I researched it a bit more and discovered two things: The first set of quills I had purchased were mutants and had been cut short by the vendor and second, people working with quills usually soak them to make them softer.
Now, to be clear, in the directions on p. 40 of my copy of Cennini he doesn't say to soak the quills. His thread may have been stronger, his quills thinner-walled, or many other things. However it looks like fly-fishers and others using these quills today frequently soak them, so I gave that a try. That made the quills softer and suddenly able to take a bit of a "squeeze" that broke my thread with the first try. Not wanting to waste my precious supplies of stoat tails, I decided to try again using hogs-bristle (the other type of brush described by Cennini).
A feather from the first set (top) next to one from the second set. I hadn't realized the feather should have this chiseled tip the top one has. Comparatively the second set also have much more usable quill.
The process is pretty much exactly as described in the previous post. I have a pack of hog-bristle brushes that I bought for scrubs in my glass painting. I took an exceptionally large one for which I had a duplicate and took it apart to claim the bristles.
An interesting observation, the bristles have a "root" end. I'm not sure it's biolgically/cosmetically a "root" but one end will be thicker and stronger on a good bristle (I also don't know why these are bristles and not hairs, but I'm a glazier-turned-brushmaker not a biologist... yet). I found it important to try and line up the bristles so I knew which end was the thicker/stronger end.
I took my soaked quill and trimmed the very end off. I used a tapered stick "of other good wood" as Cennini calls for, and shoved the quill on as far as possible. I then grabbed a clump of bristles and shoved them in the other end. Cennini described sticking bristles/hairs in individually until you can't get any more of them. This is where it became a LOT easier to know which was the strong end, as that end wiggles in more easily.
This also makes some sense for retention purposes, I think. The portion of the bristles that is at the opening of the quill is somewhat thinner. You can sneak more bristles in. If you were to try and pull the bristles collectively, however, I think the thicker ends would jam and not come out in a clump.
I tied a knot with my waxed silk thread and could see the quill took the pressure a little better. My thread snapped, after I had gotten the knot tied. On a lark, taking a note from an old martial arts movie that "wet silk never breaks" (not true!) I soaked the thread. I continued tying another knot and doing a little wrapping (that was probably too loose). Its difficult to do all this with only two hands! Same thread, knot, wrapping on the other end to secure it to the stick.
The brush was a little wild, and as Cennini told me to, I trimmed it down a bit:
As a final step I dunked it back into water and made some practice strokes. I gently tried pulling the bristles and none came out. I wouldn't give them a serious tug, but I think this is a practical brush made exactly to the primary source's standards. We will see if it holds up to vitreous paint and silver stain, however!
When I teach my silver stain glass this Pennsic, I hope to give out small vials of stain. I'd love to make a dozen of these to give out too. It didn't take too long in the scheme of things and would be a nice touch!
Hog bristles are much more clingy than stoat fur. When I dunked my bunches of bristles in water they immediately made a tight cluster. I had to work the fur with my fingers for a while to try and make a cohesive bundle. I think in general bristle brushes are going to be easier to make than stoat.
I'm not sure why soaking a hard protein-based quill makes it softer, but don't our fingernails do the same after a hot soak in the tub? Might be something to investigate when bored later.
I have typed and said Cennini's name so much lately I'm starting to feel bad that I have no idea how to pronounce it. Keh-nee-nee? Cheh-nee-nee? I highly doubt it's Sen-nee-nee. I'll need to figure that out before I teach and have to say it several times.