There is a fascinating article on Medievalist.net. I am not sure I understand it but it points to a technique used by the Celtic monks that might well be worth cultivating.
Here is my further progress on my pet portrait commission. I have tried to punch up the contrast a bit but I am still very unsure of the expressions. My source material is so scant and I barely know the annimals in question having only “met” them once. I guess I will let this dry for a bit and then finish it off.