The department and the Treasurer shall determine the interest rate or rates on the bonds. The rate or rates may be fixed or variable and shall not exceed 12 percent per annum, except that the rate on any bond may, on any day, exceed 12 percent per annum if the interest borne by the issue of which the bond is a part from its date of issuance to, and including, that day does not exceed the total interest which would have been borne by the issue if all bonds of the issue had borne interest during that period at the rate of 12 percent per annum.