Since the mid-1990s Cynthia Hahn has been one of the principal voices in the study of relics and their containers (known as reliquaries) and the questions that relic cults – a central aspect of medieval devotion – raise for both art historians and historians of religion. Major exhibits, preeminent among them ‘Treasures of Heaven: Saints, Relics and Devotion in Medieval Europe’, mounted in Cleveland, Baltimore and London in 2010, and recent scholarship on the saints, most importantly Robert Bartlett's Why can the dead do such great things? (Princeton 2013), have made the role in European history of holy people and the holy places associated with them a topic of increasing urgency. It is therefore hard to understand why Cynthia Hahn's intelligent and important book, Strange beauty, which sums up and carries forward her work of the last two decades, has been neglected in the review sections of journals of general medieval and ecclesiastical history.

Hahn's book is an extremely useful survey of reliquaries, organised according to type or genre. Hence it provides a partial update of Joseph Braun's classic Die Reliquiare des christlichen Kultes und ihre Entwicklung of 1940. But Strange beauty is much more. It raises questions that every medievalist and every art historian of the period between 300 and 1700 needs to consider – questions not only about the nature of relics but also about how bodies, objects and matter itself become holy; about how the form of an image structures devotional response; and about the fundamental issue of the nature of representation. Hahn's study has implications for art historians and historians working well beyond her cut-off date of 1204.

Organised chronologically and according to type, Hahn's book moves from early Christian reliquaries, such as purse-shaped or casket-shaped containers, which often worked to obscure the nature of their contents, to the thirteenth century, when what she correctly calls ‘shaped reliquaries’ (in contradistinction to the German term ‘redende Reliquiare’ or ‘speaking reliquaries’) became more common but by no means universal. She underlines the newer approach to relics, pioneered by Julia Smith and others, that rejects older definitions of relics as body parts and understands them instead as bits of holy matter that have been in contact not only with the tombs and bodies of the saints but with holy places – such as the Holy Sepulchre or the Field of Blood (Akeldama) in Jerusalem – as well. In line with the new art historical interest in materiality, she considers not only the form but also the materials of reliquaries. See, for example, her discussion (p. 46) of why objects that image resurrection are made of ivory, a substance that reflects as well as contradicts the nature of flesh and thus explores what it would mean for body to endure for all eternity. In a chapter on reliquaries in action, she discusses processions, including some modern Italian and French examples, and sees reliquaries not as stage props but as themselves performative (see p. 147). In such discussion, she provides much evidence that should be taken into account in recent art historical theorising (such as that of Horst Bredekamp and Glenn Peers) about the medieval life of things.

In dealing with the complicated question of the relationship between inner and outer, contents and container, Hahn employs the concept of metaphor, citing some recent literary and even post-colonial theory. This is an important interpretive move in that it takes us away from tendencies either simply to classify by type or to over-narrativise medieval objects. It is not clear, however, even to Hahn herself, that the idea of metaphor quite does the work that she wants it to do. In fact, she moves beyond it to talk of ‘somatic charge’ and of some reliquaries as ‘machines’ (p. 243). Moreover, Hahn sometimes speaks – as other historians such as Jean-Claude Schmitt have recently done – as if the reliquary is or creates the relic. Again this does not quite seem to work as an interpretive move. Medieval theologians and the ordinary devout did consider aspects other than the container – for example, provenance or location – to authenticate, and hence to create, the relic. Indeed one could argue that in some cases the relic dictates the container, even where it does so by reversal (for example, by demanding crystalline material to frame fragments of bone, dirt or flesh). If we look at the box of stones from the Holy Land, now one of the important small relic collections at the Vatican, it seems as if there is a reciprocal (not a one-way) relationship between the images in the lid and the stones. But Hahn herself would not disagree.

The fact that these theoretical questions are raised, if not answered, is what makes Strange beauty so important for historians of religion as well as art historians. Hahn is theorising the nature of representation itself. For if representation means to stand in for (as a diplomat does for a country) as well as to look like (as a portrait does for a person), saying how an object does this and what authorises it to do so is a central question for religion as well as for art. It is a question that has not yet been very well answered. Medievalists who pride themselves on their interdisciplinarity, but have tended unaccountably to neglect the profound interpretive questions raised by objects, would do well to give Cynthia Hahn's Strange beauty a major place on their graduate syllabi and in their own investigations.