This paper presents the work done as part of a master dissertation on Conservation and Restoration at the Faculty of Sciences and Technology, Nova University of Lisbon. The "Cofre n.º 31" is a French fifteenth century Book of Hours from the Library of the National Palace of Mafra, Portugal. This prayer book, composed by illuminations and devotional texts, was in bad condition due mainly to the rebinding done between eighteenth and nineteenth centuries. According to the assessment of the state of conservation, the main damages observed in the text block and binding were: volume distortion, loose and deformed gatherings, distortion and cockling of folios, and damages in the illuminations. The pigments palette was identified by µ-EDXRF, FORS, µ-Raman and µ-FTIR, which is consistent with the materials used in the fifteenth century. Nevertheless, some particular application techniques such as mixtures and overlays were found, namely in the red colours and specifically in the metallic leaves. Some of these colours also show specific degradation: darkening of the mixture of minium and vermillion; silver tarnishing with strong migration into the parchment support, especially when associated with minium; detachment of azurite, lead white and its mixtures, such as flesh tones and light colours. The main deterioration processes associated to silver and pigments can be assigned to their reactions with humidity and air pollutants, to the grain size of the pigments and to mechanical factors related to the book binding and its use. The presence of different pigments application techniques in different groups of folios lead us to conclude for the existence of a pigments palette with variations, and finally, three different illuminators and two different techniques, confirming the proposals of art history studies.