The walling is constructed with alternating courses of undressed stone and brick (opus mixtum) held together with mortar made from a mixture of binding materials (lime, pozzolana), aggregates (sand, gravel), and water. Roman mortars are being studied and investigated today, mainly owing to their great strength, which is due to a thorough working of the mixture and slaking of the lime. One important consideration in the study of the monuments is the “module”, a unit of measurement by which the proportions of the buildings and their various parts are regulated. In the case of the opus mixtum, the “module” is equivalent to the height of a band of bricks. The bricks are square or rectangular and of varying dimensions, and, owing to the cost of making them, their use was restricted to courses of walling, doorways, piers, and vaults. The walls were built upwards using scaffolding. Information about its form is provided by the holes, still visible today, which were made in the walls to receive the ends of the wooden scaffolding poles.
The Bricks
A study of the bricks, their technical characteristics, dimensions, and stamps, in association with the archaeological data, produces important information for dating the monuments. The bricks bear signs (such as X or S) made with the manufacturer’s finger, or relief stamps, usually a single letter, a cross, or a monogram. The stamped bricks were made in a mould with the stamp cut into the bottom.
The Mortar
Depending on the kind of plaster and aggregates it contains, the mortar used in the Palace defines the building’s construction phases and falls into three types.
Type 1. Greyish-white mortar. Binding medium: lime, pozzolana. Aggregates: sand, gravel.
Type 2. Greyish-white mortar. Binding medium: lime, pozzolana. Aggregates: sand, gravel, crushed tile.
Type 3. Pinkish mortar. Binding medium: lime, pozzolana, tile-dust. Aggregates: sand, gravel, crushed tile.
