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Excursus: How to blue screws (according to the British Horological Institute)
- to blue a screw head you first have to clean it up, polish it, and then remove any oily or greasy deposits from the surface.
-holding the screw (by its thread) in a pin vice or lathe; cleaning the slot with a piercing saw or junior hacksaw; removing any burr with a pivot file
-polishing the head on a piece of crocus paper or 3/0-emery, supported on a cork or other flexible sheet.
- When the polish is to your satisfaction, wash it off in meths, alcohol, or other volatile solvent.
- Then, holding it by the threaded end in a pair of old snipe-nose pliers, slowly pass it in and out through a spirit lamp flame. Watch very carefully for the straw colour beginning to appear and continue until there is a nice deep purple. By this time, with a little experience, you would already have the screw withdrawn from the flame.
- As soon as the colour is right *immediately* quench the screw in water, dry it and rinse again in solvent.
Complicated? -- the whole operation would probably be completed in less time than it took to read this paragraph, and it is FUN.
If you have a *whole batch* of screws to get to exactly the same colour, take a piece of thickish brass plate, drill a number of holes clearance size for the screw thread in it, and drop a screw into each hole.
Heat the whole plate, screws and all, over the alcohol lamp, then when the right colour is reached (or just a smidgen before), upend the plate and tip all the screws simultaneously into cold water. I dewater the screws in white spirit, then dry (with a soft tissue, not a hot air gun - this could change the colour), and give a final wipe with thin oil to bring up the blue colour to its gleaming best, and stop any future rusting.