The Beam Line 12-1 mirror is made of single-crystal silicon coated with rhodium. A cooling rail with brazed-in copper tubes is connected to the mirror via silicon pads and liquid gallium-indium. This system extracts heat from the mirror when it is illuminated by powerful X-rays. The liquid metal mediates heat transfer to the silicon pads while minimizing mechanical deformations from physical contact with the cooling rails. (SLAC National Accelerator Laboratory)