What device had artists used since the late renaissance to project images for reproduction?

The answer is camera obscura. This is an ancestor of photographic camera which means the dark chamber consist of small dark rooms with the light at a single tiny hole. The result would then be an inverted image of the scene on the opposite wall and that had been whitened.

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.