Ishi-usu
石臼/EE-shee-oo-soo/
The traditional granite stone mill used to grind tencha into matcha.
Also known as: matcha stone mill, granite mill
An ishi-usu is a hand-cut granite stone mill used to slowly grind tencha into the fine powder we call matcha. Two large discs rotate against each other at low RPM, with grooved channels guiding the ground leaf to the outer edge. The slow speed prevents heat — heat would scorch the leaves and dull both color and flavor. A traditional ishi-usu produces only 30 to 40 grams of matcha per hour, which is most of why high-quality matcha isn't cheap. Modern producers also use ceramic or electric mills, which are faster but considered to produce less nuanced matcha.
In practice
- The granite mill that grinds tencha into matcha. Two stones, the upper one turning slowly against the lower, with the leaf fed through a central hole.
- The critical constraint is speed. A mill turns at around 30–60 revolutions per minute and yields only 30–40 g of matcha per hour — one stone, one hour, one tin.
- Slowness is not tradition for its own sake. Friction heat degrades the aroma compounds and turns the colour olive, so grinding faster measurably damages the tea.
- That throughput ceiling is the single biggest reason good matcha costs what it does.
The common mistake
Assuming 'stone-ground' on a label guarantees quality. It describes the method, not the leaf fed into it — culinary matcha is stone-ground too. The grade of the tencha matters more than the mill.
Questions people ask
Can I grind my own matcha at home?
Not usefully. Domestic spice and coffee grinders run far too fast and hot, and produce a coarse, dull-tasting powder at maybe 50 microns against the 5–10 microns a stone mill achieves. Small hand mills exist but take hours per serving.
