Last night I sat down in the lounge with my mirror and declared war on the shape. It was beautifully spherical, and too shallow, and had stubbornly refused to deepen measurably. So I sprinkled some #120 grit onto the tool, sprayed it with a mixture of water, dishwashing detergent and glycerin, put the mirror on top, and began grinding with new savagery: As much pressure as my arms could sustain, 1/2 length strokes in a wide W pattern. After about half an hour of this, the mirror began suctioning itself to the tool at the end of each wet – a clear sign that it was no longer mating cleanly with the tool. The centre of the curve had deepened, so that as the mirror passed over the centre of the tool, the gap between mirror and tool would expand. And since there was a nice thin slurry of ground glass, broken down abrasive and soapy water sealing the edges, it made a nice effective suction cup. In fact, the vacuum was strong enough to support the full weight of the tool – a fact which I demonstrated by lifting the mirror about a centimeter off the workspace, and observing that the tool went up with it!
Now it’s not normally good thing for the mirror to stick to the tool, but I felt it was cause for celebration – my longer and more aggressive strokes were working to deepen the curve. I kept it up for another hour before cleaning up the workspace, propping the mirror up on its stand (I have an area of the garage set aside with a tape measure permanently out on the floor, all ready for the spit test) and testing the radius of curvature. Result: 210 cm.
So in only 90 minutes of ugly grinding, I managed to drag the project back on track. Brilliant! Now I just have to restore the bevel (This rough work wore it down pretty fast) and then revert to my usual clean 1/3 stroke to restore the spherical shape. Then I can move on to the next finest grit. Finally!