Using a surface force balance, we have examined the orientation of a nematic liquid crystal (6CB) confined between mica surfaces. By simultaneous measuring of the force and the refractive index profile, we were able to distinguish between planar, planar twisted, and homeotropic orientation of the nematic. When the mica was exposed to air for short periods (< 1 h) prior to introducing the 6CB, either planar or twisted planar orientations resulted, depending on the relative orientation of the mica axes. For longer exposure periods (greater than or equal to 3-4 h) prior to introducing the nematic, a homeotropic orientation was observed, presumably because the mica surfaces became sufficiently hydrophobized. Slow transitions (over a period of days) from a planar twisted to a planar orientation and from a homeotropic to a planar orientation could be observed in certain conditions. We propose that these are associated with anchoring transitions on the mica surfaces, resulting from adsorption of water molecules penetrating by diffusion from the ambient atmosphere.