Here are some close-ups of some of those original mortise /tenon joints at the sashes:
The contractor restoring the windows is Allegheny Restoration, from Elkins, West Virginia. They were selected because of their approach to window restoration and their experience. In addition to many other projects, they restored the windows at the Benjamin Latrobe Cathedral in Baltimore. Interestingly, Latrobe was also the architect for the first Capitol Building in DC, and the original buildings in the Allegheny Arsenal, just a few blocks away from the house in Lawrenceville.
All of the sashes have been removed and taken to the shop where they will be steamed to remove paint, glazing putty, etc. The wood parts will then be repaired as necessary, and the glass will be reglazed. In addition, new weather stripping elements will be added at the sides of the sashes and at the meeting rails of the top and bottom sashes, to minimize air infiltration. Any broken glass panes will be replaced with new "old" glass. Where original windows were totally replaced , new replacement windows will be made to match the original windows.