Pozzolana is ground volcanic ash composed of siliceous and aluminous material from Pozzuoli, Italy.
Pozzolana, also known as pozzolanic ash (pulvis puteolanus in Latin), is a siliceous or siliceous and aluminous material which reacts with calcium hydroxide in the presence of water at room temperature (cf. pozzolanic reaction). In this reaction insoluble calcium silicate hydrate and calcium aluminate hydrate compounds are formed possessing cementitious properties. The term pozzolana is derived from one of the primary deposits of volcanic ash used by the Romans in Italy, at Pozzuoli. Nowadays the definition of pozzolana encompasses any volcanic material (pumice or volcanic ash), predominantly composed of fine volcanic glass, that is used as a pozzolan. Note the difference with the term pozzolan, which exerts no bearing on the specific origin of the material, as opposed to pozzolana, which can only be used for pozzolans of volcanic origin, primarily composed of volcanic glass.
Use pozzolana with lime putty in fresco painting to make hydraulic plaster for stronger fresco walls. Pozzolana reacts with slaked lime and water to form mortar with cementious properties. The fine grind (1-100 microns) of this pozzolana is best used in the scratch and brown coats of fresco. It can also be added to give "tooth" and color to animal collagen and chalk or gesso (gypsum) grounds and in paint.