Cox's watercolour technique of the 1840s was sufficiently different from his earlier methods to need explanation to his son in 1842, despite the fact that his son had been helping him teach and paint since 1827. The materials used for his later works in watercolour also differed from his earlier periods: he used black chalk instead of graphite pencil as his primary drawing medium, and the rough and absorbent "Scotch" wrapping paper for which he became well-known – both of these were related to his development of a rougher and freer style.