Imperial Savings has made a public offering of $450 million of 8% mortgage-backed bonds, reportedly the largest mortgage-backed bonds ever offered. Last month, Imperial offered $300 million of 8.5%, mortgage-backed bonds, due Aug. 1, 1996. In April, Imperial offered $300 million in 5- and 10-year debt. The two debt financings are "part of the continued effort to lengthen the maturity of (the thrift's) liabilities," according to Chief Financial Officer Kevin Villani. Proceeds will be used for general operating purposes, including funding maturing brokered certificates of deposits.