Final answer:
The primary interior finish of Borromini's San Ivo and San Carlo alle Quattro Fontane was stucco, notable for its ability to be shaped into the complex, fluid designs of the Baroque era.
Step-by-step explanation:
The primary interior finish of Francesco Borromini's San Ivo alla Sapienza and San Carlo alle Quattro Fontane was stucco. Borromini's use of stucco allowed for the creation of intricate designs and fluid forms that contribute to the dynamic Baroque style of architecture. By employing stucco, Borromini could achieve a sense of movement and complexity in his designs, which were further emphasized by the play of light and shadow within the interior space. Stucco was a preferred material in Baroque architecture for its versatility and ability to be molded into elaborate shapes that are characteristic of the period.