I'll preface this with the fact that I know nearly nada about Tiffany.
Hello Kristi,
Please do not take this the wrong way, it is being said in jest.
I thought you knew nada about Tiffany?
Well, in regards to the iridescence.
I am very surprised you think once the iridescent surface is crated that they would put a clear coat on top.
That is the purpose of that treated surface, to be able to create a rainbow of colors once the light hits the objects.
Putting a layer of clear defeats that purpose of it. At least in my experience with Italian glass, pieces with Iridescent surfaces do not have a clear coat over them.
And from what I have seen personally, Tiffany pieces would not be coated with an extra layer of clear.