Combining leadfree and lead-bearing enamels in one piece

If you are used to working with lead bearing enamels you may wonder if you can combine your stock of lead bearing with the lead free enamels. The following passage from the Thompson Enamel Workbook may answer your questions.

Many people have inquired as to how lead bearing and lead free enamel can be used on the same piece. There are many possible ways to do this. Most enamellers prefer to fire lead bearing enamel on top of lead free enamel. Lead free transparent clears 2007, 2009, 2010, 2020, 2030 and 2040 or lead free opaque whites 1010, 1020, 1030, 1040 and 1045 can be used as base coats under lead bearing transparents or opaques. Transparent colours such as 2110 Ivory Beige, 2325 Gem, 2520 Aqua, 2915 Oil Grey or 2680 Prussian Blue are among several lead free enamels which work well when fired directly over copper. These can be used individually or in combination as a background colour and then lead bearing opaques or transparents can be used on top.

Another way they can be used together is to wetpack or stencil them side by side. In the cloisonné technique, individual cells can be packed with one or the other, in the same piece. Again, if lead free is applied underneath, lead bearing enamel can be fired on top (after firing the lead free) within the same cloison.
Certainly, lead bearing can be applied to one side of a form and lead free on the other side, if the expansions are close.

Now, to break the rule implied above, we know of several enamellists who intentionally fire lead free on top of lead bearing. This results in a textural surface which may add interest to a composition. A word of caution: if small amounts of lead bearing enamels contaminate unfired lead free enamel, dimples in the fired surface will result, and vice versa.