Coated vs Uncoated Paper

Definition and overview

Coated paper and uncoated paper are two broad categories of printing and writing stock distinguished by whether the sheet's surface has received a pigment coating after the base paper (the fiber web) is formed.

Coated paper has one or more thin layers of a coating mixture applied to the base sheet. The coating is a suspension of mineral pigment particles held together and bonded to the paper by an adhesive binder. When dried and often calendered (pressed between rollers), this layer fills the valleys between surface fibers, producing a smoother, more closed, more uniform surface than the underlying fiber structure alone.

Uncoated paper has no such pigment coating. Its printing surface is the fiber web itself, though it may still be internally filled with mineral pigment during forming, surface-sized with starch or similar agents, and calendered for smoothness. The surface remains comparatively porous and absorbent because it is not sealed by a pigment coat.

The practical consequence of this structural difference is how ink and the paper surface interact. On a coated sheet the coating tends to hold applied ink nearer the surface ("ink holdout"), whereas an uncoated sheet tends to absorb ink into its fibers. That single behavioral contrast underlies most of the print-quality differences discussed below. The coated/uncoated distinction is a category, not a single graded number: any given sheet is more fully described by measured properties such as grammage, thickness, brightness, opacity, gloss, and smoothness.

The coating layer: composition and application

A paper coating is broadly composed of two functional parts: a mineral pigment that provides the smooth, opaque, light-scattering surface, and a binder that adheres the pigment particles to one another and to the base sheet. Additional additives (for example rheology modifiers, lubricants, or optical agents) are commonly present but the pigment-and-binder pairing is the defining structure.

The pigments most widely reported in paper coatings are mineral in origin:

• Kaolin (china clay), historically the dominant coating pigment.
• Ground and precipitated calcium carbonate, which has become widely used, particularly in neutral/alkaline papermaking, and generally offers higher intrinsic brightness than kaolin.
• Titanium dioxide, used where high opacity and brightness are needed.

Binders are typically synthetic latexes (for example styrene-butadiene or polyvinyl acetate types) and/or natural binders such as starch and protein, used alone or in combination. The choice and ratio of pigment to binder influence the finished surface's gloss, porosity, and ink receptivity.

Coating can be applied to one side or both sides of the sheet. In trade terminology, a sheet coated on a single side is often designated C1S (coated one side) and a sheet coated on both sides C2S (coated two sides). Several application methods exist, including blade, air-knife, rod, and curtain coating; cast coating is a distinct process that produces an unusually high-gloss, mirror-like surface. Because the exact formulation, coat weight, number of layers, and finishing (calendering) vary by product, the precise surface characteristics of any coated grade are set by manufacturing choices rather than by the word "coated" alone.

Grades and finishes of coated paper

Coated papers are commonly organized along two axes: the finish (how glossy or matte the surface is) and the furnish of the base sheet (what fiber the underlying paper is made from).

By finish, coated stocks are usually described on a continuum from matte to high gloss:

• Matte (dull) coated, with a low-sheen surface that still benefits from the smoothness and ink holdout of a coating.
• Silk / satin coated, an intermediate finish between matte and gloss.
• Gloss coated, with a reflective surface.
• Cast-coated, at the high-gloss extreme, produced by drying the coating against a heated polished surface.

By base sheet, coated papers are broadly split into coated freesheet (also called coated woodfree, made from chemically pulped fiber with little or no mechanical/groundwood content) and coated groundwood / coated mechanical (containing mechanical pulp). The term art paper is also used, generally for high-quality coated printing stock. These groupings are descriptive families; individual products within them differ in coat weight, brightness, and finish, and naming conventions vary between regions and suppliers. This entry does not assign fixed numeric specifications to any named grade, because those values are properties of specific products rather than of the grade name itself.

Uncoated paper and the coated–uncoated continuum

Uncoated papers present the fiber surface directly and are likewise organized by furnish and by surface treatment. Common uncoated categories include bond and writing papers, offset (book) papers, text and cover stocks, and mechanical (groundwood) grades such as newsprint. Surface descriptions such as wove (smooth, even formation) and laid (with a ribbed pattern from the forming wire or a dandy roll), and finishes such as vellum (a toothy, matte surface), describe the uncoated sheet's texture without implying a pigment coat.

Although "coated" and "uncoated" are treated as two categories, surface smoothness itself is a continuum. An uncoated sheet can be made smoother by heavier calendering or by surface sizing, and such treatments narrow but do not eliminate the difference from a pigment-coated sheet. The defining line is the presence or absence of a distinct surface pigment layer, not smoothness per se. Some products (for example lightly pigmented or "film-coated" grades) occupy the middle ground with a very thin coat, illustrating that the boundary is one of degree in coat weight and coverage rather than an absolute switch.

Because uncoated surfaces are more open and absorbent, they generally accept writing media (pencil, pen) more readily and present a more tactile, less reflective appearance than coated stock. These are descriptive tendencies; the exact behavior depends on the specific sheet's sizing, filler content, and finish.

How the media is characterized and measured (standards)

There is no single instrument reading that reports "coated vs uncoated." Instead, both categories are characterized by a set of standardized paper properties. Naming the measurement standards is useful because it identifies exactly what is being reported and how; the standards below define methods, and this entry does not assign specific numeric outcomes to any grade.

