Security Paper.

Security paper is the engineered substrate behind every banknote, passport page, cheque, tax stamp, share certificate, and legal document whose authenticity has to survive contact with a determined forger. It is a tightly regulated specialty grade, normally produced in the 70–120 g/m² range, and it is defined less by its weight than by what has been built into the sheet while the fibres were still wet WPI-g-000903. Three features distinguish it from any commodity paper. First, the fibre: cotton and linen rag, or specially-treated bleached cellulose — 100% cotton for US dollars, 75% cotton plus 25% linen for euro banknotes, and a range of chemically-modified cellulose blends for cheques, passports and stamps. Second, embedded security elements introduced during forming: watermarks, security threads (metal or polymer, windowed or continuous), microprinting-bearing fibres, fluorescent fibres, iridescent planchettes. Third, chemical-reactive treatments that respond to bleach, solvents or mechanical erasure with a visible colour change, so that any attempt to alter the printed content leaves a trace. Security paper is small by tonnage and enormous by value: a few thousand tonnes globally per year, priced at US$25,000–50,000/tonne for banknote grades, against roughly US$1,000/tonne for standard uncoated woodfree.

What it's used for

Banknotes dominate the category. Global banknote production runs to roughly 150–200 billion notes per year across all central banks combined, the overwhelming majority of which are still printed on cotton-fibre paper rather than polymer. The US Federal Reserve prints every dollar bill on paper supplied exclusively by Crane Currency (Dalton, Massachusetts) — a contract held continuously since 1879. Euro banknotes are printed on paper manufactured by a short list of accredited mills including Louisenthal in Germany, Fabriano's security division in Italy, and Oberthur Fiduciaire in France. Sterling, Swiss franc, and Scandinavian currencies have migrated partially or fully to polymer, but most African, Middle-Eastern, Latin American, and South Asian central banks remain on cotton paper.

Passports are the second-largest category by value. EU and US passports use a dedicated secure-paper for the inner pages — the long-lived pages that receive visa and entry stamps. The passport paper-spec is distinct from banknote paper: it has to tolerate hot-stamping, inkjet personalisation, and laminated identity pages without delaminating or yellowing over a ten-year document life.

Cheque paper is a large-volume if slowly-declining segment: MICR-readable (magnetic-ink character recognition along the bottom edge), watermarked, and chemically sensitised so that bleach or solvent attacks on the payee or amount field leave a permanent stain.

Tax stamps — tobacco, alcohol, and increasingly cannabis — are country-specific security products with very high per-unit value and correspondingly high security features. Revenue stamps, lottery tickets, gambling vouchers, university diplomas, birth and death certificates, land-title deeds, share certificates all draw on the same family of substrates. Secure pharmaceutical labels, anti-counterfeit blister-pack wrapping, and voting ballots (in jurisdictions where physical paper with anti-photocopy features is mandated) round out the list. Bearer bonds — historically a major use — have largely disappeared with dematerialisation, but legacy instruments still trade.

Origins and history

The watermark — the single defining technology of security paper — was invented in Fabriano, in the Marche region of central Italy, around 1282. Fabriano papermakers began impressing wire-formed emblems into the wet paper web by stitching a shaped brass or copper wire onto the laid mould; as the sheet drained on the wire, the pulp thinned over the emblem, leaving a translucent mark visible when the dry sheet was held to the light. The original function was trademark — attestation that a specific mill had produced the sheet — but by the fourteenth century Italian notaries were relying on the watermark as authenticity proof for merchant contracts and legal documents. The mark is the ancestor of every modern security substrate.

Paper money followed its own timeline. The Song Dynasty issued jiaozi — redeemable paper certificates — in Sichuan from the early eleventh century, initially as private merchant notes and from 1023 as state-issued currency. Security features were limited. In Europe, Stockholms Banco issued the first European banknotes in 1661 under a charter from Queen Hedvig Eleonora of Sweden; the Bank of England followed in 1694, though its notes were partly hand-written until 1745. For the first century of European paper money, the substrate was a generic rag paper, and security relied mostly on the printer's signature and the authority's seal.

