Making a texture transparent with GL_MODULATE

I have the following setup that currently works fine:

glTexImage2D with GL_LUMINANCE (greyscale)
Set primitive material to metalic values

on draw:
glShadeModel ( GL_SMOOTH ) ;
glEnable ( GL_LIGHTING ) ;

  glActiveTexture ( GL_TEXTURE0 ) ;
  glBindTexture ( GL_TEXTURE_2D, textureId ) ;
  glEnable ( GL_TEXTURE_2D ) ;


I get a nice shinny metallic surface with a texture on it ranging from transparent to black. The black regions still have reflexions, which is exactly what I want.

Now, I’d like to make that texture “more transparent” with user input. ie: I would like to be able to change the texture from its default look to all transparent.

Does anyone have some pointers? I hope I dont have do actually modify the texture memory!!