Why is the inside of a shell so hard to break, when it's made of the same stuff as chalk?
― Shells make cracks take a "detour"
The inside of an abalone shell has a layer that shimmers in rainbow colours. About 95% of it is calcium carbonate, the same material as blackboard chalk. Yet breaking it takes more than 1,000 times the energy needed to break a crystal of the same substance. The secret is not the material but the way it is stacked.
A stick of blackboard chalk snaps with a light press of your fingers. The broken face is clean and flat.
An abalone shell picked up on the beach is another matter. Try to bend it by hand and it won't budge. Its inside flashes rainbow colours as you change the angle.
Both are mostly the same substance, calcium carbonate. So why can a shell be made hard to break when the version people make snaps so easily?
There are just two reasons
Nacre is made of thin plates, each about 0.5 micrometres thick, stacked thousands of layers high with each layer offset from the one below. A crack can't run straight. It is forced to zigzag along the gaps between plates.
Between the plates sits a thin layer of protein. Under a strong force, the plates slide a little at a time and the glue stretches like threads. The force doesn't pile up in one spot. A wide area takes the load.
To make hard ceramics, people use kilns hotter than 1,000°C. Even so, ceramics break when dropped. A shell builds a tougher material at about the temperature of seawater. The difference lies not in the temperature or the material, but in the design.
A crack that can't run straight wears itself out
Breaking means creating new broken surfaces. Each new surface costs energy, because bonds between atoms must be cut. So the longer the crack's path, the more energy it takes to break the material.
In a single solid block like chalk, a crack races along the shortest path, in a straight line. In nacre, though, when a crack hits a plate it has no choice but to turn sideways along the gap. Look at Figure 1. In the chalk on the left, the crack is a single straight line. In the nacre on the right, it is a zigzag staircase.
The plates in real nacre are very flat, with a width 10 to 20 times their thickness. With that shape, the crack's path comes to about 9 times the length of a straight break. It works much like a brick wall or concrete-block wall, where the joints are staggered from row to row.
The glue stretches, and the plates slide bit by bit
But the detour alone gives only about 9 times. Most of the rest comes from the second reason. Between the plates is a layer of "glue" made of protein and other substances, only a few tens of nanometres thick. By weight, it is just a few per cent of the whole shell.
When a strong force hits the shell, the plates slide past one another a little at a time. As they do, the glue stretches like threads and holds the plates together. The plate surfaces also have fine bumps that catch as the plates slide, adding resistance. As Figure 2 shows, when one plate slides, the plates around it are dragged along a little too.
In chalk, force piles up at the tip of the crack, and the break spreads from there all at once. In nacre, many gaps stretch a little and share the force. The tip of the crack is, in effect, put under "brakes". As a result, the energy needed to break nacre is thought to be 1,000 to 3,000 times that for a crystal of the same material.
The thickness of nacre's plates is about the same as the wavelength of visible light. Light bouncing off the top and bottom of the stacked plates strengthens in some colours and cancels in others, so the colour changes with your viewing angle. A pearl's lustre comes from the same layers, wrapped into a ball. One structure gives both toughness and beauty.
Human bone is also made by combining hard mineral grains with flexible protein fibres. A material that is only hard breaks easily, and one that is only soft can't hold its shape. By combining the two on a fine scale, living things are thought to get the strengths of both.
Summary
Nacre takes the same calcium carbonate as chalk, makes it into thin plates, and stacks them like bricks. Cracks are forced to detour through the gaps, and the glue in the gaps stretches to take up the force over a wide area. Without changing the material, only the arrangement, shells make themselves tougher by several orders of magnitude.
Hardness is set by the material; toughness is set by the stacking.
A shell is a designer that forces cracks to take the long way round.
For how an eggshell spreads force through its shape, see Why can't you crush an egg by squeezing it in your hand? For the same kind of thin-layer light, see Why are sardines and cutlassfish silver? and Why does opal, a stone, shine in rainbow colours?
- Compare the inside and outside of an abalone or turban shell. Check that only the inside shimmers in rainbow colours, and that the colours move as you change the angle.
- If you have a broken piece of shell, look at the broken edge through a magnifying glass. You may see fine layers piled up like the growth rings of a tree. The edges are sharp, so wear work gloves.
- Soak a small piece in table vinegar and bubbles appear. This is a sign that the calcium carbonate is dissolving and giving off carbon dioxide. After a few days, a soft, thin film may be left. It is what remains of the "glue" that held the plates together.
If you put a piece of chalk in vinegar in the same way, bubbles appear but no film is left. That difference is the difference in toughness. (The make-up of chalk varies by product, and chalk made of gypsum gives off no bubbles.)
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from lower-secondary science to university specialist courses
- Middle schoolCovered in lower-secondary school science
- High schoolCovered in upper-secondary "Chemistry", "Physics" and "Biology"
- High school+Advanced upper-secondary material, or textbook sidebar topics
- UniversityUniversity specialist courses not taught in high school (mechanics of materials, fracture mechanics, biomaterials)
- ResearchNot yet taught even at university as settled fact; questions researchers are working on now
Middle schoolTerms: this phenomenon has names
- Nacre: the rainbow-shimmering layer inside a shell. Thin plates of aragonite, a form of calcium carbonate, alternate with layers of protein and other organic material.
- Calcium carbonate: the main component of limestone, marble, chalk, eggshell and seashell. It dissolves in acid and gives off carbon dioxide.
- Fracture toughness: a measure of how well a material resists the spread of a crack once one exists. It is a different thing from "hardness".
