changed his focus from the readymade to the conceptual system, developing mathematical procedures for mixing colours and chance operations for their placement. The range of the colors he employed was determined by a mathematical system for mixing the primary colors in graduated amounts. Each color was then randomly ordered to create the resultant composition and form of the painting. Richter's second series of Color Charts was begun in 1971 and consisted of only five paintings. In the final series of Color Charts which preoccupied Richter throughout 1973 and 1974, additional elements to this permutational system of color production were added in the form of mixes of a light grey, a dark gray and later, a green. Richter's 4900 Colours from 2007 consisted of bright monochrome squares that have been randomly arranged in a grid pattern to create stunning fields of kaleidoscopic color. It was produced at the same time he developed his design for the south transept window of Cologne Cathedral. 4900 Colours consists of 196 panels in 25 colors that can be reassembled in 11 variations – from a single expansive surface to multiple small-format fields. Richter developed Version II – 49 paintings, each of which measures 97 by 97 centimeters – especially for the Serpentine Gallery. Sculpture Richter began to use glass in his work in 1967, when he made Four Panes of Glass. These plain sheets of glass could tilt away from the poles on which they were mounted at an angle that changed from one installation to the next. In 1970, he and Blinky Palermo jointly submitted designs for the sports facilities for the 1972 Olympic Games in Munich. For the front of the arena, they proposed an array of glass windows in twenty-seven different colors; each color would appear fifty times, with the distribution determined randomly. In 1981, for a two-person show with Georg Baselitz in Düsseldorf, Richter produced the first of the monumental transparent mirrors that appear intermittently thereafter in his oeuvre; the mirrors are significantly larger than Richter's paintings and feature adjustable steel mounts. For pieces such as Mirror Painting (Grey, 735-2) (1991), the mirrors were coloured grey by the pigment attached to the back of the glass. Arranged in two rooms, Richter presented an ensemble of paintings and colored mirrors in a special pavilion designed in collaboration with architect Paul Robbrecht at Documenta 9 in Kassel in 1992. In 2002, for the