Letter dated 8.1.1820 from Niels Rosenkrantz to Irgens Bergh, cf. Sass: Op. cit., II, p. 2 (“Thorvaldsen has almost finished the busts of the Princess and her sister. Those of the King and the Queen as well. The work is quite successful.”)
Other references
Else Kai Sass: Thorvaldsens Portrætbuster, Copenhagen 1963-65, Vol. I, p. 22, 470; Vol. II, p. 2-7, 10, 17, 180, 182.