Artificial stones utilised in Florence historical palaces between the XIX and XX centuries

Some Florentine historic buildings have ornamental elements (for instance stringcourse, metope, or coats of arms), ashlars, false “bugnato” or the complete façade, realised in artificial stone; this is mainly a characteristic of the period between the XIX and XX centuries, contemporary to the increasing use of the modern hydraulic binders (cements). The artificial stone is a mortar constituted by lime or hydraulic binder mixed with aggregate and water; this mortar can reach the hardness and appearance of a natural stone (for instance sandstone). The artificial stone can be either a mortar “worked” directly in façade or an ornamental element prepared in mould. The main purpose of this work is to characterise from the petrographical, mineralogical and chemical point of view the mixture utilised to produce the artificial stone used in some Florentine palaces (Palazzo Grifoni - Budini-Gattai, Palazzo delle Poste e Telegrafi, Palazzo dello Strozzino, Palazzo dell'Università), with a particular care to the characterisation of the binder used for its realisation. The analytical results show that all the mortars are characterised by a binder with a granular aspect and low birefringence. The aggregate is made by a silicatic sand and the binder/aggregate ratio is between 1/1 and 1/3. In some samples the presence of larnite, a calcium silicate often presents in the hydraulic mortars, and brownmillerite was surveyed. The thermal analysis of the binders allows to classify the four materials as hydraulic mortars. Mineralogical and petrographical differences were evidenced between the mortars laid in mould and those laid directly in the façades. The results obtained allow a first characterisation of the artificial stones utilised in Florence between the XIX and XX centuries.