We bought a bag of white powder plaster at Orsini builder's merchants which we were told was pure Gesso.The countryside around Manoppello and Roccamorice in particular seems to have deposits of this mineral that is a dirty yellow grey colour with quite large crystals.A company called La Farage still make Gesso in Abruzzo.
Locally several villages are named after this mineral;Gessopalena being one such place.
thanks to website panramio for photo
The mineral is burnt in a furnace and the resulting white powder is called Gesso in Italy and Gypsum elsewhere.It is actually Calcium Sulphate.The process can also create 'plaster of paris' which apparently gets its name from the plaster used by specialist French masons who came to work in English cathedrals during the middle ages.Its not something that naturally lends itself to stone pointing or building but maybe it was mixed with some other substance?
The term Gesso in England is really only used to describe a 'ground' for paintings on canvas or wood panels.
thanks to The National Gallery for this information:
Gesso is the Italian word for the white mineral gypsum. It is used as a ground or preparatory layer to ensure a smooth surface for painting or gilding on wood. It was also sometimes used for the priming of canvas.
Gesso consists of gypsum bound in size (an adhesive made from animal skin and waste). The common practice was for a coarse lower layer to be applied (gesso grosso), followed by a finer upper layer (gesso sottile). It has to be scraped and sanded to make it absolutely smooth. In some unfinished or damaged pictures parts of the gesso layer can be seen.
The gesso could be built up to form a low relief on the wood known as pastiglia. It could also be cut or incised - much of the fine details of picture frames is cut in the gesso rather than in the wood below. In panel painting north of the Alps, instead of gypsum chalk or some other form of calcium carbonate was generally used bound with size as the white ground.
I imagine that the gesso made in the north used lime putty or dry lime rather than gesso powder?