History of Coccio Pesto

Stuccoes and mortars are composed by binder, aggregates and extenders.
The main traditional binders to formulate mortars and stuccoes are limes, cement and gypsum.
According to the final use, lime based mortars and stuccoes can be composed by:
common lime or lime putty, that hardens only on open air;
hydrated lime that, as the above, hardens only on open air;
hydraulic lime, that hardens in water too.
The typical aggregates for lime stuccoes are sand and marble dust. Since from the Roman times, to those have been added, if needed, extenders able to modify the hydraulic properties of the binder by physical-chemical interaction. In this way, by increasing the hydraulicity is possible to obtain more hardness.
The extender par excellence, utilized already by Romans to add resistance to mortars and stuccoes, is pozzuolan. This is a volcanic rock, that is crushed at the same size of the aggregates.
Another material that can be considered as a pozzuolan is coccio pesto, obtained by crushing old roof tiles or old bricks made, notoriously, of cooked clay, chemically composed of aluminium silicates.
Coccio pesto was used by Romans in regions where was impossible to find the pozzuolan and is still used in places like Venice, where is necessary, in order to have a good weatherproof finish, to use stuccoes and mortars greatly hydraulics, that means more resistant.
Coccio pesto, besides giving great hydraulicity to the stucco, allows to do quite thick layers for its lightness, and gives the final characteristic salmon colored surface due to the crushed reddish clay.
Mainly, the Romans used coccio pesto for the rough coat, the first layer of plaster, and than they were going over it with two layers of lime and sand to finish it with lime and marble dust in different coats, the so called opus marmoratum (marmorino). The last coat, very thin and made of lime and fine marble dust was the opus albarium (lime stucco). This cycle, being very expensive in terms of materials and work, was used only in important structures.
In common and cheap building, normally were adopted plasterworks with less number of coats. In particular were avoided the stucco coats, while was kept the rough coat made in coccio pesto, just for its peculiar characteristic to give more hydraulicity, so more resistance to the plaster.
In the specific case of Venice, coccio pesto has always been used either for rough coats and for finishing coats. As previously said, coccio pesto gives to stucco and mortar a characteristic salmon color, more or less accentuated according to the quantity and the fineness of the clay. This color cannot be obtained with earth pigments, so the use of coccio pesto has contributed in characterizing Venice’s chromatic look.