Hylography – the process of emergence and disappearance, intentional or unintentional, of graphical form out of matter – [Link]
Here in a work by Helen (Shanks).
The technique is terra sigillata – employing a fine slip applied to the surface and manipulated in firing to various effect. In antiquity the results were fine and very distinctive, ranging from Greek Geometric, Red and Black Figure, to the glossy red tablewares popular for centuries across the Roman empire.
Terra sigillata – “earth bearing small figures” – “earth bearing a sign or seal” – Roman terra sigillata (Arretine, Samian) was often decorated with applied moulded figures.