Glassine.

Glassine is the translucent, grease-resistant, almost waxy-feeling paper that has quietly been doing essential work in half a dozen industries for more than a century WPI-g-000899. It is the paper inside the little envelope that holds your grandfather's postage-stamp collection; the interleaf between silver-gelatin prints in a museum archive; the inner liner of a pharmaceutical blister card; the sheet between layers of chocolate in a confectioner's tray; the carrier that a pressure-sensitive label sits on until the moment it is peeled. The sheet itself is made from bleached chemical pulp that has been beaten to the physical limit of what cellulose can tolerate — 90-95 °SR on the Schopper-Riegler scale — then run slowly over a Fourdrinier and passed through a ten- or twelve-roll supercalender at high temperature and high pressure. No coating. No chemical treatment. The translucency, the grease resistance, the air-tightness and the glass-like smoothness all come from mechanical densification alone. Basis weights run 35-90 g/m². Density lands between 1.1 and 1.3 g/cm³ — nearly twice that of ordinary paper — because almost every pore in the fibre network has been closed by pressure and fibrillation.

What it's used for

Glassine's original and still-dominant application is philately. The small semi-transparent envelopes used to store and trade postage stamps — called glassines in English, Pergamintüten in German — have been the universal stamp-collector's storage format since the late 1890s. Glassine is chosen for stamps because it is chemically inert (no rosin, no acidic sizing, no brightener that would migrate under humidity), smooth enough not to abrade delicate perforations, and translucent enough to identify the stamp without opening the envelope.

Photography uses glassine as archival interleaving between silver-gelatin prints and between sheets of photographic paper in cold storage. The absence of sulfur compounds, plasticisers, and coatings — all of which attack silver halide — makes it the default museum-conservation interleaf.

Pharmaceutical packaging uses glassine as the inner ply of blister cards, as the desiccant-pouch material in pill bottles, and as the protective wrap on ointment-loaded gauze. The low air permeance slows moisture ingress; the grease resistance contains waxy or oily drug vehicles without staining the outer carton.

Food packaging is the highest-tonnage outlet. Bakery interleaves between cookies and pastries; confectionery wraps around chocolates, nougat, caramel, and sticky candies; bakery bag liners for greasy items; commercial tea-bag outer envelopes; deli-counter sandwich wraps. Glassine competes here with greaseproof paper WPI-g-000904 and, for hot-oven applications, with baking parchment WPI-g-000649 — but glassine is the lowest-cost option for cold or ambient grease-barrier work. Art suppliers use glassine as the interleaf in portfolios of fine-art reproductions and as the protective cover sheet on framed prints.

Release-paper base is the single largest industrial outlet. Before a silicone coating is applied to make a label release liner WPI-g-000873, the base sheet must be smooth and dense enough that the silicone forms a continuous film without sinking into the fibre network. Glassine is the workhorse substrate for that role.

Global production sits around 300,000-400,000 tonnes per year — a small speciality by volume against the 400-million-tonne global paper market, but a high-value grade with per-tonne prices several times commodity packaging paper.

Origins and history

Glassine is a product of the late-19th-century explosion of speciality paper experimentation. German and Scandinavian papermakers through the 1880s and 1890s were systematically exploring what happened when bleached sulfite and early kraft pulps were beaten far beyond the normal refining window and then densified under heated calender rolls. Somewhere in that experimentation — the exact mill and year are contested, with credible claims from several German producers between 1886 and 1895 — it was found that a sheet beaten to the threshold of unrunnability and then supercalendered at steam-heated temperatures became translucent, grease resistant, and nearly air-tight. No acid, no sizing, no coating. The effect was entirely mechanical: fibrillation during beating maximised inter-fibre bonding, and supercalendering under pressure closed the residual pores.

The German name Pergamin became the European market standard. The word deliberately echoed Pergament — vegetable parchment, the acid-treated sheet invented by Frédéric Kuhlmann in France in 1857 — while distinguishing the new product as a chemically untreated cousin. In English-speaking markets, glassine emerged from the late 1890s, emphasising the glass-like translucent smoothness. The two terms refer to the same product; European technical literature and older German mill documentation still use Pergamin almost exclusively.

