Sample preparation is the process of altering a naturally occurring object into a representative form that is amenable to scientific inquiry. Preparation is an important preliminary to data collection, ubiquitous in the life sciences and elsewhere, yet relatively neglected in historical and philosophical literature. This paper presents a detailed historical case study involving the preparation and study of blood crystals in the nineteenth century. The case is used to highlight significant features of preparation, which aid our understanding of the epistemology (...) of sciences in which preparations play an important role. First, it shows the role of technical knowledge in efforts to characterize a scientific phenomenon or object of interest. Especially in early stages of characterization, scientists improve their understanding of what they are preparing by better understanding their preparation procedures. Second, this case contributes to recent views of characterization as a relatively autonomous domain of scientific activity. It shows how preparation functions as a site for integrating different experimental methods, and the difficulties that ensue. In light of these considerations, the case shows how characterization is capable of shaping or constraining explanatory pursuits as much as it is guided by them. (shrink)