Why Is Urushi Lacquer Dried in a Humid Place?
― Lacquer Doesn't "Dry". It "Sets" in Water
Laundry dries best on a clear, dry day. Yet lacquer craftsmen put freshly coated bowls into a damp box on purpose. This seems backwards, and the reason is that lacquer is not a paint that dries by losing water.
Think of the glossy red or black bowls used at New Year. Their surface is a tree sap called lacquer, painted onto a wooden bowl layer after layer.
A lacquer workshop has a wooden cabinet called a "urushi-buro" (lacquer bath). The craftsman wets the inside walls with water, puts the freshly coated bowls in, and shuts the door.
A paint that dries in a damp cabinet. It feels completely backwards. But lacquer left in a dry, airy room stays sticky for a very long time.
Humidity is needed for two reasons
Most paints dry as water or a solvent evaporates. Lacquer is different. It uses oxygen from the air. Small molecules join hands and form a net, and that net is what hardens.
The linking is done by an enzyme, a kind of protein found in lacquer. An enzyme can't work without water. In dry air the water escapes first, and the enzyme stops.
So the lacquer bath is not a room for drying. It is a room for keeping water in until the enzyme has finished its job. Let's go through it step by step.
How does lacquer harden?
Lacquer is a sap collected by cutting into the trunk of the urushi tree. One tree is said to give about a milk bottle's worth in a summer, around 200 grams.
Fresh lacquer has three main parts. The largest is an oil-like substance that hardens into a film. Next comes water, said to make up 20 to 30 percent of the whole. Last, there is a tiny amount of enzyme.
Look at Figure 1. On the left is lacquer just after it is applied. Oil-like molecules are mixed loosely with tiny droplets of water. The enzyme sits in these droplets. It uses oxygen dissolved from the air to make a "joint" on a molecule.
A molecule with a joint sticks to its neighbour. Repeat that again and again and you get the net on the right. The net no longer dissolves in water or in oil. This is what "hardened" lacquer is.
Why doesn't it harden in dry air?
An enzyme can't keep its shape or work unless it is surrounded by water. But a coat of lacquer is very thin, and in dry air the water inside escapes quickly.
If the water disappears first, the enzyme stops before it finishes linking the molecules. The net is left half-made, so the surface stays sticky for a long time.
Try changing the humidity of the room with the slider in Figure 2. The left column shows the most water vapour the air can hold. The blue part is how much it actually holds, and the empty part above is the "room left to hold more". The bigger this room, the stronger the pull on water in the film.
This is said to be why craftsmen keep the lacquer bath at about 20 to 30℃ and 70 to 85% humidity. Too much humidity is also bad. Only the surface sets quickly while the inside stays soft, and the film gets crinkled wrinkles called "chijimi" (shrinkage), like crepe fabric.
It takes about half a day to a day before lacquer stops sticking to your hand. But the net-linking reaction is thought to carry on slowly afterwards, for months and even years. This is why older lacquerware is said to be tougher. On the other hand, lacquer is weak against the ultraviolet light in sunlight, and if left outdoors it loses its gloss.
The skin rash from touching lacquer is caused by the very substance that forms the film. Once it is locked into the net, almost none of it comes out. So eating from a properly hardened lacquer bowl does not normally cause a rash.
Summary
Lacquer is not a paint that dries by losing water. An enzyme uses oxygen to link its molecules into a net, and that is how it hardens. That enzyme works only in water, so craftsmen use the damp of the lacquer bath to stop the water escaping.
It is not dry air that dries lacquer.
It is a tiny enzyme that keeps working in water.
For more on dry air and how water escapes, see Why Does Heating Make a Room So Dry? You can also see what happens as water evaporates in Why Does a Coffee Drop Leave a Ring-Shaped Stain When It Dries?
- Find a lacquered bowl or pair of chopsticks at home and drop a little water on the surface. If the water beads up and doesn't soak in, that is a sign the film has set into a net.
- Hang laundry out on a dry day and on a rainy day. You will see that the higher the humidity, the slower it dries. The lacquer bath uses this slowness on purpose.
