During the Tang dynasty, Islamic potters across the Middle East were producing a range of turquoise-blue and green effects with alkaline glazes fluxed with sodium and cupric oxides. The burgeoning trade between China and the Islamic world at this time is evidenced by archaeological finds, including that of turquoise-glazed earthenware at Yangzhou, already a major trading centre. However, Chinese potters developed a high-alkaline turquoise glaze rather different in composition to the traditional Islamic version, using potassium oxide sourced from saltpetre as the primary flux for producing a bright blue colour. Experimentation in glaze composition and improvement of glaze technology continued into the Qing dynasty, especially at the Jingdezhen kilns. Turquoise monochrome glazes proved particularly popular in Europe, where the colour was known as bleu-celeste and favoured by upper-class patrons including Marie Antoinette.