I hadn't heard of Jean-Etienne Liotard before, so the exhibition on the RA was a revelation. His technique is breathtaking, especially his masterly use of pastels, which rivals even Degas.
With dropped jaw I kept mouthing to myself 'how did he do that?', particularly the amazing miniatures. My favourite piece though was 'Mary Therese Liotard' (1780), a really striking etching of his daughter that he made purely for himself.