one number.
Surface area also carries an honest warning, because it responds to more than just fineness. Rough, porous, or hollow particles report a higher surface area than smooth ones of the same size, and internal pores in a particle add surface that does not help sintering the way external surface does. A very high surface area also means the powder is thirsty for water and quick to pick up impurities from the air, and it makes particles cling together into agglomerates. So a high surface area is the sinterer's friend up to a point, beyond which the same reactive surface becomes hard to handle and prone to clumping, which is why the standard way to measure it, BET gas adsorption, is such a workhorse tool.
For smooth spheres, SSA = 6 / (density times diameter). Alumina (density about 3.97 grams per cubic centimetre) at 1 micron gives roughly 1.5 square metres per gram; shrink the particles to 0.1 micron and the surface leaps to about 15 square metres per gram, ten times the sintering fuel.
Surface area and particle size are two views of one thing; more surface means more sintering driving force.
A high surface area is not purely a measure of fineness. Rough, porous, or hollow particles inflate it, and internal pore surface does not drive densification the way outer surface does, so read surface area together with particle shape.