Glassine Envelopes from JBM

Glassine is translucent, which means that it is not completely clear like a window pane, but it allows you to see the general outline and some detail of the items.

Because it’s pH neutral, acid-free and largely non-porous, glassine is ideal for storing items like art, photos, stamps and other items that may discolor with exposure to air.

When you think of paper, transparency may not immediately come to mind. But glassine is a smooth, glossy paper that is transparent. Glassine allows what’s inside or behind the paper to show through. As a result of the manufacturing process, glassine is pH neutral and acid free. And it’s resistant to moisture, air, and grease. Because it is made from wood pulp, glassine is fully recyclable and biodegradable – making it a sustainable material choice.

Glassine is made from hardwood trees, primarily varieties of aspen, oak, birch, and gum. Once the trees are grown to a specific size, the virgin timber is felled, stripped of its branches and twigs, and cut for transport. When logs arrive at the paper mill, they are completely debarked and sent to chippers, where they are cut into pieces for mechanical or chemical pulping.

The first step in the papermaking process is converting the trees into pulp (the fibrous raw material used to make paper). The pulping process cooks the wood chips under pressure in a variety of ion solutions to break the chemical bond of the lignin (the organic polymers that form the cell walls and support tissue in plants). This process separates the pure cellulose fibers from the lignin and other substances.

Removing all of the lignin from the fiber during the pulping process is a crucial step in making glassine paper. If lignin remains in the pulp during bleaching, the chemical reaction with the bleach turns the lignin into hydrochloric acid, which can make the paper brittle and discolored over time. By removing the lignin, the pulp produces a paper with a neutral pH level and higher resistance to changes in chemical, structural, or optical properties. These properties are what make glassine so unique.

The wood fibers that remain are now ready to be bleached and treated further with dyes, pigments, or additives to fit the desired grade and color before moving on to the paper machine. There, the watery pulp slurry flows onto a moving wire cloth where most of the water drains through. The fibers mat and intertwine as paper sheets begin to take form. More of the water is squeezed from the damp mat as it passes through a series of roller presses. And finally through the dryer section of the papermaking machine.

Calendering is the final step in the papermaking process. This smooths the surface of the paper by pressing it between hard pressure cylinders or rollers — called calendars.

Glassine gets its uniquely smooth, transparent, and glossy properties through a process called supercalendering. This process is completed by putting the paper through the calendaring process multiple times or, more often, by performing the pressing on a special machine with heated or cooled pressurized calendars.

Supercalendering machines are specifically designed to apply pressure, heat, and friction at high speeds through vertical, alternating stacks of hard polished steel rollers paired with softer fiber-covered cylinders (called nips). As the high-speed calenders spin, the soft rolls struggle to return to their original dimensions and buff the paper passing through the nips. The force generated by each progressive nip polishes both surfaces of the paper.

At a cellular level, the supercalendering process breaks down the capillaries of the paper fibers. This gives the paper a highly closed surface with improved density and very low porosity. As a result, supercalendaring makes glassine resistant to grease, air, and moisture – ideal barrier protection from many substances.

Many papers contain wax coatings or plastic laminates to achieve these protective benefits. Plastic fragments and silicone coatings are typically less biodegradable. Free from waxes or coatings, glassine is a sustainable alternative to laminated counterparts.

Once the glassine paper completes its journey through the supercalendering machine, it is rolled or sheeted. Glassine sheets are often used for interleaving, inserting specific-sized sheets between delicate artwork, documents, books, or photos to protect from damage. Sheets are also used for windowing of envelopes and manufacturing fully translucent envelopes.

Glassine is used most often in its translucent form because it shows what is beneath the paper. The color can be changed during the pulping process with the addition of dyes. Other additives – such as clay, titanium dioxide, or calcium carbonate – reduce the amount of light that is transmitted through the paper and make glassine more opaque.

The degree and quality of the supercalendering machine impacts how dense, and in turn how translucent, the final product becomes. The quality can vary drastically among manufacturers, with the clarity usually lost in cheaper glassine paper.

Adjustments in the supercalendering process determine the weight, caliper (thickness), clarity, brightness, smoothness, strength, and grade of the final product.

Glassine’s significantly higher density and caliper give it greater bursting and tensile strength, folding endurance, and stiffness than papers of similar weight. Glassine is generally available in densities between 40–90 grams per square meter (gsm) – more than twice as dense as other common papers.

Glassine is manufactured within a narrow caliper tolerance, meaning measurements of the thickness are highly accurate to within 0.01 millimeters.