Industrial banknote paper begins with Tumba Bruk. The Swedish Royal Bank — Sveriges Riksbank — established the Tumba mill in 1755 specifically to supply banknote paper to the crown. Tumba has produced banknote paper continuously from that date to the present, making it the oldest continuously-operating banknote paper mill in the world; it is now part of Crane Currency's Swedish operation. In the United Kingdom, Portals began supplying Bank of England paper at Laverstoke Mill in 1724, a relationship that continued for more than two centuries before moving to nearby Overton; Portals is now part of De La Rue.

The American story runs through one family. Zenas Crane founded his Dalton, Massachusetts mill in 1801. The Crane firm won the US Treasury banknote-paper contract in 1879, and every US banknote printed from 1879 to the present day has been on Crane-made, 100% cotton-rag paper. The contract — remarkable for its longevity and exclusivity — was renewed in 2017 when Crane spun out its currency business as Crane Currency (now part of Crane NXT). De La Rue entered the picture from a different direction: Thomas de la Rue founded a stationer's firm in London in 1813, and the firm expanded into banknote printing in the late nineteenth century; today De La Rue prints or supplies roughly 40% of the world's non-domestic banknote production.

Twentieth-century evolution added security threads, fluorescent fibres, and optically-variable devices. Portals is credited with the windowed security thread in the 1970s, which became standard on Bank of England notes and spread globally. Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) produced the first polymer banknote — the 1988 Bicentennial A$10 — introducing a rival substrate that has since captured perhaps 5% of global banknote stock (Canada, Australia, the UK's Churchill-series £5 and later issues, Romania, Vietnam). Scanner and inkjet improvements in the 1990s and 2000s forced another round of innovation: optically-variable devices, holographic foils, thermochromic and photochromic inks, and motion threads (SPARK, Motion by Crane, RollingStar by Louisenthal).

How it's made

Furnish is the defining choice. Cotton linters — short cotton fibres recovered from the seed post-ginning — form the backbone of banknote paper. Cotton has intrinsic wet strength, does not yellow with age, and carries a specific UV-absorption and acoustic signature that is hard to fake WPI-g-000858. Long linen fibres (flax) are added in the 25% proportion used for euro banknotes to improve dimensional stability and give the sheet its characteristic snap. Passport and cheque papers use chemically-prepared bleached cellulose, often blended with a smaller cotton fraction, and modified with strengthening additives to tolerate stamping and folding.

Forming is done on a cylinder-mould machine rather than a Fourdrinier. A cylinder-mould runs a partially-submerged wire cylinder rotating slowly in the stock vat; fibres drain onto the cylinder surface and transfer to a felt. The technique is slower (typically 30–80 m/min against 1,000+ m/min for a commodity Fourdrinier) but produces tighter fibre orientation control, better watermark fidelity, and — critically — a long forming dwell that allows security threads to be fed into the sheet as it is being assembled.

Security features are introduced at several points. The watermark is imposed by an electrotyped dandy-roll on the wet web: a portrait watermark (variable-tone, producing smooth grey-scale images) requires a bas-relief dandy surface with graded depths, which is a specialised craft skill. Security threads — polyester film strips 2–4 mm wide, metallised or carrying printed micro-text — are paid in from a bobbin at the forming section and embedded within the fibre matrix; "windowed" threads emerge at the surface at regular intervals via a bypass mechanism. Fluorescent fibres (short 1–3 mm plastic or viscose fibres dyed with UV-reactive pigment) are added to the pulp upstream of the headbox and distribute randomly through the sheet. Chemically-reactive sensitisers — typically latent leuco dyes — are applied by size-press or surface treatment; they sit dormant until bleach or solvent triggers an irreversible colour change.

Printing happens at the accredited security printer, not the paper mill: intaglio (deeply engraved plates producing the tactile raised surface of banknotes), offset for background tints, letterpress for serial numbers, foil stamping for holograms. Every reel of banknote paper is batch-tracked from forming to shipment under ISO 14298 chain-of-custody protocols; reject rates run 1–3% (versus <0.1% for commodity UWF WPI-g-000875).

