From the 1680s Ehrenfried Walther von Tschirnhaus (1651-1708) developed intricate optical instruments to focus and employ the power of light. Starting out by improving the construction of burning mirrors and the analysis of caustics, he went on to make the most powerful burning lenses of his day that promised the transmutation of materials - for some. Over the course of his pursuits Tschirnhaus interacted with savants like Huygens and Leibniz, who responded with searching mathematical, physical, and philosophical queries regarding the properties of his optical instruments. What worked practically was not necessarily reasonable. At the same time, he engaged with diverse efforts to put the power of light to practical use in industrial projects of material manufacture. Eventually, his burning lenses gave rise to a heated dispute over the nature of light and its possibilities to produce chemical reactions. This intricate web of practical and philosophical confrontations brought together diverse minds and interests, giving rise to the question what scientific use the material successes of his burning lenses may have. This issue of the proper way of putting science to use not only occupied Leibniz; Tschirnhaus himself too reflected on the nature and value of experimental philosophy.