Thread, Tape and Cloth: What Opens a Book

For a 320-page book, use 18/3 linen thread: three plies, roughly 0.45 mm finished diameter, which is the size most trade binderies keep on the machine for octavo and royal octavo work. Below about 250 pages, 25/3 or 35/3 runs cleaner and leaves less swell; above 400 pages, step up to 18/4 or sew on tapes. The thread size, the sewing support and the weight of the covering cloth together decide how much the spine swells and how far the book will open, and none of the three can be chosen in isolation. The choice of linen thread and bookcloth is the same kind of decision a garment maker faces when a seam has to hold a heavy panel: the material has to match the load, not the look.

Which linen thread for a book of 320 pages?

Linen thread is sold by a two-part number: the first figure is the size of the yarn, the second the number of plies twisted together. So 18/3 is a size-18 yarn, three plies. The convention runs backwards from intuition: a smaller first number means a thicker yarn. Common bindery sizes are 25/3, 18/3, 18/4 and 16/3, and the finished diameter of 18/3 lands near 0.45 mm, which is why it sits in the middle of the trade.

For a 320-page book, that middle is where you want to be. A 320-page octavo block is around 20 mm thick before sewing, and 18/3 will carry it without crowding the sections. If you drop to 25/3 on the same block, the thread is thin enough that the swell at the spine falls away, and the rounder has less material to work with when the book is backed. If you go up to 18/4, the swell grows by roughly a millimetre across the spine, which is fine on a book that will be rounded and backed but awkward on a flat-back case binding.

Two practical checks. First, the thread should fill the saw kerf or the needle hole without forcing the sections apart; if you can see daylight between sections after sewing, the thread is too thin. Second, the swell should be measurable, not guessed. On a 320-page block sewn at 18/3 with a moderate pull, we expect 1.5 to 2 mm of swell over the unsewn thickness. That figure is what the backing boards and the case squares are cut against.

Unbleached linen is the default because it holds a knot, resists the paste and glue it will meet, and does not stretch under the pull of a sewing frame the way a soft cotton will.

Tapes or cords for a heavy block?

A heavy block means a thick one, and thickness is what breaks sewing. Once a book passes roughly 400 pages, or its sections are made of coated or heavily sized paper, the thread alone will not hold the sections to the spine. That is when you sew on supports.

Tapes are woven cotton bands, usually 9 mm, 12 mm or 16 mm wide, sold by the roll. Cords are twisted hemp or cotton, commonly 4, 5 or 6 mm in diameter. The rule we use: tapes for books that will be cased in and read flat, cords for books that will be rounded, backed and given a hollow or a tight back. Tapes lie flat against the spine and give a broad, gentle support, which suits a heavy reference book that has to open without cracking. Cords sit proud of the spine and let the backing shoulder form around them, which is what a rounded spine needs.

For a 600-page block on 12 mm cotton tape, five tapes is a normal count. For the same block on 5 mm cord, six or seven cords. The number matters less than the spacing: supports should fall at the head, the tail, and evenly between, so that no section is unsupported across more than about 60 mm.

The support also changes the swell. Tapes add almost nothing to the thickness of the spine; cords add their own diameter plus the thread that wraps them, so a cord-sewn book swells more and needs more backing. If you are working to a fixed case size, decide the support before you cut the boards.

How heavy should case cloth be?

Bookcloth and buckram are sold by weight, given in grams per square metre, and the range that matters runs from about 180 gsm to 450 gsm.

Light bookcloth, 180 to 220 gsm, is for small formats, limp covers and slipcases. It drapes, it turns easily over a thin board, and it will not fight a 1.5 mm board at the hinge. Standard case cloth, 250 to 300 gsm, is the workhorse for octavo and quarto cases on boards of 2 to 2.5 mm. Buckram, 350 to 450 gsm, is for library and reference work, for books that will be shelved and pulled by the spine for decades.

The weight of the cloth has to be read against the board, not on its own. A 400 gsm buckram on a 1.5 mm board will pull the board out of shape at the joint. A 200 gsm cloth on a 3 mm board will show every irregularity of the board underneath. The pairing we aim for is roughly 100 gsm of cloth for every millimetre of board, which puts 250 to 300 gsm cloth on 2.5 mm board and 400 gsm buckram on 3 mm board.

There is a second number that matters as much as weight: the cloth's stretch. A woven cloth with a plain weave and a starch or acrylic back will hold its shape at the hinge and resist the pull of a heavy block. A cloth with too much give will sag at the joint after a year of opening. If you can, cut a strip 25 mm wide and pull it by hand before you commit a run.

What does each choice do to the opening?

The opening of a book is decided at the spine, and every material above writes into it.

Thread that is too thick swells the spine, and a swollen spine on a tight back makes the book open stiffly and refuse to lie flat. Thread that is too thin lets the sections shift, and the book opens with a soft, uncertain hinge. Tapes spread the load and give a wide, easy opening, which is why they suit heavy books that must be read flat on a table. Cords, by contrast, create a firmer shoulder and a more controlled opening, which suits a rounded spine that is meant to hold its shape.

The covering cloth sets the limit. A heavy buckram over a well-backed spine will open to about 160 degrees and stop there, which is what a library wants. A light cloth over a thin board will open past 180 degrees and eventually break at the hinge, because the cloth is doing work the board should be doing.

A working order for the bench

Decide the support first, because it fixes the swell. Then pick the thread to match the support and the section count. Then measure the sewn block, not the unsewn one, and cut the boards and the case from that measurement. Finally, choose the cloth by weight against the board thickness, and check the stretch by hand before the run.

None of these numbers is a rule handed down. They are the figures we have arrived at by measuring blocks on the bench, and they shift with paper, with grain direction and with how hard the sewing is pulled. Write down what you used and what the book measured afterwards. That record is worth more than any table.