Understanding the phenomena that govern complex interfacial and directed assemblies is essential for both control and scale-up of particle syntheses. The present work describes an effort to understand, control, and tune the formation of protein–inorganic calcium–phosphate supraparticles that are produced at an oscillating air–water interface created by end-over-end rotation of the synthesis solution. Supraparticles were synthesized under an array of different conditions that varied reagent concentration, the presence of additives, tube size, and rotational speed. Paired with a fluid mechanics model of the end-over-end rotation and dimensional analysis, the sensitivity of the synthesis to physicochemical and mechanical parameters was determined. Surface tension and bubble formation were found to be important criteria for changing the size distribution of supraparticles. Thresholds for the values of the Froude, Iribarren, and rotational Reynolds numbers were identified for narrowing particle size distribution. These results both guide the specific protein–inorganic supraparticle synthesis described here and inform future manipulation and scale-up of other complex interfacial colloidal assemblies.