An improved deposition machine is disclosed having a specially configured array of axially and azimuthally stacked burners, for depositing silica glass particles on an integral tube mandrel. The deposition machine includes a fire-polishing unit for fire-polishing the mandrel in-situ, immediately before the deposition process is performed, without having to cool, store and transfer the mandrel from one machine location to another. After the silica glass particles have been deposited on the mandrel, the preform is sintered. The integral tube mandrel preferably is configured to have a closed lower end, which enables the mandrel to be pressurized during both the fire-polishing step and the sintering step. This minimizes the risk of the mandrel losing its shape and it eliminates the possibility of heat rising through the tube mandrel by a chimney effect during the sintering step.