Final result preview
Make sure you are using Vray as your render engine. Once you learn the technique, you can do it in other render engines as well. Open Material editor. apply VRayMtl to one of the free sample slots. Name it as you wish. I chose Aged Metal.
Change the Diffuse Color to black. The reflection of the color depends to the type of the metal that we are creating. If you want an old copper color, then choose a brownish color, or if you want a silver metal then light gray could be a good choice. Because it is an old metal material, we need to decrease its shininess by reducing Refl glossiness to 0.7. Make sureFresnel reflections is turn on, and change Fresnel IOR to 15. Note that if it was a polished metal, we could use 20 in Fresnel IOR.
One of the characteristics of an aged metal is that the inner parts are darker than the outer parts. We will use VRayDirtmap to create this effect. Setting up the right VRayDirt is little tricky and sometimes needs to take several render test to find the best values. Here I am going to use another way to find the best parameters for my model. Instead of applying VRayDirt to reflection color, I am going to apply it to Diffuse map. set it up and at end, copy this map on reflection map slot. So temporarily change Reflect color to black. Canceling reflection will help to have faster render test. Apply VRayDirt map to Diffuse map slot.
In VRayDirt Parameters, Change occluded color to red and unoccluded color to green. Make a render test. Now you can clearly see the effect of VRayDirt map.
Reduce radius to 1. Render again. Now red color is mostly in inner parts. You can try different values according to your model.
Change occluded color to black and unoccluded color to your metal color. In my case I change to copper color. Increase subdivs to 16 to have better quality.
Let’s go back to VRayMtl and replace VRayDirt from Diffuse slot to Reflect slot.
There is one more thing that you can do to increase darkness in inner areas. Right click on the Reflect map slot and choose Copy. We are going to copy this map inside VRayDirt map itself.
Go to VRayDirt map parameters. At the bottom, find unoccluded color map slot. Right click and choose Paste. Render again.
This is nested VRayDirt map. You can continue to get darker effect. In the below photo, I compared a 2times nested VRayDirt with a 4 times nested VRayDirt map.