At a neighborhood library book sale I found a copy of The Grapes of Wrath by John Steinbeck, a classic American novel. The book has fair quality paper with a laid finish, minimal paper foxing, and spacious margins. A germ of an idea for new cover design presents itself, so I prep the book by removing the existing cover boards.
To prep a book for a new binding, the first step involves removing the old covers from the text block. To separate the hardcovers, cut slightly in front (about 10mm) of the book’s hinges on the cover boards. Lifting the cut edges separates the text block from the cover boards and exposes the back spine. Pull the end sheets off the text block.
On inspection, I see the spine is covered with mull embedded in a hardened glue coating. Mull is a net-like backing to give added strength to the spine structure when joined to the covers. The folded signature gatherings can be seen through the mull and glue. On the head (top) of the spine are a cloth headband and bookmark ribbon. All of this must come off.
The next step requires the removal of the mull, headband, glue, and ribbon to separate the signatures in the text block. The text block is placed between 2 mill boards cut to the block size and placed in a book press. A methyl cellulose mixture is applied to the spine. The mixture is left for 10-15 minutes to soak and loosen the old hardened glue.
When the glue softens, I use a spatula to scrape the spine. This part of the removal process can be tedious. Depending on the type of glue used on the spine, it can take more time than you think to clean off the old mull and hardened glue attached to the signatures. While working the book’s spine, I noticed slits in the signature folds. At this point, I figure out this was really a notch perfect binding instead of a saddle stitch binding.
The signature gatherings have regular half-inch slits along the back fold. Liquid glue forced into the slits, effectively glued the signatures together. Saddle stitch signatures typically have sewn-in stations that bind the page gatherings together. So the lesson learned here? Be more observant to the interior of the book. A sewn signature will have a center page that will show the thread in the sewing stations. This should be visible unless the text block is tight or the spine is rounded.
This book also has signature keyword numbers for the first 10 signatures. These numbers tell where a signature ends and a new one begins. It also informs how many pages are in each signature. This will help to accurately separate the signatures from each other. This signature structure has 24 pages and there are 20 signatures total.
Based on the structure of the signatures there are three ways to bind. The saddle-stitch method would require individual paper guards adhered to each signature in the gathering. With this book being 300+ pages that is a time consuming effort. A perfect binding glues together individual pages, so the folds must be cut straight losing at least a quarter of an inch in the spine. Or use an overcast stitch on the folds. The small margin space between the text and spine fold would become smaller if cut and alter the design proportions of the page layout. With either binding style, the spine is much tighter and the book will not open completely flat. But the additional quarter inch in the spine margin keeps the layout open. 
I opt for keeping the signature folds for the time being.
In separating the signatures, the paper frays. The glue slits in the signature folds are flared out and hardened. So it is important to take time to clean off the fragments and reduce the swell of the folds. It helps fit them together in the binding process. Setting up the workflow organization, label and number the signatures with light adhesive tabs to keep the gatherings in order for sewing. The next thing to do, deal with the swell of the spine. Using a fine grit sandpaper, the paper folds are lightly sanded to knock off the frayed edges.
Then I use a rounding hammer to flatten the flared glue slits on the folds. The rounding hammer has a wide, slightly, curved face used to round the backs of a sewn spine. Covering the signature with a piece of bond paper, I tap the spine folds of the individual signatures with light glancing blows. It is important to hold the hammer parallel to the flat surface of the signature to prevent marking the pages. I lay thick paper over the top of the signature to protect the text paper from being marked in the flattening process.
Once done with flattening the signature folds, I look at any repairs needed on the folds. Tears happen, especially if the signatures are hard to separate. 
There are a couple of holes that need attention. I use a natural, unbleached mulberry paper patch for strength. It also matches the book’s laid paper in color and texture. Tear the mulberry paper in a rough, irregular shape slightly larger than the hole. The irregular torn shape will meld into the book’s paper and disappear once applied and dry. I use wheat paste to coat the patch and apply over the hole.
I place remay, a woven fiber sheet, between the pages of the signature. Remay is a non-stick surface. The remay will separate the two sheets and prevent the patch from sticking to the opposite page as it dries. A second patch is pasted over the hole on the turned page. The second patch consolidates the repair and makes it stronger. Once both sides of the hole are patched, the signatures are placed under light weights to dry. When dry, the binding of the book can begin.
A Binding Passion 