Why Can an Eraser Rub Out Pencil Marks?
― An Eraser Wears Itself Away to Wrap Up the Marks
An eraser doesn't "dissolve" the writing, and it doesn't "suck it up" either. A pencil mark is just black dust sitting on top of the paper. The eraser sticks that dust to its surface, then sheds a bit of itself, carrying the dust away as crumbs.
You make a mistake in your notebook and rub it out with an eraser. The black mark vanishes cleanly, and dark crumbs pile up on the desk.
But writing on the same page in ballpoint pen won't come off no matter how hard you rub. The paper just gets fuzzy and starts to tear.
Both are marks on paper — so why does only the pencil come off? And why does the eraser itself get smaller every time you use it?
It comes down to just two reasons
Pencil lead is made of graphite, a form of carbon built from many thin sheets stacked together. Rubbing peels off these sheets, which catch on the rough fibres of the paper. They never soak into the paper itself.
An eraser's surface is soft and slightly sticky. It grips the black dust more strongly than the paper does, and that grabby bit of surface tears away as a crumb. A fresh, clean surface appears underneath.
So an eraser is less "a tool that removes marks" and more "a tool that wins a tug-of-war over the dust." And after winning, it throws the dust away along with a bit of itself. Let's look at each step.
Where exactly is the pencil mark?
The main ingredient of pencil lead is graphite. In graphite, carbon atoms form flat, honeycomb sheets, and those sheets stack up like a deck of cards. The bond between one sheet and the next is very weak.
When you press the lead against paper and drag it, these sheets slide right off. The thin flakes that peel away get rubbed into the rough fibres of the paper and snag on the surface. That's what a pencil mark actually is.
Look at the left side of Figure 1. The black dust just sits "on top of" the gaps between fibres — it hasn't worked its way inside the fibres. All that holds the dust to the paper is mechanical snagging on the rough surface, plus a very weak attractive force that appears whenever two surfaces sit close together.
The ballpoint ink on the right of Figure 1, though, is a liquid. It soaks into the gaps between paper fibres and dyes the fibres themselves. Once that happens, rubbing the surface only shifts colour around — you'd have to scrape away the fibres themselves to remove it. That's why ballpoint writing won't erase.
How does the eraser grab the dust?
When you rub an eraser on paper, its soft surface moulds closely to the paper's rough texture. The graphite flakes stick to the eraser more strongly than they stick to the paper — it's like a tug-of-war over the dust between the paper and the eraser.
But if the surface kept holding onto the dust forever, you'd end up rubbing the paper with that black surface instead. That would just smear the mark and make things worse.
So an eraser is deliberately made to "wear away" easily. The dust-covered surface tears off as you rub, curling into a crumb. As shown in Figure 2, the crumb rolls off the paper, still wrapped around the black dust. Underneath, a fresh, unsoiled surface appears.
An eraser shrinking every time you use it isn't a flaw — it's the whole mechanism. If you tried to do the same job with a hard rubber that didn't wear away, black dust would just pile up on the surface and smear the mark wider.
Most white erasers today are made of PVC (polyvinyl chloride) resin mixed with an oily softening ingredient called a plasticiser. This ingredient gradually migrates into any other plastic it touches. That softens the other plastic's surface, so it sticks as if melted. The paper sleeve on an eraser exists partly to prevent this.
The English chemist Joseph Priestley is said to have written, around 1770, that rubber could rub out pencil marks. Before that, people rolled up soft bread and used it to erase writing. It's said that the English word "rubber" for eraser comes from this discovery — something you "rub" with.
Summary
A pencil mark is just flakes of easily-shed graphite sitting on the paper's surface. An eraser grips those flakes more strongly than the paper does, then tears away a bit of its own surface as a crumb to carry them off. Colour that has soaked into the fibres, like ink, can't be removed this way.
An eraser isn't erasing the writing.
It's picking it up — and throwing itself away with it.
For why pencil graphite peels away so easily in the first place, see "Diamond and Pencil Lead Are Both Carbon — So Why Are Their Hardnesses So Different?" And for the odd properties of rubber, see "Why Does a Rubber Band Get Warm When You Stretch It?"
- Draw lines of the same pressure on white paper with a soft pencil (like a 2B) and a hard pencil (like a 2H), and compare which is easier to erase.
- Look at eraser crumbs with a magnifying glass. Check whether the black dust is only on the surface, or rolled up inside the crumb too.
- Compare erasing with a grimy, blackened corner of an eraser versus a fresh surface cut with a craft knife. On the dirty surface, the mark may smear instead of disappearing.
Softer, darker pencils leave more graphite on the paper, so see for yourself whether they take more passes to erase. Use a craft knife only with an adult nearby.
Want to know more? — terms, formulas, and links to the school curriculumWe label each part by level, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school chemistry or physics
- HS+Advanced high-school material, or textbook sidebar content
- Univ.Not taught in high school — university-level specialist material (interface chemistry, polymer materials, tribology)
- ResearchNot yet settled textbook fact even at university — an active research question
MSTerms: this phenomenon has names
- Graphite: A material in which carbon atoms form flat honeycomb sheets that stack together. The bond between sheets is weak, so they peel apart easily. The main ingredient of pencil lead.
