Hello,
I like your testure, even if it must be quite slow to render.

I guess the problem comes from max_trace level. In your function:

>   function {(f_sphere(x/4.2+2, y/2.8-2, z/1.6-12, 3.9)*f_bozo(x*6.2,
> y*6.4-32, z*.18+16))+
>   f_granite(x/15-13, y/25-13, z/700-74)}//+f_granite(x/30, y/30, z/30) }


bozo and granite add real bumps on the underlying sphere, so the ray that
are shot near the edge of the cloud may have to go through more than 5
surfaces before hiting the sky sphere and you get a black dot. if you add
the statement:

global_settings {
  max_trace_level 15
}

it should disapear. Note that 15 is an example, and it may adjust to yor
scene and image resolution.

Pascal.

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