Precious metals are use in fuming a glass piece, but how does one fume a recognizable design on to their glass work?

This is a really crazy process, not fully known to the world.  This is a trade secret.  But, what we think is it is form of carbon tech.  Where the blower somehow puts a graphic on the glass after they get a lot of carbon on the glass.  Then they fume it.  The fume only sticks where the carbon is not.  This can give a very clean image.  But we do not fully know the process