A blender that handles vegetables easily will often refuse to reduce a spoonful of dry spice to powder. The two appliances look similar and are solving different mechanical problems.
Blenders are designed around liquid
A blender jar is tall and wide, and its blade relies on liquid circulating the contents back down into the cutting zone. That circulation is what makes it work at all.
Dry spice has no such circulation. The material is thrown outward, settles against the walls above the blade and stops moving entirely.
Increasing the quantity does not help, because the problem is the flow pattern rather than the load. The powder simply sits where the blade cannot reach it.
The grinder jar changes the geometry
A dry grinding jar is small, narrow and steep-sided, so material sliding down the walls returns directly to the blade rather than settling in a ring.
The blades are usually blunter and set closer to the base, since the job is impact fracture rather than slicing. Spice is shattered, not cut.
Higher rotational speed in a smaller chamber also means each particle is struck far more often per second than in a large jar.
Why heat is the limiting factor
Grinding converts most of the motor's energy into heat, and volatile aromatic oils begin to evaporate as the powder warms. A long continuous grind loses a noticeable amount of aroma.
Cooks work in short bursts with pauses between, which keeps the jar cool enough that the oils stay in the powder rather than escaping into the air.
Traditional stone grinding avoided the problem entirely, since a slow stone surface generates very little heat and crushes rather than pulverises.
Why wet grinding is a separate task again
Wet spice pastes and fermented batters need a different action, working the material between surfaces under pressure rather than striking it at speed.
Blade grinders tear the material and heat it, which affects fermented batters in particular because temperature influences how they behave afterwards.
Stone wet grinders survive in southern kitchens for exactly this reason, since the texture they produce is difficult to match with a blade.
What grinding fresh actually changes
Whole spices keep their aromatic oils sealed inside intact cell structures. Grinding ruptures those structures and exposes the oils to air, where they evaporate and oxidise steadily.
Pre-ground spice therefore loses potency over weeks rather than years, and the loss is largest in the most volatile top notes rather than in the base flavour.
That is why Indian kitchens grind small quantities often, and why blends are frequently made in batches meant to last a month rather than a season.