This is fascinating work! Small comment: you mentioned that the practical lens concentration limit is about 50% of the intensity at the surface of the sun and that we likely achieved that 1000 years ago. But then you say that magnesium combustion reaches 3370 Kelvin. Since the sun is about 5800 Kelvin and the total radiation goes with the fourth power of the absolute temperature, that would mean magnesium would only be 1⁄9 the intensity of the sun. So that would mean that magnesium combustion would not have surpassed a good magnifying glass.
This is fascinating work! Small comment: you mentioned that the practical lens concentration limit is about 50% of the intensity at the surface of the sun and that we likely achieved that 1000 years ago. But then you say that magnesium combustion reaches 3370 Kelvin. Since the sun is about 5800 Kelvin and the total radiation goes with the fourth power of the absolute temperature, that would mean magnesium would only be 1⁄9 the intensity of the sun. So that would mean that magnesium combustion would not have surpassed a good magnifying glass.