How Flax Fibre Is Graded into Linen Yarn

Sorting

Physical Assortment of Raw Scutched Flax

Scutched flax arrives at the classification table as bundled ribbons of bast fibre extracted from the flax stem following field retting and mechanical scutching. Classers evaluate bundle length, cleanliness, color, softness, and structural cohesion before any mill equipment touches the lot. They split sample heads by hand to check how individual fibre bundles divide, gauging how thoroughly pectinous gums broke down during retting.

Under-retted fibre stays stiff, green, and heavy with attached shives, whereas over-retted material turns a dull dark grey and loses bundle tenacity.

Grading relies on visual and tactile standards established across major production regions, matching raw bundle traits to target yarn counts. Classers draw individual fibre bundles lengthwise to estimate mean technical staple length, which ranges from 500 millimetres to over 900 millimetres for high-grade long line stock. Fineness is estimated by tactile flexibility and bundle diameter, where finer bundles yield higher metric fibre numbers and allow drafting down to finer yarn counts without excessive end breakages.

Conditioned at 20 degrees Celsius and 65 percent relative humidity, long line flax bundles designated for fine wet spinning maintain a minimum bundle tenacity of 38 centinewtons per tex under ISO 2370 testing.

Raw flax lots divide immediately into long line fibre and scutched tow. Long line fibre consists of full-length, parallelized bundles recovered intact from the scutching turbine, commanding premium market pricing due to its capacity for fine wet spinning. Scutched tow comprises short, tangled, or broken fibres knocked out during scutching, mixed with residual woody core fragments known as shives.

Scutched tow undergoes carding and combing for dry spinning or coarse wet spinning, selling at a substantial discount to long line fibre.

Defect identification at the sorting table prevents low-grade stock from contaminating high-count spinning channels. Classers flag structural anomalies that impair carding, hackling, and drafting mechanics.

• Shive contamination indicates incomplete mechanical separation of the woody core, leaving rigid particles that cause end breaks during fine wet spinning and form slubs in finished yarn.
• Decay discoloration marks fibre lots exposed to excessive rain during field retting, signaling fungal attack that destroys cellulose chains and reduces bundle tenacity below industrial spinning thresholds.
• Coarse bundle coarsening occurs when incomplete bacterial retting leaves thick pectin bonds between elementary fibres, restricting drafting capability to coarse dry-spun yarn counts below metric count twenty.
• Tangled head bases result from careless pulling or binding during harvest, causing severe fibre loss and excessive tow generation when heads enter the hackling machine pin field.

Primary trade classifications establish clear quality tiers for European and Asian scutched flax lots entering international trade streams.

When the feel of a bundle reveals brittle resistance rather than supple yield under tension, the lot belongs in a coarse dry-spinning blend regardless of visual lustre.

Strand

Hackling Mechanics and Fibre Separation

Hackling converts raw, coarse flax bundles into refined, parallelized slivers while separating long line fibre from hackled tow. Continuous mechanical combing draws flax heads through progressively finer pin fields mounted on revolving leather or rubber aprons. Steel pins penetrate the bundle heads, splitting thick technical fibres along their longitudinal middle lamella into finer elementary assemblies.

The operation removes remaining shives, combs out short unaligned fibres, and forms continuous, uniform slivers suitable for drawing and roving.

Machinery operators balance hackling intensity to maximize long line yield while achieving required fibre fineness. Coarse pin fields at the inlet remove heavy shives and split large bundles; ultra-fine pin fields at the exit perform final drafting and alignment. Excessive pin density or aggressive belt speeds break sound fibres, elevating tow percentage and eroding commercial margin.

Fibre bundles that split cleanly along natural pectin lamellae without transverse snapping maintain maximum continuous length through progressive hackling pin fields.

Evaluating hackling yield follows a strict sequential protocol on incoming scutched flax parcels prior to allocating lots to spinning lines.

1. Sample three representative bales from the delivery lot, taking ten bundle heads weighing approximately one hundred grams each from distinct depth layers within each bale.
2. Condition the sample heads in a standard atmosphere of 20 degrees Celsius and 65 percent relative humidity for twenty-four hours to equalize moisture regain at twelve percent.
3. Weigh each conditioned bundle head on a calibrated balance to record initial unhackled mass.
4. Feed bundle heads manually into a laboratory hackling machine operating at standard pin progression speeds, maintaining fixed clamp pressure on the root ends.
5. Reverse bundle heads in the clamp mechanism to process top ends through the identical pin sequence.
6. Collect and weigh the resulting long line hackled dress, expressing long line yield as a mass percentage of initial unhackled head weight.
7. Collect, weigh, and record hackled tow from underneath the pin aprons to establish the exact line-to-tow ratio for lot cost allocation.

