Many of the screws had surface rust. To bring back the original look, the screw has to be completely re-surfaced. This involves removing the rust and bringing back the polished surface through various grades of wet dry sandpaper and then a buffer wheel. It is then carefully cleaned in solvent to remove any polishing compound. The highly polished metal surface is heated with a heat gun. The surface will first turn a straw -yellow color and then progress to a blue and then deep purple blue color in rapid succession. At just the right moment the screw is quenched in oil to stop the color turning process. If the part is over-heated, the deep blue disappears into a light sky-blue. The process must be done seamlessly from start to finish. If the screw is over-heated the color cannot be reversed. The part must be stripped, re-polished and heated again. One can usually get a satisfactory result when re-doing an original part. However, it is important to remember that if you are replacing any parts, for example replacing missing screws, check first to see if that part will blue upon heating. Most newer screws are made with modern, low carbon alloys that will not respond well to this process; they will not turn blue.