There's an artwork called Accumulation #1 by Alexander Opper, made in 2010, the year the Johannesburg Art Gallery's foundation collection was inaugurated. It's an abstract "drawing" made from a century of accumulated dust he hoovered from the cornices of three of JAG's exhibition halls. In his commentary on the work, Opper notes that the dust constituted an invisible "lived 'archive' of the park and the city surrounding the museum".It stands in counterpoint to the other kind of accumulation the gallery is more famous for: its art.Although, as Opper wryly pointed out at the time, sometimes the art collection seems almost as invisible as the dust.Since the collection's centenary, JAG's chief curator and head, Antoinette Murdoch, has dedicated significant energy to securing funds from the council to repair the century-old building. Last year, she secured R15-million. The first phase of repairs is almost complete, and she says more are likely to follow.The establishment of the gallery was largely driven by Florence Phillips, wife of Randlord Lionel Phillips. Its intention was to bring some culture to the crassly commercial gold-rush town, and its focus was unashamedly Eurocentic.Both that intention and the contents of the gallery have changed over the past century, which is celebrated with the launch tonight of six exhibitions and a beautiful book, Constructure: 100 Years of the JAG Building and its Evolution of Space and Meaning, which focuses on the history of the building itself, designed by Edwin Lutyens.Few Johannesburgers are aware of the artistic gems that lie in the heart of the city. The centenary exhibitions aim to rectify that.Murdoch's own chronological exhibition runs through the gallery's collection. Some of the pre-Raphaelite works from the foundation collection make an appearance, and so does an exhibition of African objects from JAG's collection of Southern African art, widely regarded as the best in the world.The smallest exhibition, entitled Encore, includes the controversial Picasso, and works by Claude Monet, Gerard Sekoto and Penny Siopis.The celebrations herald an opportunity not only for Joburgers to rediscover JAG's value, but, as Murdoch says, for the gallery to "really analyse and understand new audiences", from the nursery school across the park to international galleries.When the dust from the renovations settles, there will be little doubt that, should another artist in 100 years' time hoover it up, it will testify to a strong start to JAG's second century.JAG's centenary exhibitions run until March. More information from Tiny Malefane on tinym@joburg.org.za, 011-725-3130/80/52..