Clipping through stl file of porous material I've an .stl file of a cube of a porous material. The material is basically many overlapping spheres. Since I imported it as an .stl file, it is one object to blender.

I want to have an animation, that clips through the material. Can anyone help?

I tried using outside lamps and camera clip, but then the inside is not illuminated. Also, once a sphere is "cut open" by the clipping, you can see through it. I don't want that. Solidify doesn't do the job, because the spheres have different radi.

I also tried using a mixed shader with emission/diffuse and light path, so the spheres would illumiate each other, but that doesn't work, because the whole strucutre is one object to blender.
You will want to use a boolean modifier.  Start by adding a cube that overlaps with your object.  Select your object, go to the modifiers tab, and add a boolean modifier.  Set the target object to be your cube, and select 'difference' as your operation.  

Before modifier: 



After modifier:

In this screen shot I've set the cube draw type to wire so you can see what the modifier does.  You could simply move the cube to a different layer.