Bookbinders use unbleached linen thread almost exclusively for sewing sections. Of course, there is nothing stopping you using whatever you want. However, natural flax spun to make linen is recognized for its strength and longevity. In the early 20th century a few authors, such as Douglas Cockerell and Edith Diehl, recommend ligature silk as used by surgeons. The heavier silks available which are used for sewing headbands could be a replacement for ligature silk, which is no longer used in the medical profession. But for sewing sections linen thread is hard to beat.
Linen (the fabric and thread) is made from the fibres of the flax plant. The process for making linen from flax is complex and labour intensive, and thus the production of linen has declined dramatically in favour of synthetic fibre.
Bookbinders linen thread is almost always specified using the lea numbering system and a ply. A very commonly used thread is 25/3, which means it’s 3 strands of 25 lea twisted together to form the thread. The lea is the number of yards in a pound of linen divided by 300. For instance, 10 lea would have 3000 yards per pound of linen and if this was wound 3 ply (10/3) it would produce 1000 yards of thread.
The lea systems should not to be confused with the weight systems used in the thread trade. Some common systems are Weight, Denier and Tex. A 40 Weight (written 40 wt) thread means that 40 km of this thread weights 1kg. Denier is the weight in grams of 9000 meters of thread. Tex is the weight in grams of 1000 meters of thread.
The numbering system is like the Standard Wire Gauge in the smaller the number the thicker the thread. Thus 18/5 is much thicker than 40/3, these being about the extremes used by bookbinders for sewing.
Currently available linen threads from the main bookbinding suppliers are 16/3, 18/5, 18/3, 18/2, 25/3, 30/3 and 40/3.
Bulk thread is usually sold in 3 main ways, 50g spools, 250g cops and 50g skeins. A spool is almost self-explanatory in that it is a small cylinder with a length of thread coiled around it. A cop is cone shaped spool of thread coiled around a spindle and usually used in industrial sewing, and thus the larger weight. A skein is loosely wound length of thread. Wikipedia describes “a skein of cotton yarn is formed by eighty turns of the thread around a fifty-four inch reel”. However, cotton and linen skeins are different, but the example is a useful descriptor.
It is useful to know some of the parameters for the commonly used threads. Note that the thickness varies and is hard to measure. In the table I have assumed 25/3 is 0.45mm in diameter (based on my measurements of many samples) and I assume the density of the thread is the same. The ratio of the lengths is proportional to the square root of the ratio of the diameters. These numbers match fairly closely with what I have measured, and I think is a useful guide for thread selection. If there is a systematic error, it is that the thicker threads have a slightly lower density and I underestimate the thickness.
Most bookbinding needle sizes are based on the American Wire Gauge (AWG) system. This makes sense since needles start life as a spool of wire. In the AWG the diameter increases with decreasing AWG number. AWG 10 (2.59mm dia.) is much thicker than AWG 20 (0.81mm dia.). It’s a logarithmic system with a complex formula to determine the diameter from the AWG number. Two useful sizes are AWG 15 (1.45mm dia.) and AWG 18 (1.02mm dia.). Having explained all that, most needle sizes never exactly match the equivalent AWG size, but it’s a good starting point.
John James is a needle manufacturer based in Worcestershire, “the Needle Capital of the World”. John James was founded in 1840 and is now part of a larger company called Entaco. What is most interesting about John James is they manufacture needles specifically for bookbinders. They have two sizes, 15 and 18.
Size 15 (4/0) 69mm length x 1.83mm diameter
Size 18 (1/0) 62.5mm length x 1.09mm diameter
Just like 25/3 is the most commonly used thread size, the most useful needle is size 18, and the two go together like peas and carrots. What makes a needle a bookbinding needle? Many different types of needles can be used. Darning needles will be found in sizes very similar to the JJ bookbinding needles and are perfectly acceptable. A bookbinding needle is on the long side to help with kettle stitch and not too sharp to reduce the chance of cutting or puncturing paper when you don’t mean to.
Length (m) = Lea x 604.8 x Weight(kg) / ply
Conversion of thread weight systems
Weight to Denier is 9000/Weight
Weight to Tex is 1000/Weight
Denier to Weight is 9000/denier
Denier to Tex is Denier x 0.111
Tex to Denier is Tex x 9
Tex to Weight is 1000/Tex