The firing temperature was probably between 1,180 °C and 1,280 °C, with the range over 1,250 °C giving the best green or blue colours. In some cases at least there appear to have been layers of glaze and also multiple firings to achieve a deeper glaze effect. The glaze is made opaque by the presence of plant ash and tiny bubbles of gas, which give a lustrous effect. With the whiter body clays pieces may be translucent. The pronounced reddish colour of unglazed areas comes from the end of the firing, as the heated clay comes in contact with fresh air let into the kiln, and the iron present turns into ferrous oxide. Many pieces have crazing or crackle in the glaze, but much less than in the closely related Guan ware. A technique sometimes found before about 1400 was to add spots or splashes of a mixture rich in iron oxide with an appearance of randomness; these fired a dark brown. Both Chinese and Japanese tradition have developed a range of terms to describe the glaze colours and qualities; some of the Japanese ones have the advantage of being anchored to specific pieces in Japan. The term "kinuta" (砧青瓷) meaning "mallet", probably after a particular mallet vase, represents the most admired blue-green colour from the Song period, and is often used in English, while "tenryūji" has a "a faint yellowish-green tone", and is from the Yuan and Ming. The "shickikan" type is from the middle Ming, after the glaze became more transparent. As with other celadons, for the Chinese the similarity of the colour to jade, always the most prestigious material in Chinese art, was an important factor in their appeal, and something the potters attempted to increase. Most shapes are simple but very elegant. The size and decoration of larger fine pieces increases from the Yuan onwards, with some very large vases and lidded wine jars being made by the 14th century. The "mallet" vase was a special favourite at Longquan, often with handles formed as animals or dragons. Funerary vases, made in pairs, also often feature charmingly stylized animals, usually tigers and dragons, curled around the shoulders of the vessel. These were used in southern Chinese burial custom to store provisions for the afterlife. Another distinct Longquan style was a dish with two or more fishes in low relief swimming in the centre, either in biscuit or glazed; these sometimes have holes drilled for metal handles, as mentioned in a late 14th-century source.