How hot can the suns rays get ?
Hot enough to make
Boiling Lava !
I think the images say enough. The rather large 30cm Fresnel lens was scavanged from a discarded overhead projector. The rock is common slate. This rock is fossilized ooze from the floor of a long forgotten ocean . It appears not to contain fossils of anything interesting. It was necessary to view the focused sun spot with arc welding googles. You can notice that alongside the moltern zone, iron oxide in the rock has been baked ,the Fe203 oxide giving it a distinctly red coloration. This slate rock has a lower fusion temperature than say granite, because of its high lime content. This lens cannot fuse quartz or more reflective material. Monumental lithography is a possible use of this idea?
When viewed with welders googles, the molten rock bubbles as entrapped gas is forced out. The rock reaches bright yellow heat at the lens' focus. This experiment was done during a bright day in Melbourne at about local midday.