That book is a nice read. Google books is fantastic for research.

I had not heard the term "gain" before in terms of joinery. Pg 174 of Hodgson's book refers to it. Looks like a general term for a housing or even what we've called a "dap" in the past. Must check the glossary on that one.

The old masters must have used all sorts of things to wet / dye a snap line: berries, flowers, maybe even blood if something was freshly killed...

Although I've put this link up in the past, It's relevant to snap lines (charcoal in this case):

http://books.google.com/books?id=2W1Lq_nvlCQC&printsec=frontcover&lr=#PPA15,M1