Improper setting of hackling pins damages elementary fibres, causing severe loss of bundle length and generating excessive short tow. Misaligned clamp bars crush root ends, creating weak zones that fail during drafting and cause high end breakage rates at the spinning frame. A three percent reduction in long line hackling yield shifts processing economics unfavorably, forcing mills to reassign affected lots from fine warp yarns to heavy weft or blended dry-spun products.

Drawing

Sliver Attenuation and Spinning Pathways

Doubling and drawing combine multiple hackled slivers into a single homogenous ribbon while progressively reducing linear density. Combined slivers pass through successive sets of drafting rollers operating at increasing surface speeds. Doubling cancels out local thickness variations across individual hackled slivers, delivering count consistency throughout long production runs.

Faller bars fitted with fine steel pins control short fibre movement within the drafting zone, preventing uncontrolled slippage and drafting waves that create thick and thin places in the sliver.

Conversion to roving introduces light protective twist to the drawn sliver, imparting sufficient mechanical cohesion for winding onto bobbins and unwinding at the spinning frame. Wet spinning and dry spinning represent two distinct chemical and mechanical pathways for converting roving into finished linen yarn.

Where Does Roving Twist Shift Wet Drafting Limits?

Roving bound for wet spinning passes through a hot water bath heated between 60 and 70 degrees Celsius prior to reaching the drafting rollers. Hot water softens residual pectinous binder matrix surrounding elementary flax fibres within the roving bundle. Softened pectins allow elementary fibres to slide smoothly past one another during final drafting, achieving fine linear densities impossible under dry conditions.

Wet spinning produces smooth, dense, lustrous yarns with high tenacity, suitable for fine apparel, warp applications, and high-density woven fabrics.

Dry spinning bypasses water treatment, drafting roving directly at ambient temperature and humidity. Without pectin softening, elementary fibre bundles remain locked together, restricting drafting to coarse yarns. Dry-spun yarns exhibit higher bulk, lower density, hairy surface morphology, and lower tensile strength, finding application in heavy upholstery, decorative fabrics, carpet backing, and industrial twines.

Spinning line operators control process variables to match yarn specifications and prevent yarn defects.

• Trough bath temperature governs pectin softening rate, where temperatures below 55 degrees Celsius fail to liberate elementary fibres, causing drafted roving to snap under drafting roller pressure.
• Drafting zone setting regulates distance between back and front rollers, which must exceed maximum technical fibre length to prevent fibre breakage while maintaining control over short elementary fibres.
• Spindle speed control sets production rate and controls balloon tension, where excessive speed induces high yarn tension and elevated end breakage on fine counts above metric count fifty.
• Ring traveller selection determines winding tension and yarn compactness, requiring lighter travellers for fine wet-spun yarns to prevent tensile failure during cop building.

Persistent end breaks on fine wet-spinning frames result from uneven field retting rather than incorrect drafting roller gauge settings or hot water bath temperatures.

Laboratory

Analytical Methods and Quality Standards

Verification of raw flax parameters and yarn metrics relies on standardized testing procedures that eliminate environmental variance. Sample conditioning under ISO 139 mandates twenty-four hours exposure to 20 degrees Celsius plus or minus two degrees, and 65 percent relative humidity plus or minus four percent. Testing unconditioned linen fibre skews linear density and tenacity readings due to flax’s high moisture regain capacity, which ranges between 8 and 12 percent under standard atmospheric conditions.

Yarn count determination follows ISO 2060, measuring the mass of skeins wound on a wrap reel of fixed circumference. Metric count (Nm) expresses length in metres per gram of mass, whereas the traditional Lea system states the number of 300-yard hanks contained in one pound of yarn mass. Converting metric count to Lea count utilizes a fixed mathematical factor of 0.643.

Single yarn tensile properties are evaluated per ISO 2062 using constant-rate-of-extension tensile testers, recording breaking force in centinewtons and elongation at break as a percentage.

Contractual specifications under ISO 6741 mandate adjusting invoice weights for commercial moisture regain fixed at twelve percent for flax yarn shipments.

