the blackening will depend on how you hold it over the hear source.
The method comes from a 1/35 armour modeller "guru" called Adam Wilder, and American who now works with Mig Jimenez in Spain. There's a magazine from the UK called AFV Modeller, one of the best out there, where the article came from. I'll check tonight for any further comments about it.
AFV Modeller are smart by selling articles for £1 from their online shop in PDF format......