CORNING, N.Y., Nov. 9 -- Corning Inc. said it has started a public offering of $600 million of senior unsecured convertible debentures to be used for general coprorate purposes. The company said the offer is part of its existing $5 billion universal shelf registration statement. The debentures are expected to have a seven-year maturity, due in November 2008 and redeemable at Corning's option after November 2004. The proceeds from the offering will be used for general corporate purposes, including working capital requirements, normal capital spending and the possible funding of future acquisitions, Corning said.