You can use any glass in a

[Stephen Richard]

You can use any glass in a microwave kiln.  Some (fusing compatible) glasses will work better than others.  The number of layers is not so important as the area.  The closer you get to the maximum size of the shelf, the more problems you will have with annealing.  There is no real reason why you could not put a small 5-layer piece in and have it come out successfully, if you give sufficient space around it for the expansion that will result.

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/