Artist Mandy Johnston proposes that her "work refers to a specific historical and cultural context". But, you'd be hard-pressed to immediately identify the contexts - they're not always visually apparent in her work. As the title of her latest exhibition In the Presence of Absence at Absa Art Gallery, Joburg, suggests she's aware of the contradiction, her art presents - it' s one that confronts our society.The exhibition deals with violent traumas, such as slavery in the 1600s in South Africa.Comprising monochromatic drawings, prints and sculptures with titles like Disarming Spaces and Absence Can Be Measured, she's fixated with capturing invisible conditions that influence the present but leave no physical trace.Johnston attempts to make these "traces" real through dated photographic methods - using light and paper - and crafting new objects from old ones.She is "inspired by [the historically rich Cape] west coast where traces of mining and early Khoisan culture can still be excavated. I'm interested in why some things are more valued than others."This is explored in the salt print Restraint. Her process involves wetting the paper in a weak solution of salt and dyeing the other side with silver nitrate. When exposed to light the paper darkens, exposing the image. Her method symbolises the idea that she's not recreating images but revealing latent traces.Restraint invokes memories of slavery on the west coast in the mid-1600s. The image depicts a rusted iron neck ring used to shackle African slaves.In other works Johnston is like an archeologist unearthing fossils by manipulating 19th-century photographic processes. She revels in the way "the materials become shadows of themselves and through the artist's intervention carry a secretive history of their origin".The story behind her techniques and the history of the west coast during the 17th century make the exhibition an exciting journey of discovery.At Absa Art Gallery, Joburg, until October 28..