I use several enamelling techniques. 

In champlevé the metal is etched and the enamel is inlaid.
In cloisonné the design is created with wire and built up on the metal
In grisaille the first layer is black and subsequent layers of white are built up to create highlights.
In sgraffito an unfired layer is scratched through to reveal what is underneath, and then fired to fix the design. 

There are many variables in enamel, such as stress factors between the hardness of the glass and the softness of the metal. There are stress factors in layering enamels which all have different coefficients of expansion, and can often crack or otherwise refuse to bond. 

Of course, the variables are also what makes it exciting!
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