To create a dirty glass material we will need to use two layers. The first one is the glass layer and on top of that we will be adding a basic layer for the dirt. To blend those two layers together we will be using a surface imperfection map.

Glass layer

• Create a new material and change the default layer (basic) to glass
• Load the surface imperfections map to the roughness channel and increase the roughness value

Dirt layer

• Click on the plus icon to add a new layer on top
• Change the diffuse color to near white (220,220,220) to simulate the color of dust
• Click on layer weight and add the same texture
• This will be used to mix the two layers together
• Adjust the gamma / contrast / brightness values to fine-tune the weight map
• If the look of the dirt layer is too strong, reduce the layer’s weight percentage to your liking

Download Dirty Glass Material
Credits: Using textures from CG Bookcase