Oxidizing Using Gun Blue

The product is particularly useful when I have a piece of metal that is made of both sterling silver and copper as shown in the initial photo. I used to think I could just oxidize the copper and then solder on the silver pieces; however, as you likely well know, once the torch hits the metals the oxidation changes. Now, for these mixed metal pieces I use Perma Blue since it doesn't appear to affect the sterling silver, but does oxidize the copper. Although it's a bit difficult to see in a photos, following is a comparison first of copper and sterling silver scraps oxidized with liver of sulphur. You may note that both metals changed. The second photo is of similar scraps oxidized with Perma Blue. I see no change in the sterling silver scrap.