Overview of Materials in Cycles - Blender Cycles Materials and Textures - page 42

3. Add a Voronoi Texture node (Press Shift + A and navigate to Texture | Voronoi

Texture). Connect the Fac output to the Density input socket of the Volume
Scatter node. Set the Voronoi scale to 3.800 .

4. Add a Math node (Press Shift + A and navigate to Converter | Math) and paste

it in the link between the Voronoi Texture and the Volume Scatter nodes. Set
Operation to Less Than and second Value to 0.100 .

5. Add a second Math node and paste it right after the first node. Set the Operation

to Multiply and second Value to 50.000 . Here is a screenshot of the output of a
Voronoi Texture node for your reference:

The output of a Voronoi Texture node used as Factor for the density of the Volume Scatter node