According to his trade labels, I (or J) Hill worked from an address in Oxford Street, London. The silhouettes ascribed to him are painted in black watercolour, with no gum Arabic. The brushstrokes are not particularly fine, and the detail is roughly drawn. Foskett mentions a J Hill as a miniaturist on ivory who could be the same person as the profilist I or J Hill.
Source: McKechnie (Author of, British Silhouette Artists and their Work 1760-1860)
Hill, I. or J. (McKechnie Section 2)