The mural painting preserved in the former monastery of Santa Caterina was made using the buon fresco or a fresco technique: after sketching the preparatory drawing directly on the wall to be painted, the painter spread the pigments on the damp plaster so that they crystallized with the lime binder during drying and formed a whole with the surface below.
An essential prerequisite for the success of the work was therefore the speed of construction, since the dry plaster would have created a layer that was almost impermeable and unsuitable for the adhesion of the color.
To overcome this problem, between the 13th and 14th centuries the practice of spreading the plaster "per giornate" (for days) was introduced, i.e. limited to the portion of the fresco that was intended to be created in one day and with the mortar still damp.
Still in use in the seventeenth century, this practice is also found in the fresco of Santa Caterina in which, for the main scenes of the Annunciation and the Last Supper, it was possible to count four "days": two for the Virgin Annunciation and the Archangel Gabriel, respectively, and two for the central episode, where the line of demarcation runs at the center of the composition.
The study conducted during the restoration also made it possible to identify the different ways in which the preparatory drawing was reproduced on the wall: tracing from cardboard with a thick, rounded tip, dusting and details carried out freehand with a brush. For the frames, ready-to-use guide models, the so-called patrones, were used.
The mapping of the engravings also made it possible to identify a "pentimento" ("second thought") on the part of the painter: in fact, in the preparatory drawing a canopy over the figure of the Virgin was initially foreseen, but this detail was not then executed in the final version of the painting.
Crivellaro 2009–2010, pp. 98-99)