Buy a small specialist glass drill, drill the hole at each end of the crack to pierce the first and second layer. Then call up your local car windscreen repair place, and ask them to pump their special clear glue into it. you will be left with two tiny marks where the holes were, and a crack you cant easily see. If its a long crack you may need more than 2 holes, confer with the company.
the-checkout-tech.com, December 2011