The sintering of films differs from that of bulk gels in several ways. The initial state of a film is generally denser and less crosslinked than a bulk gel made from the same sol, and these factors enhance the densification rate of the film. The substrate constrains the shrinkage of the film, leading to high stresses that retard densification and can influence phase changes. The substrate is a site for heterogeneous nucleation, and crystallization makes densification more difficult, so the competition between sintering and crystallization is particularly important for films. Fast heating favors densification over crystallization, so rapid thermal annealing usually produces denser films. The high surface to volume ratio of a thin film makes it susceptible to degradation by reaction with the substrate and the atmosphere, so choosing compatible materials and avoiding over-firing is essential.