Re: Peacock Herl vs Peacock Black Ice dub
Old peacock herl or herl that has been exposed to too much light or a very dry environment tends to get brittle. You can "test" the viability of the strands prior to tying by pulling on them. Generally, it's the area near the tips that is most fragile.
I know someone who used to make a bath of glycerine and water and soak bundles of herl in it before any large tying sessions, but I've never done that... if I found it too brittle to tie with, I moved on to another piece. You can save the brittle stuff to use as winging on small streamers, or tails on zug bugs or whatever.
The suggestions about making a rope are good, but that tends to mat the herl down and it loses some of it's lustre. I do it on large (size 6 and up) princes, but that's about all. You can counterwrap/rib the herl with 14/0 black thread or clear thread- that allows it to remain 'bushy' but strengthens it.
I learned that herl actually has a "direction"- if you look closely when you wrap it, the fibers stick out in a specific direction from the 'stem' and wrapped correctly, the fibers stand up and the stem winds tighter against the previous wrap.
One other comment- I mentioned how too much light can make the herl brittle. The way you naturally "bronze" herl is to expose it to direct sunlight, just don't overdo it.