The Kraft process, the chemical procedure used to separate the cellulose in wood from the lignin which binds the cellulose fibers together and provides to structure of a tree, surprisingly (at least to me) was not invented by a German named Kraft. It was actually invented in 1884 by a German named Carl Ferdinand Dahl. The first pulp mill using the process came on line in Sweden in 1890-1. Wikipedia, Encyclopedia Britannica, and Uncle Tom Cobley and all maintain it is called Kraft (strength in German) because it makes strong paper. I wonder. The US Patent for the process makes no reference to Kraft or strength ( — if you do look at this you’ll find an extravagant illustration of the pitfalls of optical character recognition technology). The Oxford English Dictionary tells us that the term kraft paper is derived from the Swedish kraftpapper (which no doubt does mean strong paper) so I suspect we’ve got the cart pulling the horse here, and Mr Dahl’s process picked up its name because it was first used in Sweden to make the strong brown stuff we now call kraft paper. The Kraft process doesn’t have to result in brown paper though: 80% of the pulp produced chemically in the USA uses the Kraft process.

The Kraft process tweaked the earlier soda process, and is sometimes referred to as the sulphate process, after the sodium sulphate it uses.

It certainly takes a deal of craft to follow this diagram, which can be enlarged by clicking on it.

Wood chips, steamed to expand the water cavities in them are mixed with a combination of black liquor and white liquor, and the mixture is then cooked in a digester. After a few hours the chips fall apart into cellulose and lignin plus other byproducts, including turpentine. The black liquor, which is actually produced during this process, was in the past mostly vented into the river which is always to be found next to a paper mill. Eventually we came to realize that this wasn’t exactly good for the fishes, who tended to turn up dead as a consequence.

The cellulose from the digester goes to the blow tank, so called because the cellulose is really blown in there. After that the cellulose fibers are screened, washed, and bleached. Various chemicals, including surfactants, defoamers, dispersing and fixing agents are added to help the pulp perform in production. The pulp delivers from the end of the machine and its driers as a continuous thick blanket. This is cut into sheets and baled for shipment. Most pulp is produced in specialized pulp mills, though there are still a few paper mills which produce their own pulp. Notable among these is Glatfelter — a much appreciated manufacturer of book papers. A visit to their Spring Grove plant in Pennsylvania starts with a visit to their forest! Here’s their video of the pulping process:

If you don’t see a video here, please click on the title of the post so as to view it in your browser.