In 4/61 Mauern-Positiv-Negativ und Weg (4/61 Walls-Positive-Negative and Path) Kurt Kren uses slides - photographic planes - which show different surfaces and finishes of a wall, which have been erroded by time. Kren stacks the positives and negatives of these pictures quickly one after the other, so that a moving relief-type structure develops through this combination of "two ...