Setts Of The Scottish Tartans $20

[figheadair]

Correct me if I'm wrong Peter, my guess is that it's a combination of having in-person access to the portrait, studying the tartan as painted very carefully, noting a number of frustrating inconsistencies committed by the artist (or were these inconsistencies present in the actual cloth?) and using encyclopaedic knowledge of historical tartan design to overcome these obstacles and arrive at an educated best-guess estimate of what the original fabric looked like.

It's pretty much the life of the historian, for now we see through a glass, darkly.

Absolutely spot on Richard. There is no alternative to having access to the portraits, as well as specimens of the period to study, and the time to do so.