Simultaneous temperature measurements from both sides of the sample during laser heating on side 1 with laser 1 (a) and on side 2 with laser 2 (b) from 63 to 180 GPa. Temperatures measured from side 2 (i.e. upstream of the X-rays, see Fig. 1) are reported on the vertical axis, and temperatures measured from side 1 are reported on the horizontal axis. Circles indicate temperature measurements obtained during laser heating from side 1 only (a) and diamonds represent the temperature measured on both sides of the samples while laser heating from side 2 only (b). The dark thick line is representative of an absence of gradient in the sample, i.e. equal temperature from both sides of the sample. The dashed line refers to isotherms, shifted from the thick line.