Haha! I am glad you added that last line as I ended up more confused. Phone boxes? B&W photography?

I do think I get it now though. If you spill red wine and then rub some bicarb into it it goes a purplish blue but this fades over time to an increasingly grey colour that eventually disappears altogether. The blueness is being 'washed out' by the light.

I'm going to mess about a bit and see what I can come up with. Thinking about it the type of ink used to draw this map would most likely be ferric tannate, or what was called iron gall ink. The tannin was extracted from oak galls - you know those ball things that form when wasps lay their eggs in the leaf buds? This is reacted with a source of iron and produces a blackish/purplish/grey that when fixed with a gum forms a very persistent ink. The map would probably have been drawn by one of those monks or whatever they are in that weird college place on that island in the west. They tend to wander about a lot, drinking in the local taverns and bothering people about how talented their kids are. Them posh buggers up in the city pay a fortune to send their kids there you know.

So yeah ferric tannate it is