Damascene inlay, a technique that dates to ancient China and Egypt and also practiced in Renaissance Europe involves cutting grooves that are wider as you cut deeper into the iron.

I suspect the Insular Celtic metalsmiths also used the technique but I'm not likely to get permission to take apart a period piece :-)

Technique looks kinda like this

/___\

You then take a softer metal such as gold, copper, silver in round wire form and lay in the groove like this:

/O\ The wire is actually taller than the groove is deep

Then you use a hammer to gently flatten the wire so the softer metal "flows" into the undercuts and is held in place.

A good reference is Oppi Untracht's Metal Techniques for Craftsmen


Mike