In art history, increasing attention has recently been paid to artistic forms of marking, visualising, and reflecting on spatial transition zones and social boundaries within space, or the division of space (e.g., doors and portals, altar screens, curtains, geometric floor designs). The starting point of the joint project are decorated iron grilles, a previously unstudied yet functionally comparable group of objects, which were employed in the 12th century to separate the sanctuary from the lay area in churches.

The grilles frequently feature doors that can be opened and closed by members of the clergy, a privileged group responsible for regulating access to the sanctuary.
However, when laypeople look through the latticework at the altar, the saints, or the biblical scenes on the apse wall, these grilles inevitably obstruct an otherwise unobstructed view. Conversely, when the viewer's gaze is directed towards the lattices and the ornaments attached to them, the elements situated in the background-altar and apse paintings alike-become blurred.

From this high-medieval group of objects, we aim to develop a broader medieval perspective on ornamental membranes as permeable or semi-permeable liminal spaces, and to open the discussion toward further ideas on non-iconic visual, architectural, or metaphorical spaces of passage, pause, or lingering. This raises questions about the ways in which optical progression and the layering of images and objects in time and space affect the viewer's experience. Likewise, very impermeable structures, such as marble altar screens, can exhibit distinct depth features and suggest a non-existent visual access.

In the theology of the Church Fathers, ornament was conceptualised in the spiritual context of divine ineffability. Therefore, we interpret ornamental structures as 'membranes', that is, as media for personal, social, and physical exclusion. At the same time, they also offer the experience of partial participation and can serve as stimulating devices for the individual's reflection on the human knowledge of God.