Abstract: A process for making a honeycomb sandwich panel in a single furnace cycle which is particularly applicable to making a glass honeycomb sandwich panel as might be utilized in the construction of a glass mirror substrate. In one embodiment, lengths of glass tubing are sandwiched between glass faceplates. On heating to softening temperature, the faceplates are compressed between rigid plates until they are sealed against the open tube ends. Then the sealed tubes are inflated, or the external gas pressure decreased, so the tubes expand and seal also against each other. At the same time the softened faceplates are pressed against and conform to the shape of the rigid restraining plates. One way to achieve inflation of the tubes is with a gas manifold communicating through a hole in the back faceplate and into each tube. A pressure differential is maintained while the glass is cooled until it becomes rigid.