Had lots of problems with this - keeping the 'dots' on while firing. Eventually I bought some thicker glass tac, which worked much better. I did lose one or two, but at least the rest stayed on. Link to my Microwave Fusing Blog
This piece had kitchen foil between the black glass base and the top dichroic glass. I think it accentuated the dichroic colours.
before firing - dots on already fired glass piece
after firing: - I overfired this .... the dots 'splatted' and the shape became oval - but I like it