The short answer
Paper is generally considered porous. It is made of a tangled network of plant fibers, and there are tiny gaps between those fibers that liquids and air can move through. Some papers have coatings, wax or plastic layers that make the surface much more resistant to liquids, but even then the paper underneath is still fibrous.
What does porous mean?
A porous material has small openings, or pores, that allow liquid or air to pass through or be absorbed. A nonporous material does not have openings like this and blocks liquids and air, at least at the surface and under ordinary conditions.
Strictly speaking, few materials are perfectly nonporous at a microscopic level, and technical definitions vary by field. In everyday use, though, people sort materials into “porous” (absorbs) and “nonporous” (does not absorb).
Why paper is porous
Paper is made by spreading wood or other plant fibers into a thin mat and drying it. The fibers lie in a criss-cross pattern, leaving a labyrinth of small air spaces between them. When a drop of water, ink or glue touches the sheet, the fibers draw it in by capillary action, which is the same effect that lets a paper towel soak up a spill.
Other things affect how much a sheet absorbs:
- Sizing. Many writing and art papers are treated with sizing, a material that makes the surface less absorbent so ink and paint sit on top rather than sinking in. Without any sizing, ink tends to spread and blot, as with blotting paper or tissue.
- Fiber density. Tightly pressed papers absorb more slowly than loose, fluffy ones.
- Thickness. Thicker sheets can take more liquid before it soaks through.
- Surface smoothness. Smooth surfaces tend to give sharper lines and details.
For more on how paper is made, see our article on why paper looks white, which also covers fillers and coatings.
Coated, glossy, waxed and laminated paper
Not all paper behaves the same way.
- Glossy or coated paper, like many magazine pages and photo papers, has a thin mineral or polymer layer that slows absorption. Liquids may bead up or dry slowly instead of soaking in.
- Waxed paper and plastic-laminated paper resist liquids much more, because wax or plastic blocks the pores.
- Parchment or baking paper has a surface treatment that resists moisture, which is why it is used in kitchens.
Even with these treatments, the cut edges of the paper can still absorb liquid, and heavy or long exposure can damage the sheet. So “coated” does not mean “waterproof.”
Porous vs nonporous examples
Commonly porous: paper, cardboard, untreated wood, fabric, unglazed ceramic, sponge, cork, unsealed stone.
Commonly nonporous: glass, most metals, most hard plastics, glazed ceramic, sealed or varnished surfaces.
People often ask whether plastic is porous. Most hard plastic is treated as nonporous for practical purposes, although some flexible plastics and foams are an exception. As for the “least porous” material, it depends on how you measure, but dense materials such as glass and many metals are typically given as examples of very low porosity. It is a common misconception that every hard surface is nonporous; some stone, concrete and tile can absorb liquid.
Why it matters for crafts
- Ink and markers. On absorbent paper, ink can feather or bleed through. Choose smooth, sized paper for fine lines.
- Watercolor and wet media. Thin or unsized paper buckles when wet. Use paper made for wet media.
- Glue. Wet glue can cause thin paper to wrinkle or show through. Use thin layers and let it dry flat under a light weight.
- Sealing finished projects. Decoupage and papier-mache use glues or sealers to make paper more resistant to liquids. Follow the directions on the product label and work with ventilation if the label recommends it.
- Printing. Printers and inks work best on paper matched to the ink type. Our guide to craft paper types helps with matching.
A quick absorbency test
Place a tiny drop of water on a hidden corner or scrap and watch it. If it spreads and soaks in quickly, the paper is highly absorbent. If it beads up or sits on the surface for a while, the paper is coated or well sized. Test your pens in the same way before starting a full project.
Safety
- Use any sealers, varnishes or spray adhesives only as the label directs, in a ventilated space.
- Keep craft glues, sealers and small paper pieces away from young children and pets.
- Cut with sharp tools carefully and keep them out of children’s reach.
FAQ
Is paper waterproof? Not on its own. Treated papers can resist water for a time, but they are not fully waterproof.
Is cardboard porous? Generally yes, as it is also made of fibers. See how cardboard is made for the structure.
Is notebook paper porous? Yes, though many notebook papers are sized to reduce bleeding. Fountain pen and gel ink can still show through on thin pages.