I am trying to make a transparent bottle with a transparent label (i.e. I
want only the text of the label to be visible). When I trace the label
without the bottle everything is OK, only the text is visible. However,
when I trace this with bottle there is a distinct shadow on the surface of
the glass where the transparent part of the label begin. How to get rid of
this artefact?

The label is produced by the following code:
#declare label =
   difference{
    cylinder { <0,0,0>, <0,60,0>, 31.855
       scale 0.1
             }
     cylinder{ <0,0,0>,<0,60,0>, 31.853
         scale 0.1
     }
   pigment { image_map { gif "label.gif"
                    map_type 2
                    transmit 255, 1.0
                    transmit 0, 0.01
                    //once
                    interpolate 2
                    }
      scale 20*y
      rotate 84*y
       translate -8.5*y
   finish{
           reflection 0
           diffuse 0
           ambient 0
             }
}

The gif image is black text over the white background.

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