Abstract: A tray for uniformly mixing fluent material contained within a plurality of vessels, the tray having a restraining plate, restraining the vessels at a first location thereof against movement, and a vessel engaging element arranged to engage the plurality of vessels at a second location thereof and being spaced away from and rotatably movable relative to the restraining plate for shaking all the vessels over a uniform path when the vessels are thereby engaged and restrained. The frequency of shaking is adjustable and its duration can be set for a specific period of time.