The instantly-recognizable opalescent blue of the Jun glaze is in fact primarily due to an optical effect, rather than to an actual blue element within the composition of the glaze. After high firing, the pieces in the kilns were cooled very slowly, resulting in the development of tiny globules of lime-rich glass within the silica-rich glaze matrix, a process known as ‘liquid-liquid phase separation’. 

The purple splashes are the result of the deliberate application of copper oxides to the surface of the unfired glaze, a decorative technique which appears from the end of the 11th century. In the 2001 excavation of the Liujiamen Jun ware kiln site in Shenhou, Yuzhou city, Jun ware shards decorated with large red and purple areas were found in the late Northern Song strata. See ‘Liujiamen junyao fajue jianbao’ (Brief of the Excavation of Jun Ware at Liujiamen), Wenwu (Cultural Relics), 2003, no. 11, fig. 13 and 19. From the same excavation, a shard of a small bowl with rounded sides and a slightly inverted rim, strongly reminiscent of the current bowl was also found in the late Northern Song stratum. A line drawing of this shard is illustrated ibid., p. 34, fig. 15.7.

Small Jun-glazed bowls with splashes both on the inside and out are highly sought after. Examples of 'bubble' bowls, mostly of below 9 cm. in diameter, include one in the Palace Museum, Beijing, illustrated in The Complete Collection of Treasures of the Palace Museum - 32 - Porcelain of the Song Dynasty (I), Hong Kong, 1996, p. 246, pl. 222 (8.3 cm. diam.); another in the collection of the Percival David Foundation of Chinese Art, now on loan to the British Museum, illustrated by S. Pierson, Song Ceramics: Objects of Admiration, London, 2003, pp. 60-61, pl. 20 (PDF 45B) (8.6 cm. diam.); and one in The Metropolitan Museum of Art, illustrated by S. Valenstein, A Handbook of Chinese Ceramics, New York, 1975, p. 87, no. 80 (50.145.316) (8.6 cm. diam.). Two further bowls are illustrated by J. Ayers, The Baur Collection, Geneva, 1968, vol. 1, nos. A31 and A32 (both 8.5 cm. diam.). See, also, the bowl sold at Christie’s Hong Kong, 30 May 2012, lot 4051, and another bowl, also from the Linyushanren Collection, sold at Christie’s Hong Kong, 2 December 2015, lot 2808.

Small blue cup with splashed glaze, Northern Song dynasty, about AD 1000–1127. Jun ware, Yuxian, Henan province. Stoneware with opalescent blue and purple glazes; 3,1 cm. diam. © 2017 Trustees of the British Museum

A purple splashed Jun bowl, Northern Song-Jin dynasty, 12th-13th century; 3 3/8 in. (8.6 cm.) diam, from the Linyushanren Collection, sold 6,400,000 HKD at Christie’s Hong Kong, 2 December 2015, lot 2808.

Cf. my post: A purple-splashed Jun bowl, Northern Song-Jin Dynasty, 12th-13th century