The Italian word scagliola (pronounced with a silent ‘g’), was in use in the Middle Ages long before the arrival of artificial marble.  ‘Scagliaiuola’ was the name given in parts of central Italy to the mineral rock from which gypsum plaster (the basic ingredient of Plaster of Paris) was obtained.  Only later did it become associated with the production of ‘scagliola’ artefacts.  The Florentine naturalist G. Targioni Tozzetti wrote in 1769:

   ‘[in addition to alabaster]…the majority of scagliola (i.e. the raw mineral rock) also comes from Spicchiaiuola (a town near Volterra), and is calcinated in Florence to make tables, altar fronts etc…’ (see reference).

Tozzetti went on to say that ‘scagliaiuola is none other than selenite’.  Selenite is a translucent form of gypsum found in abundance in the central and northern Apennines.   It has a laminated structure that allows it to be split into sheets.  The Romans used these sheets to make window panes, a practice which continued into the Middle Ages in Bologna, a major centre for selenite extraction.  The name ‘scagliola’ was undoubtedly derived from the physical characteristics of raw selenite, which is brittle and breaks easily into flakes and chippings (scaglie in Italian).  Once calcinated, the material loses its translucency and behaves like Plaster of Paris.

During the eighteenth century the word gradually came to be applied to imitation marble and hard-stone work, though by no means uniquely.  Several other Italian terms survive, many of them specific to different regions; these include meschia or mestura (mixture) from Modena and Florence, pasta di marmo (marble paste) from Naples, and pietra di luna (moon stone) – which again refers to the natural stone rather than to its product.  There is also the generic marmo di poveri (poor man’s marble) – no longer appropriate by today’s standards.

In the English-speaking world the term ‘scagliola’ has always referred to both  architecturally formed and inlaid work.  This is not the case in Europe, and particularly in German-speaking areas, where art historians and commentators have tended to differentiate between architectural and inlaid scagliola, seeing only the latter as true scagliola.  In Germany, architectural forms of scagliola are called Stuckmarmor or (particularly in Austria) Kunstmarmor (stucco or artificial marble), and inlaid work Scagliola.   In France the same distinction is made, using stuc marbre and scagliole or scaiole respectively.  Italians sometimes use the expression finto marmo (artificial marble) to describe architectural scagliola.  In the nineteenth and twentieth centuries, ‘Bossi work’ was sometimes used to describe the practice of inlaying white marble table tops and chimney pieces with delicate neo-classical designs in coloured scagliola; many of these fashionable pieces were made by the Italian Pietro Bossi in Dublin at the end of the eighteenth century.

The opening footnote from Erwin Neumann’s article on Scagliola (quoted alongside) gives a good description of the ‘arbitrary’ nature of the division between architectural and inlaid scagliola, which Neumann himself considered to be problematic.
     One could add that on both sides of the division the same materials are used and manipulated in much the same way to create the appearance of polished marble and hardstone; also that many scagliolists past and present have been  active in both types of manufacture.