Early-20th-century adoption was rapid in two sectors: philately, where stamp dealers seized on glassine envelopes as a safe, cheap, see-through storage medium; and confectionery, where the rising industrial chocolate trade needed a grease-barrier wrap that could take colour print on the outside. Tervakoski Mill in Finland, founded in 1818 on the Vanajavesi lake system, became one of Europe's dominant glassine producers through the first half of the 20th century and remains a major site today under the Delfortgroup. Löbau and Kämmerer (Osnabrück — now part of Smurfit Kappa) were the historic German centres. In North America, Glatfelter in Pennsylvania and Fraser Paper in Maine ran significant speciality glassine production.

The second major wave came with pressure-sensitive labels. The Avery-Dennison label industry, commercialising through the 1950s and scaling aggressively through the 1970s and 1980s, needed a silicone-coated release liner to carry adhesive-backed labels to the point of use. Glassine, being cheap, dense, and smooth, became the default silicone-coating base sheet. By the late 1980s a substantial fraction of global glassine capacity was being consumed as release-liner base rather than as a finished packaging paper.

A third wave — still running — arrived in the 2010s and accelerated through the 2020s as brand owners sought plastic-free, PFAS-free packaging. Glassine, being compostable, recyclable, and greaseproof through mechanical densification alone, became a credible alternative to plastic film and fluorochemical-treated paper wraps. Current major producers include Nordic Paper (Bäckhammar mill, Sweden), Delfortgroup (Kösen mill in Austria and Tervakoski in Finland), Ahlstrom (Finland), and Berry Global (which acquired Glatfelter in 2023).

How it's made

Glassine starts with fully bleached long-fibre chemical pulp — historically bleached sulfite, now more commonly bleached softwood kraft — at an ISO brightness target of 88-94%. Softwood is preferred because its longer tracheids develop the dense inter-fibre bonding network that mechanical densification depends on; eucalyptus-heavy furnishes can be used for lower-specification grades but do not reach the highest translucency and grease-resistance levels.

The pulp is then beaten to an extreme refining level — 90-95 °SR on the Schopper-Riegler freeness scale. For context, standard printing paper is beaten to 25-35 °SR; even heavily refined greaseproof paper stops around 80-85 °SR. The glassine target is the practical maximum for cellulose: beat any harder and the pulp becomes so slow-draining that it cannot be formed into a runnable sheet. The purpose of this extreme refining is fibre-surface fibrillation — mechanical abrasion of the fibre wall that exposes microfibrils and maximises the hydrogen-bonding surface area between adjacent fibres. High fibrillation is what ultimately closes the inter-fibre pore network.

Sheet forming is on a conventional Fourdrinier at slow speed — typically 200-400 m/min, where a commodity printing-paper machine runs at 1,000-1,800 m/min. The low speed is dictated by the sluggish drainage of the heavily refined furnish. Basis weights are set in the 35-90 g/m² range; 40-60 g/m² is the most common commercial target. The web is press-dried aggressively and passed through a normal multi-cylinder dryer section to a moisture target of 5-7%.

The defining operation is the supercalender. This is a ten- to twelve-roll stack, alternating hard chilled-steel rolls with softer cotton-polymer or filled rolls, run at 80-120 °C surface temperature and linear pressures up to 350 N/mm. The sheet passes through each nip in turn, densifying progressively from a bulk around 1.4 cm³/g at the calender inlet to 0.75-0.9 cm³/g at the outlet — a density of 1.1-1.3 g/cm³, near the theoretical maximum for a paper network. Each nip closes more of the residual inter-fibre porosity; by the last nip, almost no continuous air path remains through the sheet. That is why glassine is translucent (light is no longer scattered at fibre-air interfaces) and air-tight (Bendtsen permeance drops below 100 ml/min) at the same time.

No coating, no sizing press, no chemical after-treatment. The entire functional performance comes from the combination of extreme beating and supercalendering.

