Why do sushi cooks mix the vinegar into the rice in a wide wooden tub, a handai, rather than a metal bowl?

Answer: The bare wood soaks up excess moisture, so the rice turns glossy, not soggy

The bare wood soaks up excess moisture, so the rice turns glossy, not soggy ✓ — Unvarnished wood drinks up the extra moisture released while the cook cuts the vinegar in and fans the rice, so the grains end up glossy and separate instead of wet and pasty. A metal or glass bowl absorbs nothing, so that moisture stays with the rice. The tub's wide, shallow shape also spreads the rice out so it cools evenly.

The wood gives the rice a cedar fragrance that the vinegar cannot — A clean handai is not there to perfume the rice. Sushi rice is already seasoned with a balanced mix of rice vinegar, sugar and salt, and a strong wood aroma would compete with the fish; what the tub changes is texture.

The wood chills the rice to fridge temperature in a few seconds — Sushi rice is never chilled that far or that fast. It is shaped at roughly 30 to 40°C, close to body temperature, so the tub's job is a gentle, even cool-down while the wood takes up moisture.