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"The plates were originally engraved in steel to ensure fine lines before being hand-colored on the small prints which measured only approximately 4 × 6 inches...

Once the black-and-white images were printed, they were passed to teams of colorists who applied watercolors by hand."

Natural science illustration returns reasonable results. I think most are done traditionally in some type of watercolors.

I hate to be a contrarian, but I've always hated them. IMHO, the extra-fancy-style illustrations are just that. Now that we have wildlife photography, these seem to regress and appear much worse (again, IMHO) because they are literally an illustration of reality and not a snapshot: https://www.youtube.com/watch?v=NwhVJ7cv9B4&t=36s