There are two ways.
Make a mix of 50/50 water and amonia drop the handle in and leave for a few hours then rinse off thoroughly
Or.
This way is dangerous
You will need a blow torch, nitric acid and a dark wax polish, heat proof gloves, goggles and pliers.
Hold the handle with pliers and dip into the nitric acid, heat for about 15 seconds then allow to cool a little and dip into the wax burn off the wax allow to cool and then rub with hessian or rough cloth
Dom