The cloister ( Chiostro piccolo ) of the famous Carthusian Monastery located at 7 kilometers from Pavia (Lombardy) was built between 1460 and 1470, presumably after designs by G. Solari.It includes 50 arches decorated by terracotta ornaments by R. De'Staulis, G. A. Amadeo, F. Solari and C. Mantegazza. Unaltered manufactures and weathering products have been investigated by x-ray powder diffraction, scanning electron microscopy, and under the polarizing microscope; pore-size distribution and behavior after water absorption/evaporation were also measured. The manufactured and fired bodies show uniform granulometry sorted by previous sieving of the used raw materials. Presence or absence of illite and/or cristobalite is a suitable criterion for firing temperature: 700° <ds> 750°C and 800° + 850°C have been valued for terracotta of N-E and S-W cloister sectors, respectively. The fabric is becoming looser and more porous depending upon the degree of decay and resulting in a loose sandy material and scabbing. Gypsum of foreign origin is sometimes accompanied by traces of other salts. The degree of decay correlates to micro-climatic conditions, i.e., exposure. It is lower for those manufactured items fired at higher temperature. Other factors to be considered are previous replacements and/or preservation treatments.