Tealeaves release their tannin, tea’s naturally astringent element, only at 140° or above. So infusing green tealeaves with cold water brings out their innate sweetness, enhanced even further when you pile your glass or ceramic kyusu (the traditional small Japanese teapot, made of earthenware, stoneware or porcelain) with ice on top of the tea leaves.
This explains why sun tea, which is also brewed without boiling water, has such a sweet, mellow flavor. Ice-brewing works best on a hot, muggy day, because the tea is brewed as the ice melts. With a good green tea, like In Pursuit of Tea’s sencha or gyokuro, you can fill your teapot with ice as many as three times, but the second and third infusions might not measure up to the grassy sweetness of the first one.