The bar is clamped in a metal vise, so the vise is very slightly heated and that side of the bar is much cooler.
Taken using the Topdon Android attachment.
Haven't used sharpie and masking tape before, but I've used kapton at relatively high temps (500F). Contact pain for metal typically starts around 113F (45C), but I was referring to right where you were cutting, not where you were holding.