Missing Board

This particular book was leather bound and missing its back board.

Normally, my first response would be to rebind the book. If the book had any important book plates, I would soak that board in a warm water bath, until the book plates floated off. Then I would reattach them to the new board when dry. However, this book had a nice book plate that was painted with fugitive inks. A water bath would have destroyed it. I  had to find another way to replace the board.

Since the front board was not attached, I used it as a guide to cut a new piece of board (make sure to account for the added thickness of materials at the squares). I sanded and back cornered the new board so it would tight against the shoulder. Then I cut new endpapers for the back cover, and attached them to the text block with paste.
I toned a piece of PC4 paper using and acrylics/methyl cellulose mix, to match the original leather. While that dried, I built up the corners on the remaining board.
I took the new board and covered it with the PC4 paper and put it in the press, between blotters, to dry flat. Since this paper is relatively thick, I also infilled the board with a 10 pt card (after pressing).
Because the original spine was a tight back and was still intact, I couldn't bring the new material around the spine. Therefore, I attached the new board as I would the originals, with Japanese tissue at the hinge and joint.
I attached the boards at the inner hinge first, using a strip of Japanese tissue and wheat starch paste. (This has to be done one at a time. Make sure the paste has ample time to dry). I reinforced this by adding additional tissue to the joint with PVA. I also reconstructed the head caps. I glued out a piece of tissue, overlapping on to both boards, and wrapped it around a piece of cord, the width of the spine and the height of the squares.
Finally, I toned all the tissue to match. This is the end result. (Middle: original leather board, Bottom: new PC4 board).