Specs that distinguish it

The numbers that matter when comparing security-paper specifications:

• Basis weight — 70–120 g/m², with 80–110 as the banknote band.
• Cotton/linen content — 75–100% cellulose-from-cotton-or-linen. Confirmed by fibre microscopy and FTIR analysis.
• Folding endurance (Köhler-Molin) — >10,000 double folds. Banknote paper must survive 5–10 years of wallet use; commodity UWF fails by 50–500 folds.
• Wet strength — >50% of dry tensile. The sheet has to survive laundering, sweat, and occasional river rescue.
• Tensile strength — 2,500–6,000 kN/m MD/CD. Extraordinary for a sheet this light.
• UV-reflectance signature — under 365 nm UV excitation, genuine banknote paper reads as dark (non-fluorescent) because cotton is free of the optical brightening agents that saturate commodity paper. Commodity uncoated woodfree WPI-g-000890 reads as bright-blue fluorescent. Any cashier who slides a bill under a UV lamp is running this exact check.
• Dimensional stability — <0.1% expansion across 50–80% relative humidity. Commodity papers run 0.3–1% over the same range.

Variants and family

• Banknote paper, 100% cotton — US dollars, most African and Latin American currencies, Indian rupee. Pure cotton linter furnish, portrait watermark, windowed security thread.
• Banknote paper, 75% cotton / 25% linen — euro banknotes, most EU member-state pre-euro currencies, Swiss franc historical series. The linen fraction improves snap and dimensional stability.
• Polymer banknote substrate — biaxially-oriented polypropylene film (Guardian substrate from CCL/Innovia Security; Durasafe from Louisenthal). Not paper, but the direct competitor; included here because polymer specs are often quoted side-by-side. Canada, Australia, the UK current series, Bulgaria, Vietnam all use polymer for some or all denominations.
• Passport paper — bleached chemical pulp with cotton fraction, specific security-fibre mix, chemistry tuned for inkjet personalisation and hot foil stamping. ISO/IEC 7810-compliant for ID-3 documents.
• Cheque paper — MICR-readable, watermarked, bleach-reactive. Typically 90–110 g/m² with surface sizing compatible with magnetic toner.
• Tax-stamp paper — country-specific; tobacco and alcohol stamps carry the highest per-unit security-feature density of any substrate.
• Security certificate paper — diplomas, bearer instruments, legal documents. Lower security-feature count than banknote paper but retains cotton fibre and watermark.

Buying notes

Three failure modes worth knowing about, even though the commercial procurement of security paper is restricted to central banks, accredited security printers, and authorised government-account holders operating under ISO 14298 chain-of-custody:

Cotton-authenticity failures. A mill supplying banknote-grade paper must maintain verified cotton-linter supply with full provenance. Post-production fibre microscopy audits occasionally catch mills substituting cheaper chemical cellulose into a nominally-100%-cotton lot; the substitution degrades fold endurance, alters UV signature, and compromises the security check cashiers and ATMs rely on. Every lot requires fibre analysis against spec.

Watermark uniformity drift. The dandy roll that imposes the watermark wears over time, and a worn dandy produces a watermark with softer contrast than a fresh one. Central banks specify watermark contrast as a tolerance band; a production lot whose watermark drifts outside the band is rejected regardless of every other spec being in range.

Security-thread placement variance. Security threads are specified to lie at a fixed position (e.g. 25 mm from the left edge of the finished note, ±1 mm). Forming-machine drift can push placement out of tolerance on one side of the web; the lot is rejected.

For commercial buyers — cheque printers, diploma-stock procurement, certificate printing — chain of custody documentation is the non-negotiable: every reel is sequentially numbered, transport is bonded, waste is destroyed under audit. A vendor that cannot produce chain-of-custody paperwork is not a security-paper supplier, regardless of the marketing.

Related reading

• Bond paper — the cotton-rag cousin, the grade from which many security-paper conventions descend.
• Offset paper — the commodity printing sheet that security paper is explicitly engineered not to resemble; useful for comparing fluorescence, fold endurance, and cost per tonne.
• Watermark — the thirteenth-century Fabriano invention that remains the signature security feature.
• Security thread — the windowed polyester strip embedded during forming, invented by Portals in the 1970s.