Sulfite Wood Pulp

Plant fibre is cellulose. It is cemented together by lignin. Mechanical pulping tears fibre and keeps the lignin; chemical pulping dissolves the lignin and frees whole fibre. Write that distinction down before you start.

Tilghman's process uses sulphurous acid under pressure in a sealed digester. That is genuinely dangerous and is not reproducible in a classroom. This blueprint substitutes sodium carbonate (washing soda) in an open pan at atmospheric pressure. The chemistry differs — soda is alkaline, sulphite is acidic — but the mechanism demonstrated is the same: a hot chemical bath selectively dissolves lignin and leaves cellulose. Wear eye protection; soda ash is an irritant, not a poison.

Weigh out two 10 g samples of linen rag, torn small. Keep them identical.

Soak sample A in plain water for 20 minutes, then tear and mash it. This is groundwood in miniature — all the fibre, all the non-fibre.

Dissolve 10 g sodium carbonate in 500 ml hot water. Stir until clear.

Simmer sample B in the soda solution for 20 minutes. Do not boil dry.

Record the liquor colour every 5 minutes. It darkens to brown as lignin and gums dissolve out. The colour in the pot is the material leaving the fibre.

Rinse under running water until the water runs clear and the pulp no longer feels slippery.

Sample B should be paler, softer and separate into individual fibres more readily. Examine both with the magnifying glass and note fibre length.

Squeeze and dry both, then weigh. Sample B has lost mass — that lost mass is what Tilghman dissolved. Record the percentage.

Disperse each pulp in water and couch a small sheet onto watercolour paper. Press and dry.

Tear each dried sheet. The chemically pulped sheet should tear with more resistance and a longer fibre pull.

Put both sheets on a sunny windowsill for a week and compare yellowing. Lignin is what browns. This is the experiment that explains crumbling newsprint.

US 70,485, Benjamin C. Tilghman, granted 5 November 1867, filed 26 October 1867. The granted title is “Improved mode of treating vegetable substances for making paper-pulp” — it never uses the word sulfite.

Tilghman is the same Philadelphia chemist who patented sandblasting (US 108,408, 1870). He is properly Benjamin Chew Tilghman; his own signature reads “B. C.”, though patent headers and catalogue metadata have variously misprinted it as B. G. and Benjamin J.

He held British and American patents across 1866, 1867 and 1869. Commercially the process was slow to take: the Swedish chemist Carl Daniel Ekman ran the first successful sulfite mill, and it was decades before chemical pulp displaced groundwood for quality papers. Groundwood never went away — it is still what newsprint is made from, which is precisely why newspapers still yellow.

Why this matters beyond paper. Selective dissolution — take out the binder, keep the structure — is a general materials strategy. It reappears in retting flax, in de-lignifying bamboo for textiles, and in modern biorefinery work that treats lignin as the product rather than the waste.