William I (1797-1888), second son of King Frederick William III of Prussia (see lot 152) and Queen Luise of Prussia (see lot 153), became regent for his elder brother King Frederick William IV (see lot 152) in 1858, and King of Prussia in 1861. He was proclaimed Emperor in 1871. He had married Augusta of Saxe-Weimar in 1829.
For other enamels by Bastanier, including portraits of King Carol and Queen Elizabeth of Romania, dated 1894 and set into a presentation guest book, see Sotheby's, Geneva, 14 November 1985, lot 347.