Principal and complementary space-charge gratings are formed in photorefractive bismuth silicon oxide with use of 785-nm light. An electric field is optionally applied in the direction of the grating for hologram evolution by either drift or diffusion of charge carriers. For write times on the order of the decay time of the principal grating, no complementary behavior is observed. For much longer write times, a complementary space-charge grating is introduced both in the presence and in the absence of the applied field, and is initially hidden due to screening by the principal grating. Uniform illumination in the presence of the field reveals the complementary grating. Time-resolved data show its growth and decay, with a response rate much lower than that of the principal grating. A two-level electron-hole transport model explains the observations; one level participates in the establishment of the principal grating by majority carriers, and the other in the establishment of the extended-lifetime complementary grating by minority carriers. A scheme for multiplexing normal and extended-lifetime complementary gratings is presented. © 1990 The American Physical Society.