Exposure to light hardened the bitumen and made it insoluble, so that when it was subsequently rinsed with a solvent only the sufficiently light-struck areas remained.
ɛkspˈoʊʒɚ tə lˈaɪt hˈɑːɹdənd ðə bɪtˈuːmɛn ænd mˌeɪd ɪɾ ɪnsˈɑːljuːbəl, sˌoʊ ðæt wɛn ɪt wʌz sˈʌbsᵻkwəntli ɹˈɪnst wɪð ɐ sˈɑːlvənt ˈoʊnli ðə səfˈɪʃəntli lˈaɪtstɹˈʌk ˈɛɹiəz ɹᵻmˈeɪnd.