The use of Prussian blue in nineteenth-century Japan has been extensively researched, particularly in relation on the "blue revolution" in ukiyo-e prints but its use by Chinese artist has not received the same degree of attention. A commodity traded by the East India Company, this pigments was used to "improve" the colour of tea, but in or about 1825 the trade abruptly ceased. It now seems fairly clear that the cessation of the Prussian blue trade coincided with the setting up of a Prussian blue factory at the northem gate of Canton, and that knowledge of the industrial process was possibly acquired covertly from a London manufacturer. The pigment has been identified chemically among pint fragments collected during the dis-binding of an album of Chinese botanical watercolors sent from Canton to the Horticultural Society of London by tea inspector John Reeves between 1817 and 1830. This finding suggests that conservators should not exclude the possibility of finding Prussian blue on Chinese work dating from at least the early part of the nineteenth century.