Middle schoolHigh schoolChecking with a formula: how many times longer is the crack's path?
The energy needed to break something is proportional to the area of the new broken surface. So we calculate how many times longer the crack's path becomes with the brick-like stacking, and compare it with the measured toughness.
| In symbols | E ＝ γ × A n ＝ 1 ＋ w ÷ （2 × t） |
| In words | Energy to break ＝ energy per 1 m² of surface × area of broken surface. Path multiplier ＝ 1 ＋ plate width ÷ (2 × plate thickness) |
| Where the formulas come from | Griffith's idea that breaking requires the energy to create new surface. The multiplier comes from the shape in Figure 1: for each row descended (thickness t), the crack detours sideways by half a plate (w ÷ 2) |
| Symbol | Meaning and unit |
|---|---|
| E | Energy needed to break (joules) |
| γ | Energy needed to create 1 m² of broken surface (joules per square metre) |
| A | Area of the broken surface (square metres) |
| n | How many times longer the crack's path is than in a straight break |
| w | Plate width (micrometres) |
| t | Plate thickness (micrometres) |
| Plate thickness t | About 0.5 μm |
| Plate width w | About 5–10 μm (here 8 μm) |
| Measured toughness (energy needed to break) | About 1000–3000 times that of a crystal of the same material |
| Thickness of the nacre layer in an abalone shell | Taken here as 1 mm (1000 μm) |
| Twice the plate thickness | 2 × 0.5 ＝ 1 |
| Divide the width by that | 8 ÷ 1 ＝ 8 |
| Path multiplier n | 1 ＋ 8 ＝ 9 |
| Part of the measured value (taking the low end, 1000 times) that the detour can't explain | 1000 ÷ 9 ≒ 111 |
| Number of plate layers stacked in a 1 mm layer | 1000 ÷ 0.5 ＝ 2000 |
The detour alone raises toughness only about 9 times. The remaining gap of roughly 100 times is thought to be covered by the work of the plates sliding and the glue stretching (reason 2). And in a shell 1 mm thick, about 2,000 layers of these plates are stacked up.
High schoolHigh school+Why "hard" and "hard to break" are different things
High schoolCalcium carbonate is an ionic crystal. When an ionic crystal is forced and its rows of ions slip, ions of the same charge face each other and repel, and the crystal splits cleanly along that plane (cleavage). Unlike a metal, whose atoms can slide and change shape, it has no way out, so it is hard but brittle.
High school+In nacre, the crystal itself stays brittle. But by dividing the crystal into small plates and putting soft organic material in the gaps, the material as a whole gains a way to give, deforming as it slides. A material that combines hard and soft parts is called a composite.
UniversityCrack deflection and plate pull-out
In fracture mechanics, how easily a crack spreads is described by the stress intensity factor and the energy release rate, and a material's resistance is called fracture toughness. In nacre, the main toughening mechanisms are thought to be "crack deflection", where the crack is turned aside at plate boundaries, and "plate pull-out", where plates are drawn out while the organic layer stretches. Nacre is also known to show "R-curve" behaviour, in which resistance rises as the crack grows. There is an optimal aspect ratio for the plates, and shear-lag models have been used to discuss how plates that are too slender break themselves.
📖 Derivations of the formulas and further reading: Fracture toughness (Wikipedia, in Japanese) ／ Nacre (Wikipedia, in Japanese)
ResearchWhat is still not clear
- How does a shell make the plates so uniform? A protein framework is thought to set the orientation and thickness of the crystals, but the detailed steps are still being studied.
- Which mechanism does how much? Estimates of the share from the bumps on plate surfaces, the thin mineral bridges linking plates, the stretching of the organic layer and so on vary with the way they are measured.
- Can it be copied artificially? Research on nacre-inspired ceramics and glass has advanced, and some versions are reported to be far tougher than the original material. But no one has yet found a way to make it at room temperature, as large and as fast as a shell does.
So what this article says is an "explanation within what is known today". The numbers also vary with the species of shell and how wet it is, and dry nacre is said to break more easily than wet nacre.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: acids and alkalis, the reaction of limestone with hydrochloric acid | Bubbles in vinegar |
| High school | Chemistry: properties of ionic crystals | Why it is hard but brittle |
| High school+ | Physics / chemistry: composite materials, thin-film interference | Role of the glue, rainbow light |
| University | Mechanics of materials, fracture mechanics, biomaterials | Crack deflection, plate pull-out |
| Research | Biomineralisation, bio-inspired materials design | How plates are made, artificial nacre |
| ― | Links to daily life | Joints in brick walls, strength of bone |
- Jackson, A. P., Vincent, J. F. V., Turner, R. M. (1988) The mechanical design of nacre. Proceedings of the Royal Society B 234: 415–440
- Currey, J. D. (1977) Mechanical properties of mother of pearl in tension. Proceedings of the Royal Society B 196: 443–463
- Barthelat, F., Tang, H., Zavattieri, P. D., Li, C.-M., Espinosa, H. D. (2007) On the mechanics of mother-of-pearl: A key feature in the material hierarchical structure. Journal of the Mechanics and Physics of Solids 55: 306–337
- Wegst, U. G. K., Bai, H., Saiz, E., Tomsia, A. P., Ritchie, R. O. (2015) Bioinspired structural materials. Nature Materials 14: 23–36
- Wikipedia, "真珠層" (Nacre, in Japanese)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work. The broken edges of shells, and the edges of pieces dissolved in vinegar, can be sharp, so take care not to cut your hands when observing.