Defect analysis quantifies structural irregularities that diminish fabric appearance and mechanical performance.

• Slub count index quantifies localized thick places where un-drafted fibre clusters pass through spinning rollers, measured via optical clearers on automatic winding equipment.
• Nep density count tracks small, tightly matted knots of tangled short fibre formed during aggressive carding or hackling, recorded as nep count per one thousand metres.
• Mass irregularity coefficient records short-term count variation using capacitive evenness testers, reported as CV percent across fifty-metre test sample lengths.
• Shive residue content measures remaining non-cellulosic woody particles by mass following chemical digestion of bast fibre samples in hot sodium hydroxide solution.

Quality assurance standards mandate specific acceptance limits across major commercial yarn categories.

Standard delivery contracts incorporate ISO 6741-1 rules, specifying that weight discrepancies exceeding one point five percent of certified conditioned mass trigger automatic landed price adjustments and lab re-testing at the seller’s expense.

Valuation

Yield Arithmetic and Landed Cost Dynamics

Raw flax price accounts for only a portion of final yarn manufacturing cost, with hackling yield, spinning waste, and processing route dictating final landed cost per metre of cloth. Purchasing low-grade scutched flax at a discount frequently increases total yarn cost if poor hackling yield elevates raw material consumption per kilogram of finished yarn. Sourcing managers evaluate total clean yield through hackling, carding, drawing, roving, and spinning frames before committing to raw material contracts.

Consider a practical cost model converting scutched raw flax into Nm 39 wet-spun yarn (25 Lea). Take a 10,000-kilogram lot of Courtrai scutched long line flax purchased at 4.80 Euros per kilogram landed mill gate. Assume hackling yield produces 62 percent long line dress and 30 percent hackled tow, with 8 percent invisible moisture and dust loss.

The hackled tow sells back into coarse dry-spinning channels at 1.50 Euros per kilogram. Processing through drawing, roving, and wet-spinning frames incurs 2.20 Euros per kilogram in direct conversion cost while generating 5 percent spinning waste.

Calculating effective raw material cost requires accounting for tow credit and spinning loss balance:

Initial raw lot cost: 10,000 kg multiplied by 4.80 Euros equals 48,000 Euros.

Hackled long line yield: 10,000 kg multiplied by 0.62 equals 6,200 kg long line dress.

Hackled tow credit: 10,000 kg multiplied by 0.30 multiplied by 1.50 Euros equals 4,500 Euros credit.

Net material cost for long line dress: 48,000 Euros minus 4,500 Euros equals 43,500 Euros for 6,200 kg, or 7.02 Euros per kg of hackled line fibre.

Spinning yield after 5 percent process loss: 6,200 kg multiplied by 0.95 equals 5,890 kg finished Nm 39 yarn on cone.

Total processing cost: 6,200 kg input multiplied by 2.20 Euros conversion cost equals 13,640 Euros.

Total finished yarn lot cost: 43,500 Euros material plus 13,640 Euros processing equals 57,140 Euros.

Final yarn cost per kilogram: 57,140 Euros divided by 5,890 kg equals 9.70 Euros per kg on cone.

Applying this Nm 39 yarn to produce a plain weave linen cloth weighing 160 grams per square metre at 150 centimetres finished width requires calculating raw yarn consumption per linear metre of woven fabric. Fabric construction includes 22 ends per centimetre warp and 20 picks per centimetre weft, yielding a total yarn mass of 240 grams per linear metre including 5 percent weaving crimp and take-up waste.

Yarn cost per linear metre: 0.240 kg multiplied by 9.70 Euros per kg equals 2.33 Euros per linear metre of grey cloth.

If raw flax hackling yield drops from 62 percent to 54 percent due to weak fibre strength or poor retting uniformity, net material cost for long line dress rises from 7.02 Euros to 8.24 Euros per kilogram. This shift increases final yarn cost to 11.08 Euros per kilogram on cone, driving fabric yarn cost up by 0.33 Euros per linear metre before weaving conversion costs apply.

Every five percent drop in hackling line yield increases clean sliver raw material cost by roughly nine percent at the drawing frame input.

How far commercial mills can adjust wet-spinning trough temperatures and drafting roller gauge settings to compensate for variable retting degree without triggering severe end breakage on high-speed ring frames remains an active operational debate between fibre classers and spinning supervisors.