Mentions of wall and vault mosaics were rare (Ronczewski 1903 and Ricci 1914) until Carlo Cecchelli wrote the first definitive work on the subject (Cecchelli 1922). Since then scholars such as Bovini (1954) and Stern (1959) have sought the origins of wall mosaics. Others, like Parlasca (1959, 66–8) and Becatti (1961, 274–6, 291–4) have made valuable contributions to the subject in the form of essays within longer works. Neuerburg's (1965) work on fountains and nymphaea is particularly useful as a source because so many nymphaea are decorated with wall and vault mosaic. Another recent contribution to the subject is that of D. Joly (1965). A study of these works and of the monuments themselves reveals the following broad facts:
that wall mosaics, as distinct from floor mosaics, are largely composed of glass rather than stone tesserae;
that the earliest wall mosaics (those of the first half of the first century A.D.) incorporate, in addition to glass tesserae, a number of other materials, such as shells, pumice, twisted glass rods, pieces of glass and a substance known as Egyptian blue (Sear 1972–3, 29, n. 2);