In an alloy containing 2% cobalt, balance copper, it is possible to quench a nonmagnetic solid solution from 1000°C and then produce a ferromagnetic precipitate in a nonmagnetic matrix by aging at several hundred degrees. The magnetic properties of this precipitate can be correlated with the size and shape of the precipitate particles. Considerable information about the precipitation process can be obtained in this way. In these experiments, saturation magnetization, torque, rotational hysteresis, remanence, coercive force for zero magnetization, and coercive force for zero remanence (which behaves in a surprising fashion) have been observed during the course of aging. In particular, the effect of a field during aging in producing magnetic anisotropy (``magnetic annealing'') has been studied and is shown to occur entirely during the growth of the particles, after the precipitation is complete.The precipitation process is viewed as one in which the particles as they grow are first superparamagnetic, then stable single domains, and finally multidomain. This point of view is consistent with all of the varied phenomena observed.