To my knowledge the only primary source on making medieval brushes is Cennini's Ill libro del'Arte where he describes, short and sweet, the making of minever and hog bristle brushes.
Throughout the centuries the parts that make a brush apparently haven't changed much : a handle, a ferrule [now metal, sometimes plastic, then a piece of quill] and a bunch of hairs. Pretty straightforward. During munchkin-naptime today I grabbed beeswax, linen thread, a couple of sticks [you -always- have sticks in the house when there is a preschooler], a Xacto knife, a couple of feathers and some sable fur since I'm not too keen on having to shoot squirrels for the proper minever hair :)
It's a fumbly sort of business and arranging the hairs in a way that'll qualify in the slightest as a brush shape is the hardest part of it but I think I did okay. Here're a couple of pictures
The ingredients ...well, most of them. I forgot to take a picture of the beeswax and the thread ...meh.
Here the handle is already whittled into shape [you've gotta watch how much you're slicing off - you end up with a very very thin stick really fast *G*], the hairs have been fiddled into shape and tied to the handle and the quill ferrule is cut and good to go over the top.
Look ! It's a brush ! While the handle is not the Cennini recommended length of 9 inches [I love my handles short] it does look like a real brush. It's also not really unimportant to mention it actually works and keeps its shape fairly nicely :o)
So encouraged I made another one - a little fatter this time with a broader tip.
Now ... where to get squirrel fur ?