Vicky, The enamel is not actually shrinking away from the edges of
the copper. Most likely it is burning out, leaving a black edge on
the copper. This must be filed away carefully after each firing.
Also, make the enamel thicker along the edges—just dust on extra
enamel along the edges using your tiny little sifter.
Regarding what you refer to as holes in the top coat revealing the
flux underneath. This is referred to as “breakthrough,” and is caused
by differences in the rates of expansion between the two enamels
I don’t know what brand of enamels you are using. I use Thompson,
and always check the expansion rates when layering enamels. In order
to avoid breakthrough, the enamel with the higher expansion rate goes
on the bottom.
Check the expansion rates of the fluxes you are using, and if you
are planning on using a top coat with a high expansion rate, be sure
to use a flux with an equal, or higher rate.
If you are using Thompson enamels, be sure to get the Thompson
workbook. It is very inexpensive and contains a wealth of information
about enamels. Also, it has a chart with all the expansion rates
listed. Hopes this helps.