Typically, seventeenth-century New England joinery uses mortises about 5/16″ wide, set in from the flat front face about the same thickness…so here I have set a mortise gauge to strike the width of the mortise.
The standard practice in my shop is to chop most of the mortise with a mallet and a 5/16″ mortise chisel. In something this size it’s not necessary to bore out the waste first, the green oak chops very easily.
I align the chisel in such a way that the bevel is plumb, this helps knock the chip upward as the chisel is struck down. I alternate the chisel’s position, so I chop a V-shaped opening in the middle of the mortise’s length. Then I gradually widen this opening.
To finish off the ends of the mortises, I often use hand pressure. In this view, I’ve risen up onto the balls of my feet, and come down with my whole body to drive the chisel. Then I can pry the waste up from the bottom of the mortise.
The pictures here are the beginnings of a set I am doing to illustrate the making of a joined stool. There are many more steps to chopping mortises, but these few are the gist of it. The moisture content of the oak is important, usually it’s fairly wet inside when I chop these joints. The stock in the photos was planed wet from the log less than a month before…