Lumps as key to identify the stone used for lime in historical mortars

Mortars are artificial materials realized by man along the past ages and to be used in architecture with different functions. The study of their composition is therefore not only aimed at the knowledge of the components that constitute them, but also allows us to investigate the material culture that produced them. It is well-known that a mortar is constituted by binder, aggregate, water and possible additives. As for the binder, it is important to evaluate the presence of fragments exhibiting a whitish colour and sometimes an inconsistent aspect, generally named "lumps". The presence of these fragments indicate that lime was realized according to a traditional technology with slaking of the quick lime according to the so-called "melting" method The white pasta (lime putty) is then cast (through a net that holds the bigger underburnt/overburnt lime fragments) to a pit where it is left for the aging process. The study at the optical microscope in transmitted light allows to recognize the nature of such lumps making it possible to recognize the following typologies: lumps of the binder not amalgamated into the mixture; underburnt lime fragments (burning’s relicts of limestone); overburnt lime fragments; overburnt lime fragments hydrated and carbonated after the setting reaction. Such petrographic approach to the study of the "lumps" is a valid tool to identify of the kind of carbonatic stone used to realize the binder. We should also recall that an excess of these fragments indicates a poor production technology. In particular, the type of lumps indicates which stage of the production cycle has been less cured.