The Ostermans offer a demonstration. France cuts and polishes a glass plate; she pours the collodion – a cold, viscous, amber liquid – onto it and rocks it back and forth until the entire thing is covered. In the dark, she dips it in silver nitrate, loads the glass (still wet) into the back of a 5inx4in camera and sends Mark outside to make a four-second exposure – a portrait of a V&A curator who is standing by. He returns and as the image appears in the developing tray, it begins to lift away from the glass. “Oh no, it’s peeling!” France says, as she tries to make it stay. She washes it, fixes it, washes it again and the portrait floats off entirely, a ghost in the water. It’s only three microns thick – thinner than the wrapping on a packet of cigarettes – and this so-called accident has us mesmerised. Eventually, she catches it and tenderly strokes it back onto the glass. “Collodion is kind of like skin,” she says, “you can smooth it but you can scratch it, too.”