Thin brass sometimes will break after time goes by, due to the amount
of pressure strain put on the eyelet. But the tone transference is
"better". Eyelets can also be purchased which are thicker than others
normally used, to help avoid the cracking.
Plastic is a generic term for so many different materials that it is
difficult to answer. I have used Delrin for a backup for harp strings
made both of nylon and of metal wound strings. Sometimes I've used
them inserted just above the hole the string goes through and then put
a V-groove so the string lies in the groove rather than slip to one
side as the wood gets eaten away over time. Sometimes I have used a
larger than the hole rod of Delrin and drilled a hole through the
center. In both cases I've purchased several sizes of Delrin rod and
adjusted as one would adjust when using different sizes of eyelets.
Some plastics might be too brittle and crack, or be soft, not
enough strength to prevent the string from digging into the plastic.
Some plastics might tend to "deaden" the tone a bit -- or mellow it
out, depending on the harp.
Installation of eyelets is easy, be sure to try a few in a sample piece
of wood of the same material as the string rib or soundboard, so the
hole drilled will not be too small or too large. I try to use an
eyelet deep enough not to pull out easily, but not too deep that the
bottom edge will extend beyond the inner string rib and wear on the
string itself, or push the eyelet out of its hole. The same care needs
to be taken when using a rod of plastic.
There are round "eyelets" of bone, or ivory or plastic which are
musical accessories and can be used. If you've enough time and
creativity you could produce some very nice decorative "shoes" to put
above the hole and act as a protector to the wood (for that is why we
use eyelets in the first place!). I've seen some nice cast metal ones,
and some rough bent staple-like ones.
Best of luck with your decision! Betty Truitt