refinishing a knoll risom chair  

I'm guessing
Clear lacquer.

I guess
it's possible for birch to get that dark, over time. . .
Of course, whenever I see very uniform color, I think of some sort of toner or stain. Like Heywood Wakefield -- the pigment in the translucent finish brings the various shades of wood into close uniformity across the piece -- which is surely why that finish is used, on birch and maple.

It is definitely a finish...
It is definitely a finish over the wood, not a stain that penetrates the wood. My paint store guy said it is probably shellac, because that was the finish of that time period (late 40's early 50's). I just want to be sure before I apply it to the frame. The last owner sanded (in spots), polyurethaned it, and it was covered with runs, drips, sawdust in the finish. A real mess. I hope to bring it back.