The gold Isis medal was presented to Mr. Robertson, for his improved method of executing paintings in watercolours. By a combination of processes, some original, others already known to a few artists, he has succeeded in giving to his pictures a purity of tint, and a strength and solidity of colour, which renders them almost a perfect representation of paintings in oil. A specimen was exhibited to the Society, which has been executed several years, and has been cleaned, when dirty, by means of spirit of wine ; and neither by time, nor by the use of this menstruum, have the colours been at all impaired.
^Robertson, Charles John (1828). "No. Ii. Painting in Water-Colours". Transactions of the Society, Instituted at London, for the Encouragement of Arts, Manufactures, and Commerce. 47: 38–41. JSTOR41327106.