Separation enamel is a fun enameling technique that yields interesting, sometimes unpredictable and sometimes wonderful results. I have not done much of it, but I think it's worth exploring. You can read an article about it here: http://www.glass-on-metal.com/pastart/separation.htm .
Here is an example of a bowl I made using separation enamel. Two colors of transparent enamel were sifted over the flux, followed by a layer of two colors of opaque, and then a layer of two colors of transparent again. The separation enamel was painted on in stripes at the edge of the bowl. It was allowed to dry thoroughly while the kiln temperature was increased to 1600 degrees F and then fired for about 3 minutes. You can see that the separation enamel "ate" through all the layers down to the flux