The Taiwanese artist claims his chalk drawing is the largest in the world – but that's hard to verify. According to the Observer's art critic Laura Cumming: "I could see no sign of Chih-Sheng’s immense drawing in the central gallery until I ran a finger beneath a balustrade and found the chalk line transferred to me" Photograph: Tony Kyriacou/Rex Features
The Hayward show, Cumming writes, "is strictly concerned with non-visibility and what was known in the 60s as the dematerialisation of the art object" Photograph: Tony Kyriacou/Rex Features
The Italian provocateur's police report concerning the theft of an invisible artwork from his car Photograph: Hayward Gallery
The American conceptual sculptor hired a professional witch to cast a curse on an 11-inch sphere resting 11 inches over the seemingly blank pedestal. Photograph: Tony Kyriacou/Rex Features
"Bruno Jakob’s works are a challenge to cynics, made as they are with not much more than canvas or paper exposed to the elements. There are no images but each bears faint traces of its making that inspire unexpected landscapes in the imagination"
Photograph: Tony Kyriacou/Rex Features
Bruno Jakob photographed holding up one of his blank canvases to a horse Photograph: Hayward Gallery
Visitors to Hein's installation are given a pair of digital headphones that vibrate whenever they knock into one of the invisible walls of a maze Photograph: Anders Sune/Hayward Gallery
