Decortication

A bast stem is a composite. The fibre bundles sit in the cortex, glued to each other and to the woody core by pectins. The objective of decortication is to recover those bundles as long, clean, undamaged fibre, because length, fineness, and strength are what the fibre is bought for, and because any woody fragment left clinging to it is a defect that persists into yarn, textile, or composite.

The principle is fracture followed by scraping. Fluted rollers bend and crush the stem repeatedly, snapping the brittle core into short pieces without severing the flexible fibre running alongside it. Beating or scutching then knocks those pieces loose, and the fibre — which is long, aligned, and tough in tension — is drawn away from the debris. Some equipment combines both actions; the sequence rather than the machine is what defines the method.

Two streams leave a decortication line, and the woody one is not a disposal problem. Shive is a substantial, saleable material with established markets — calling it waste misstates both the material flow and the economics of a fibre plant.

Fibre

The recovered bast bundles. Long, aligned fibre and shorter tangled tow are commonly separated and sold into different markets, since length and fineness drive value.

Shive

The fragmented woody core. Used as animal bedding, as a raw material for particleboard and composite panels, in bio-based construction binders, as a fuel, and as mulch.

The three crops are not handled identically. Flax is traditionally retted then broken and scutched to preserve long line fibre. Hemp is decorticated both after retting and, for technical and composite end uses, green from unretted stems, which yields a coarser fibre where length matters less. Jute is customarily retted and stripped, historically by hand, with mechanical decortication in use where labour and scale make it viable.

Decortication cannot make fibre better than the crop and the retting delivered. Fineness and intrinsic strength arrive with the stem; retting sets how separable it is. What decortication decides is how much of that potential survives — how long the recovered fibre is, how much woody residue clings to it, and how much of it ends up downgraded to tow or lost into the shive stream.

The failure modes are symmetrical, which is what makes the step hard to run. Insufficient treatment leaves shive bound to the fibre, and shive content is a graded defect. Excessive treatment fractures the fibre bundles, shortening them and pushing long line fibre down into the tow fraction — a quality loss that reads as a yield loss on the invoice.

• Fibre broken during beating and carried out with the shive stream
• Long line fibre degraded into shorter, lower-value tow
• Shive retained on the fibre, appearing as a defect in the graded lot
• Dust and short fines removed at aspiration
• Losses inherited from under- or over-retting, which decortication cannot correct

Decortication itself is a dry mechanical process whose main demand is electrical energy; the water and effluent questions in a bast fibre chain belong overwhelmingly to retting, which precedes it. In material terms the line is close to fully valorised, because the woody fraction — which is the larger of the two streams by mass in these crops — has real bedding, panel, construction, fuel, and mulch markets rather than being discarded.