A method and apparatus for assembling fiber optic devices in multi-fiber layers to avoid the problems of misalignment and interstices. The furnace apparatus includes a mold for receiving the fiber devices and a packing piston for placing each layer of fiber devices in position. Heating is maintained in the furnace at a level so that heat penetrates from the fibers already packed into the new layer sufficiently to enable the new layer to adhere, but no more, so that the piston may be withdrawn before it adheres to the new layer.