• Grammage (basis weight) — the mass per unit area of the sheet. ISO 536 specifies the determination of grammage in grams per square metre; TAPPI methods cover the equivalent measurement in North American practice.
• Thickness, density, and bulk — ISO 534 specifies the determination of thickness (caliper), density, and specific volume of paper and board. Because a coating adds mass and can be calendered, coated and uncoated sheets of the same grammage can differ in thickness and bulk.
• Brightness — ISO 2470-1 specifies the measurement of the diffuse blue reflectance factor, reported as ISO brightness; the method is defined for white and near-white pulps, papers, and boards.
• Opacity — ISO 2471 specifies the determination of opacity by a diffuse reflectance method (paper backing), quantifying how much printing shows through from the reverse side.
• Gloss — TAPPI/ANSI T 480 specifies specular gloss of paper and paperboard measured at 75 degrees, a geometry chiefly applied to coated papers; a 20-degree geometry is used for very high-gloss surfaces such as cast-coated stock, and ISO 8254 covers 75-degree gloss with a converging beam.
• Smoothness / roughness — surface smoothness is reported by air-leak methods (for example Bekk, Bendtsen, Sheffield, and Parker Print-Surf), each defined by its own procedure.
• Whiteness and color are reported via CIE whiteness methods, and trimmed sheet sizes follow ISO 216 (the A and B series built on a 1:√2 aspect ratio) or regional size systems.

Taken together, these methods let a coated grade and an uncoated grade be compared on common terms rather than by category name alone.

Role in printing and print quality

The coating's central effect on print quality follows from ink holdout versus ink absorption.

On a coated sheet, the sealed pigment surface tends to keep ink films near the surface rather than letting them soak into fibers. Ink that stays on the surface spreads less and scatters less light within the sheet, which supports crisper halftone dots, a wider apparent color gamut, and higher perceived saturation and contrast. The smoother, more uniform surface also lays ink more evenly.

On an uncoated sheet, ink penetrates the more porous fiber surface. Ink drawn into and spreading along fibers enlarges printed dots and can soften edges and reduce maximum ink density and gamut. This mechanical and optical enlargement of the printed dot is the subject of the separate dot gain entry; coated and uncoated stocks typically sit at different points on that behavior, with uncoated papers generally exhibiting greater dot gain. Likewise, how much of a reverse-side image is visible relates to the sheet's opacity, treated in the show-through and opacity entries.

Because these are tendencies rather than fixed constants, the actual result on any job depends on the specific paper, the ink or toner system, and the press or printer. The coating changes the surface's optical and absorption behavior; it does not by itself guarantee any particular numeric outcome.

Interaction with printing processes

The coated/uncoated distinction interacts differently with each printing process, and the relevant processes are described in their own entries; this section only summarizes the interface.

• Offset lithography is where the coated/uncoated distinction is most traditionally emphasized, because oil-based offset inks dry substantially by absorption and oxidation and the surface's holdout strongly affects dot sharpness and density.
• Inkjet printing places a water-based or pigment-based liquid ink onto the sheet. A generic offset coating is optimized for oil-based ink and is not the same as a dedicated inkjet-receptive coating, which is engineered to absorb and fix aqueous ink and to control bleed; this is why inkjet-specific media (including inkjet photo paper) differ from conventional offset coated stock. Pigment-based and dye-based inks interact with these receptive layers differently, as covered in the corresponding ink and process entries.
• Electrophotographic (laser/toner) printing fuses dry or liquid toner with heat and pressure rather than absorbing a liquid, so surface smoothness and heat behavior of the coating matter more than porosity, and not all coated stocks are designed for the fuser.
• Dye-sublimation and other specialty processes rely on receptive layers matched to their transfer chemistry.

The key point is that a coating is tuned to a particular ink/toner chemistry; "coated" alone does not imply suitability for a given process, and process-specific media exist precisely because the requirements differ.

Relationship to adjacent paper properties and defects

The coated/uncoated property does not act in isolation; it is intertwined with several other media properties and with certain print defects that are documented separately.

• Weight and caliper. A coating adds mass and can change how a sheet is calendered, so two sheets of equal grammage may differ in thickness and bulk. These are described in the paper weight and caliper entry and measured via ISO 536 and ISO 534.
• Opacity and show-through. The pigments used in coatings (and internal fillers) scatter light and raise opacity, which reduces how much printing is visible from the other side. See the opacity and show-through entries; opacity is measured per ISO 2471.
• Brightness and whiteness. Coating pigments strongly influence measured brightness (ISO 2470) and whiteness, which in turn affect apparent contrast and color rendition, as discussed in the brightness and whiteness entry.
• Dimensional stability and curl. A coating changes how the sheet takes up and releases moisture, and asymmetric coating or moisture response is one factor associated with the paper-curl defect, which is documented in its own entry.
• Print mottle and show-through. Uneven ink holdout or uneven surface absorption can contribute to print mottle, another defect covered separately.

In each case, the coated/uncoated character of the medium is a contributing variable, while the defects and the individual properties are described in full in their own reference pages to avoid duplication here.