my name is Renato, I am a character design, game designer and owner of a small Brazillian company called Nuclear Scorpions.
I see a lot of people with some problems when creating metals. Metals are very tricky some times, but when you get it it becomes one of the coolest and most pleasure stuffs to create.
With PBR shaders on the market we can get some really cool good looking metals around there, and here I am giving some tips, inside you can double click on the numbers and see some infos. Here is a small demo scene I uploaded at Sketchfab:
Pure metals has no diffuse on it, all it colors comes from the reflection, this is how our physical world works, and it is going to be exactly how you can give a cool look on a PBR shader.
Each render or engine may work a little different on the sliders, but the results are always be the same.
Diffuse color - The diffuse color is basically the color over the surface of the model, there is not much secret on it.
Metalness - Metalness is the trick here, how does it works? It simply creates a specularity based on the diffuse color, so if you put it on the highest value and have a black diffuse, it will not work. If you put a cool gold color on the diffuse, it will generate a golden specularity on it. That is the secret while using Metalness. At the time that I didn't know how to use it, I randomly picked some number until reach some cool results.
Specular color- In the PBR, specularity is the reflection on the surface. It is going to be blurred and affected by the Roughness/Glossness
In a non-physical shader, reflection and that specular shining ball on the surface are separated. A blinn for example.
Roughness/Glossness - This is the magic of metals, if you make a cool roughness/glossness map you will have a nice metal. This basically creates a disturbed surface that simulate the light/reflections hitting the surface and bouncing in a irregular way (making that blurry).
The difference between Roughness and Glossness are the values, they are inverted.
On Roughness the black areas are the most shinning, on glossness, the white parts are.
You just have to know which one the program you work on uses. Unity 3D, for an example, uses glossness, while Unreal 4 uses roughness.
If you need a gloss on a roughness engine, you just open the texture and invert the colors, simple as that.
Hope you enjoyed it and that it helps you all.
Best regards everybody