At first glimpse, when curators set up an exhibition on the Old masters, they give to their audience an occasion to go back in time. Mediators are supposed to put the curators' knowledge and understanding of the artists and their time within reach of the public. However, these actors rely on a plurality of representations and practices that fit together only because they are not completely explicit nor closely negotiated. This paper starts from an exhibition on Fouquet, a painter and illuminator from the XVth century in order to consider an exhibition, and time itself, as " boundary objects ", around which curators, mediators and audience gather. The exhibition illustrates a specific conception and a professional practice of the history of art, that are both all about piecing together a past we don't know anymore. Curators are preoccupied with rebuilding the temporalities of the works of art ; but that's not necessarily what the audience is concerned with, nor what the mediator should be working on. Indeed, they might concentrate on bringing back to life a story that revolves " around " the work, for example the history of lifestyles.