Control crest curvature and twist
There are a couple of options for getting nicely curved crests for streamers and salmon flies - one which can be considered more elegant, the other a little more crude and 'risky'.
Elegant:
Steam the feather! Put a kettle on the stove with a narrow nozzle that creates a steam ray. Grab a suitable crest with a pair of tweezers or pliers and put it shortly into the steam. The feather will almost always loose twist and end up curving nicely - and look a lot better too! This curvation will almost hold forever, and should a feather have twist after this treatment, you just dump it.
Crude:
Soak a bunch of feathers in luke warm water and take them out. Put the wet feathers on a suitable curve of a glass bottle or jar, flatten and arrange them nicely and let them dry. When they are completely dry, you can remove them and stroke them gently to loosen the fibers. The curve put into the feather this way is artificial. You can make them almost any shape you want, and inherently twisted feathers can be straightened. But once the feather touches water or steam, it will find its true shape - which can very well be quite different from the one you put into it.
If the crests still have curvature or twist problems, do not give up hope... you can still make some marked changes in their curvature by nicking the stems with your fingernail. For instance if the crest needs more curvature, you can work your way down along the crest pinching the stem between your thumbnail and index finger pad. Use your thumbnail on the side of the crest that you want to be "concave". If you overdo it it works in reverse too. Just knick the other (convex) side to straighten out curvature.
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