The copyright would only apply to wording of the effect. You can rewrite and publish the method of any effect without infringing copyright as long as the words were your own - youd be a social pariah in the magic community if you did though.
You could check to see if the original effect was patented (try
here) although it is unlikely since anyone can look up a patent (a fee applies I think) and so anyone could look up the patent and see exactly how the gimmick is made - which isnt too good for magic props.
You should make double sure that the effects isnt marketed under a different name by a different company (even if they did what you intend to do) if they got there first you would be accused of ripping them off and still end up a social pariah.
There is probably no harm in saying what the effect is since people are unlikey to attempt to have it made or market it themselves. And if you enquire on several forums then it will become known within the community that you intend to put this together, anyone else jumping in would be seen as ripping you off.
Good luck though whatever you decide.