Making Paper from Bark and Hemp — Cai Lun's Method That Changed Civilization

Cai Lun's original formula used a mixture of tree bark (primarily mulberry — Broussonetia papyrifera), hemp rags, old cloth, and discarded fishing nets. The key insight was that any plant material containing cellulose fibers could become paper — the variety of source materials was itself an innovation, making paper production possible anywhere.

Strip the inner bark (bast) from mulberry branches, cutting branches that are 1-3 years old for the longest, strongest fibers. Collect hemp rags and old cloth — cotton or linen, cut into small pieces approximately 5 cm square. Remove any buttons, seams, or non-fiber material. The cleaner the raw stock, the whiter the finished paper.

Submerge the bark strips, hemp rags, and cloth pieces in a large vessel of water and soak for 3-7 days. This retting process softens the fibers and begins breaking down the lignin, pectin, and other non-cellulose binding materials that hold the plant structure together. Change the water daily to prevent foul odors from anaerobic decomposition.

The fibers are ready when they can be pulled apart easily by hand. Mulberry bark separates into long, silky strands; hemp and cloth disintegrate into fluffy masses of individual fibers. Drain and rinse the softened material thoroughly — any remaining non-fiber material will create dark specks in the finished paper.

To further dissolve the lignin and separate individual cellulose fibers, boil the retted material in an alkaline solution. Han dynasty papermakers used wood ash lye — water filtered through hardwood ash, which produces a solution of potassium carbonate and potassium hydroxide. Pour water through a basket of ash several times to create a strong lye solution.

Place the fibers in a clay pot, cover with lye solution, and simmer over a low fire for 4-8 hours. The alkaline solution dissolves the remaining lignin, hemicellulose, and pectin, leaving nearly pure cellulose fibers. The fibers will change from brownish to a lighter cream color as the non-cellulose material dissolves. Rinse thoroughly in clean water after cooking — residual alkali will weaken the finished paper over time.

Spread the cooked, rinsed fibers on a flat stone or wooden board and beat them with a wooden mallet or stone pestle. The beating separates individual fibers and fibrillates them — splitting the fiber walls into hair-like fronds that interlock when the sheet is formed. This fibrillation is what gives paper its strength: the tangled, interlocked micro-fibers bond together through hydrogen bonding as the sheet dries.

Beat steadily for 30-60 minutes, turning and re-spreading the fiber mass periodically. The pulp is ready when a pinch of fibers rubbed between the fingers feels slippery and smooth with no detectable lumps or bundles. Over-beating produces a dense, brittle paper; under-beating produces a weak, lumpy sheet. The ideal consistency is a creamy, uniform slurry.

Suspend the beaten fiber pulp in a large vat of clean water — approximately 2-3 handfuls of pulp per 20 liters of water. Stir thoroughly to create a uniform suspension. The consistency should be thin enough that individual fibers float freely but thick enough that a handful of the suspension held up to light shows a dense cloud of fibers. Add more water for thinner paper, more pulp for thicker.

The paper mould is a rectangular bamboo screen (lian, 帘) stretched across a wooden frame. The screen is woven from thin bamboo strips spaced approximately 1 mm apart — close enough to catch fibers but open enough to drain water quickly. The mould determines the sheet size and surface texture. A removable wooden frame (deckle) sits on top of the mould to contain the pulp during sheet formation.

Stir the vat to re-suspend settled fibers. Holding the mould with deckle on top, dip it into the vat at an angle, then level it horizontally beneath the surface to capture a uniform layer of fiber. Lift the mould straight up out of the vat in one smooth motion — the water drains through the bamboo screen, leaving a thin mat of interlocked fibers on the surface.

Tilt the mould slightly in each direction to distribute the fibers evenly and encourage drainage. The sheet should be uniformly translucent when held against light — thick spots indicate uneven dipping. Remove the deckle carefully and set the mould on a tilted rack to drain further. A skilled papermaker can form one sheet every 30-45 seconds in a rhythm of dip, lift, drain, and stack.

Once partially drained, transfer each wet sheet from the mould onto a smooth wooden board by flipping the mould face-down and pressing gently. The sheet releases from the bamboo screen onto the board. Stack multiple wet sheets with smooth boards between each one — this is couching (from the French 'coucher', to lay down), though the Chinese technique predates the European term by a millennium.

Press the stack under heavy weight — stones, a wooden press, or simply stacking heavy boards on top — for several hours to squeeze out excess water and compact the fibers. Pressing also improves fiber bonding by forcing the fibrillated fibers into closer contact. The sheets emerge from pressing as damp, flexible mats approximately 0.1-0.3 mm thick.

Peel each pressed sheet from its board and brush it onto a heated wall or smooth drying board. In the Han dynasty, papermakers used sun-heated stone or plaster walls — the sheet was brushed flat onto the warm surface and left to dry, shrinking slightly and bonding to a smooth finish. Drying takes 2-6 hours depending on temperature and humidity.

Once dry, peel the sheet from the wall. The side that was against the wall is smoother — this is the preferred writing surface. For an even smoother finish, burnish the surface by rubbing it with a smooth stone or polished bone. Cai Lun's paper was noted for being smooth enough for brush calligraphy, strong enough to fold repeatedly, and light enough that a messenger could carry hundreds of documents that would have required a cart-load of bamboo slips.