We investigate diffraction from a thermally fixed grating in a photorefractive medium with an ion concentration that is large compared with the acceptor density. A numerical solution to the standard band transport model with mobile ions and the coupled-wave equations are used to study steady-state solutions. For the first time to our knowledge, transient responses are investigated during recording at elevated temperatures and during readout at room temperature. We consider the effect of large applied fields, which reduces the electronic screening and reveals the fixed ionic grating, on diffraction efficiency. The combined effects of self-diffraction, beam coupling, electrical revelation, and in some materials increased sensitivity to the applied field can result in a large diffraction efficiency even from a weak fixed grating. Results showing how transient solutions are affected by absorption in the photorefractive medium and by the magnitude of the incident intensity are also provided.