"I thought you knew nada about Tiffany?" Hee hee, well, um...I'm a quick learner?
I have as a result of this discussion (for some reason it captured my interest) spent a few hours looking into the matter, but it's true, I don't have much experience with Tiffany. I do have a fairly strong knowledge of and interest in glass manufacture, though, and that helps.
When talking about a layer of clear, I was specifically talking about Tiffany "paperweight vases," called that because the process is similar to making paperweights. From
http://www.studiosoft.it/antiquetiffany.htm (elsewhere I've read the same thing):
"...Some items decorated in this manner were cased with a layer of clear glass. Such pieces are sometimes called Tiffany paperweight glass. Aquamarine glass, made in much the same way, was embedded with marine decoration, wavy fronds of green with fishes or pebbles, in heavy green glass intended to simulate the sea."
Some of these are favrile, some aren't. Even if on the outside, the iridescence may be subtle enough that it doesn't obscure the design underneath.
One thing I've read is that Tiffany made their iridescent glass differently from others (though I'm sure others have used Tiffany's method by now). They incorporated metal oxides into the glass which colored the glass itself, but also reacted with a fine spray of tin oxide, which when reheated brought out the iridescence. This is opposed to spraying multiple metal oxides on the outside to create the effect. The result of Tiffany's method would be different iridescence depending on the color beneath. (There are mixed descriptions on the 'net, but I found a Google book called
Fakebusters II: Scientific detection of fakery in art and philately that describes the chemical difference in the outer and base layers between Loetz and Tiffany.)
Let's not forget the little blurb saying Tiffany didn't make Favrile paperweights - a statement supported by a Google which revealed only the questionable spotty type we've seen already, and one in a museum store that wasn't even made by Tiffany. Plus a lot of paperweight vases.