Abstract: An apparatus, using rigid surfaces or trays, to convey indexed items through a controlled environment when the items require a specific retention time within the controlled environment and to attain the desired retention time by traversing back and forth within the controlled environment as many times as necessary, within the constraints set by the height of the controlled environment, to achieve said retention time. The apparatus includes mechanisms for lifting or lowering trays from one level to another, at the end of each traverse, through a plurality of levels, which maintain the trays in a horizontal orientation while being lifted or lowered and to accomplish this as a smooth, continuous, and reversible process, unaffected by the heat or cold of the controlled environment.