Specs that distinguish it

The numbers a buyer should verify before committing to a glassine spec:

• Basis weight — 35-90 g/m² family range; 40-60 g/m² is the common commercial window (ISO 536). Lighter grades go to release-base and stamp-envelope stock; heavier grades to confectionery and pharmaceutical wraps.
• ISO brightness — 80-95% (ISO 2470). Natural glassine sits at the lower end; OBA-brightened "white glassine" reaches the high end.
• Density — 1.1-1.3 g/cm³ (ISO 534). This is the single spec that most clearly separates glassine from other speciality papers: commodity paper runs 0.6-0.9 g/cm³; even heavily calendered SC paper reaches only 1.0. Density below 1.05 g/cm³ on a glassine spec sheet indicates inadequate supercalendering.
• Bendtsen air permeance — <100 ml/min (ISO 5636-3). Functionally air-tight. Standard uncoated paper runs 500-5,000 ml/min. Greaseproof paper runs 150-400 ml/min. A glassine reading above 150 ml/min means the supercalender has not fully closed the pore network.
• Grease resistance — Kit test — 8-12 on the TAPPI T-559 scale (the rating runs 1-12). This is the highest grease-resistance rating achievable in paper without any chemical additive; only fluorochemically treated or silicone-coated papers exceed it.
• Bekk smoothness — 800-3,000 seconds (ISO 5627). Very smooth. A coated woodfree art paper runs 400-1,000 seconds.
• Opacity — 40-60% (ISO 2471). Deliberately low. Standard paper is 88-95%. The translucency is the feature.
• Moisture — 5-7%. Above 7% the sheet limps and dimensional stability in die-cutting suffers.

Variants and family

The glassine family is narrower than bond or kraft, but meaningful sub-grades exist:

• Natural glassine — the classic bleached-but-not-OBA-loaded off-white sheet. Default for philately, photo interleaving, and most food-contact applications where regulatory migration limits rule out brighteners.
• Bleached white glassine — OBA-loaded for a whiter, more vivid appearance. Used in premium philatelic envelopes and in packaging applications where print contrast on the finished wrap matters.
• Coloured glassine — dyed in pastel or deep colours for speciality confectionery wraps, decorative interleaving, and gift packaging.
• Release-base glassine — lightweight (35-45 g/m²), extreme-density glassine manufactured specifically as a silicone-coating substrate for pressure-sensitive label liners WPI-g-000873. Smoother and more uniform than packaging grades because the silicone coating demands a defect-free surface.
• Pergamin — the same product under its traditional European name. Still standard in German, Austrian, and Scandinavian technical documentation.
• Supercalendered kraft (SCK) — a close cousin made from unbleached kraft pulp using the same beating-and-supercalendering process. Not a glassine — the brown colour and residual lignin set it apart — but shares the translucency, density, and release-base characteristics in a niche market for silicone-coated unbleached release liners.

Buying notes

Three red flags worth catching on a glassine spec sheet:

Bendtsen air permeance above 150 ml/min. Anything above 150 ml/min indicates the supercalendering has under-performed — insufficient nip pressure, insufficient temperature, or a furnish not beaten to full °SR target. The sheet will no longer pass as an air-barrier layer in blister packaging; grease will migrate through under warm load; and if the product is sold as release-base, silicone coating will sink into the porous surface. Require a recent mill Bendtsen certificate on the specific reel.

Brightness drift in natural glassine. Natural (unbrightened) glassine should run ISO 80-86% brightness with a tolerance band of ±2 points across a lot. Drift wider than that indicates inconsistent pulp bleaching or contamination in the broke system, producing mottled off-white sheets that are unacceptable in museum-conservation or premium-philatelic applications.

Moisture above 7%. Glassine is hygroscopic and runs best at 5-7% moisture. Above 7% the sheet goes limp and curled; die-cutting becomes erratic; dimensional stability in envelope folding and release-liner coating suffers. Reject any reel above 7.5% on delivery.

FSC Mix is the realistic baseline across the global glassine supply; FSC 100% is available at a 5-10% premium from Scandinavian producers (Nordic Paper, Tervakoski) and less universally from Austrian and North American mills. For EUDR-governed supply chains, confirm chain-of-custody on each lot.

Related reading

• Release paper — glassine's largest industrial outlet: the silicone-coated liner that carries pressure-sensitive labels.
• Baking parchment paper — the hot-oven cousin: silicone-coated paper for bakery pans, where glassine alone is not heat-stable enough.
• Greaseproof paper — the adjacent food-wrap grade: also heavily beaten, but not supercalendered to full translucency.
• Tracing paper — the cousin translucent grade: translucency achieved through chemical treatment rather than mechanical densification.