The brass serves to conduct and spread the heat in a uniform way, you can spoon or move the brass around on the part to heat one part more than the other to obtain an even color. Clean sand also works. The emphasis is on clean. Any oil or grease will give color variations. A very fine sand paper finish(2500) grit gives a better blue than a mop polish. The type of steel also affects the color Chrome or nickel does not help things and stainless steel is usually hopeless. It is simple to try and if you are not happy with the results it is not to difficult the remove the blue and try again. Every time this happens the polish also improves! The trick is to keep every thing clean and heat slowly. The atmosphere seems to have some effect on the color as well. The old folk always said to use a charcoal fire for the best results. A hot air gun also works as a heat source but I have not found the results to be quite a good as a flame.
An electric muffle also works, you can take the temperature up a few degrees at a time to get the color you like, especially when the part has thick and thin sections.
Last edited by Buchanan; 05-18-2016 at 09:58 PM.