We present a class of mixed-reality displays which allow for the 3D exploration of content in public exhibitions, that we call Revealable Volume Displays (RVD). They allow visitors to reveal information placed freely inside or around protected artefacts, visible by all, using their reflection in the panel. We first discuss the implementation of RVDs, providing both projector-based and mobile versions. We then present a design space that describes the interaction possibilities that they offer. Drawing on insights from a field study during a first exhibition, we finally propose and evaluate techniques for facilitating 3D exploration with RVDs.