We present the results of laboratory experiments across a sharp finger interface using heat and salt as the properties which contribute to density. Values of the stability ratio Rp as low as 1.2 were achieved so the results approach the range of Rp which is of interest in the ocean. The buoyancy flux ratio, corrected for vertical heat conduction, Rf* was found to depend on Rp, but at Rp = 1.2, Rf* is still only 0.65 ± 0.1. Values of Stern's number A = βFs(l – Rf)/ν(αTz – βSz) were found to depend on the magnitude of the step in salinity across the interface as well as on Rp. Most of our experiments were performed with small contrasts in salinity between the layers and Stern's number was found to increase as this salinity difference decreased. On the basis of our measurements of A, we believe that pure vertical heat conduction will not be significant in the ocean.