a soft agglomerate
A soft agglomerate is a loose cluster of particles clinging together by weak forces, the kind that falls apart when you rub it or stir it into liquid. Think of clumps in a bag of flour or lumps in cocoa powder: the grains are stuck together, but a whisk or a sieve breaks them right back into free powder. Soft agglomerates form all the time simply because fine particles are attracted to one another and to bridges of moisture, so almost every dry fine powder is really a powder of agglomerates rather than of truly separate particles.
The defining feature is that the particles are held only by weak surface attractions, van der Waals forces and capillary bridges of adsorbed water, not by chemical bonds or sintered necks. That is the crucial contrast with a hard aggregate: a soft agglomerate can be broken back into its primary particles by modest energy, a few minutes of milling, ultrasonic agitation, or good dispersion in a well-chosen liquid, whereas a hard aggregate cannot. Soft agglomerates form during drying, when a wet powder dries and capillary forces pull the particles into clumps, and during storage, as fine powders slowly clump in the jar. They are a nuisance but a curable one.
Soft agglomerates matter because, if they are not broken down before shaping, they behave like large particles and pack unevenly, and each surviving agglomerate leaves a larger-than-normal pore in its place after firing, the same defect a hard aggregate causes, only here it was avoidable. The whole point of the milling and dispersion steps is to break the soft agglomerates apart so the true fine particles pack uniformly. The honest and hopeful message is that, unlike a hard aggregate, a soft agglomerate is fixable: with the right milling or a well-dispersed suspension you can knock it back into free particles and recover the fine, uniform powder you paid for.
A dried nanopowder looks like coarse grit because its fine particles have clumped into soft agglomerates. A few minutes of ultrasonic agitation in a well-chosen liquid shatters the clumps and the measured particle size drops back to the true nanometre scale.
Weakly held clumps break apart with modest energy; the point of milling and dispersing is to undo them.
Soft and hard clumps look alike in a jar but behave oppositely. A soft agglomerate breaks apart with milling or good dispersion and can be cured, whereas a hard aggregate is chemically bonded and cannot, so telling them apart tells you whether the powder is salvageable.