- Wear (abrasion): The gradual scraping away of a surface caused by rubbing. Eraser crumbs form through the eraser's own wear.
- Plasticiser: An ingredient mixed into resin to soften it. It gives plastic erasers their softness, and can migrate into other plastics.
MSHSCheck it with a formula: how much graphite sits on one pencil line?
We can estimate the amount of graphite an eraser carries away from the size of the line and graphite's density. Since the line's thickness isn't fixed, we'll assume 0.5 micrometres here for the calculation.
| In symbols | m = ρ × L × w × t , N = t ÷ d |
| In words | Weight of graphite = density × line length × line width × line thickness. Number of stacked sheets = line thickness ÷ spacing per sheet |
| Where the formula comes from | Weight = density × volume (the definition of density from middle school). Treat the line as a thin rectangular block and get its volume by multiplying length, width and thickness |
| Symbol | Meaning and units |
| m | Weight of graphite on the line (g) |
| ρ | Density of graphite (g/cm³) |
| L, w, t | Length, width, thickness of the line (cm) |
| d | Spacing between graphite sheets (nm) |
| Density of graphite | about 2.2 g/cm³ |
| Line length · width | 10 cm · 0.05 cm (0.5 mm) |
| Line thickness (assumed) | 0.00005 cm (0.5 micrometres = 500 nm) |
| Spacing between graphite sheets | about 0.335 nm |
| Area of the line | 10 × 0.05 = 0.5 cm² |
| Volume of the line | 0.5 × 0.00005 = 0.000025 cm³ |
| Weight of graphite | 0.000025 × 2.2 = 0.000055 g |
| Converted to milligrams | 0.000055 × 1000 = 0.055 mg |
| Number of stacked sheets | 500 ÷ 0.335 ≒ 1490 sheets |
| Fraction of a 1-yen coin (1 g) | 1 ÷ 0.000055 ≒ 18000 |
The graphite on a 10 cm line weighs only about 1/18,000th of a 1-yen coin. Even so, it's roughly 1,500 sheets thick, which is enough to look black. This tiny amount of dust is about what an eraser wraps up in a single pass.
HSHS+Why does graphite peel off, and why does it stay on paper?
HSInside graphite, carbon atoms form hexagonal sheets held together by covalent bonds. Between one sheet and the next, though, there's no covalent bond — only a weak intermolecular force (van der Waals force) holds them together. This lets the layers slide apart easily, which is why pencil lead can "paint" onto paper.
HS+The graphite flakes that transfer to paper are held in place mainly by that same intermolecular force and by mechanical snagging on the rough fibres. Because an eraser is soft, it can press into the paper's fine texture and increase its contact area with the flakes. More contact area means a greater total intermolecular force, letting the eraser pull the dust away from the paper.
Univ.The science of adhesion and wear
In interface chemistry, "which side the dust sticks to" is compared using the work of adhesion — the work needed to peel two surfaces apart. The work of adhesion can be found from surface and interfacial energies via the Dupré equation. The eraser's transformation into crumbs is what tribology calls rubber wear, known to proceed through wrinkle-like waves (Schallamach waves) that form on the surface and tear away. An eraser's softness comes from the plasticiser lowering PVC's glass transition temperature below room temperature.
📖 For the derivation of the formula and further reading: Van der Waals force (Japanese Wikipedia) ／ Plasticiser (Japanese Wikipedia)
ResearchWhat's still not fully understood
- The breakdown of forces holding graphite to paper. How much comes from mechanical snagging versus intermolecular force varies with the type of paper and its moisture, and is said to be hard to measure separately.
- The fine mechanics of rubber wear. Under what conditions a soft material tears into crumbs of what size is still being studied through both experiment and computation.
- Where the crumbs end up. Researchers are looking at how much ends up in the environment as tiny plastic particles, its effects, and the possibility of switching to plasticiser-free materials.
In other words, even this article's content is "the best explanation we have for now." Even an everyday piece of stationery still holds physics and chemistry we haven't fully worked out.
Links to the school curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Year 1 science, "Materials around us" (density) | Check it with a formula (weight = density × volume) |
| HS | Basic chemistry, "Covalent crystals · intermolecular forces" | Why graphite's layers peel apart so easily |
| HS+ | Chemistry, "Polymer compounds" | Eraser softness and plasticisers |
| Univ. | Interface chemistry · tribology | Work of adhesion, rubber wear |
| Research | Surface science · environmental chemistry | Breakdown of adhesive forces, fate of eraser crumbs |
| ― | Everyday connections | Why ballpoint pen won't erase, why it sticks to pencil cases |
- Eraser (消しゴム, Japanese Wikipedia)
- Plasticiser (可塑剤, Japanese Wikipedia)
- Van der Waals force (ファンデルワールス力, Japanese Wikipedia)
- Henry Petroski, The Pencil: A History of Design and Circumstance (Shobunsha edition, 鉛筆と人間, 晶文社)
- Joseph Priestley, A Familiar Introduction to the Theory and Practice of Perspective (1770)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding. Line thickness varies greatly with paper type, pencil grade, and writing pressure, so the calculation is only one example.