- Check the day's humidity on a weather forecast or a hygrometer. A heated room in winter can drop to 30 to 40%. For lacquer, that is air that is too dry.
* Do not touch raw lacquer or the lacquer tree itself, as they can cause a rash. Limit your observations to finished lacquerware.
For those who want to know more ― terms, formulas and links to textbooksIt states clearly which level each part belongs to, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school chemistry and biology
- High school+Advanced high school material, or a textbook sidebar
- UniversityNot taught in high school; university specialist courses (polymer chemistry, enzyme chemistry)
- ResearchNot yet taught as settled knowledge, even at university; researchers are still studying it
Junior highTerms: this phenomenon has names
- Urushiol: the main substance that forms the lacquer film. In 1917 the chemist Toshiyuki Majima confirmed it as the main component and worked out its structure. It is also the cause of lacquer rash.
- Laccase: the enzyme in lacquer. It uses oxygen to oxidise urushiol and starts the process of molecules joining together.
- Humidity (relative humidity): the share of water vapour the air actually holds, compared with the most it can hold (the saturated vapour amount).
Junior highHigh schoolCheck with a formula: how many times faster does water escape in a dry room?
The speed at which water leaves the film depends on how much "room left to hold more" the air has. We compare the lacquer bath (80% humidity) with a heated winter room (40% humidity). Both are taken to be at 25℃.
| In symbols | E ＝ k × ( ρs − ρ ) ＝ k × ρs × ( 1 − H ) |
| In words | Water loss rate ＝ coefficient × (maximum vapour − current vapour) ＝ coefficient × maximum × (1 − humidity) |
| Where it comes from | This is Dalton's law of evaporation: the rate of evaporation is proportional to the difference in water vapour between the air just above the surface (almost fully saturated) and the surrounding air. The bigger the difference, the more the vapour spreads outward. |
| E | Speed at which water leaves the film |
| k | A coefficient set by things like wind strength (taken as the same in both rooms) |
| ρs | Saturated vapour amount (the most water vapour 1 cubic metre of air can hold, in grams) |
| H | Humidity (as a fraction; 80% is 0.8) |
| Saturated vapour amount at 25℃ | About 23 grams (per cubic metre of air) |
| Humidity in the lacquer bath | 80% (0.8) |
| Humidity in a heated winter room | 40% (0.4) |
| Vapour held by lacquer bath air | 23 × 0.8 ＝ 18.4 grams |
| "Room left" in the lacquer bath | 23 − 18.4 ＝ 4.6 grams |
| Vapour held by winter room air | 23 × 0.4 ＝ 9.2 grams |
| "Room left" in the winter room | 23 − 9.2 ＝ 13.8 grams |
| Ratio of water loss rates (coefficient k is common to both, so it cancels) | 13.8 ÷ 4.6 ＝ 3 |
In a dry winter room, water leaves the lacquer film three times as fast. The time the enzyme needs to finish linking the molecules doesn't shorten much with humidity, so only the water disappears first. The lacquer bath can be seen as a box that holds this "3 times" down to "1 time".
High schoolHigh school+Oxidation and net-like polymers
High schoolAn enzyme is a "catalyst" made of protein. It speeds up a reaction but is not changed itself. However, its shape is held by the surrounding water and temperature, so it stops working if it dries out or gets too hot. The lacquer reaction is explained as urushiol being "oxidised" by oxygen.
High school+When molecules join one after another to form a giant molecule, this is called "polymerisation". In lacquer, the molecules don't just join like a chain. The chains are also tied together sideways, forming a "network polymer". Network polymers hardly melt when heated, and they don't dissolve in solvents either. This is why lacquerware resists acids and alcohol.
UniversityA copper-containing enzyme and two-stage hardening
Laccase is a kind of oxidase with four copper atoms (a multicopper oxidase). It takes electrons from the two hydroxyl groups (the catechol part) on urushiol's benzene ring and reduces oxygen to water. Urushiol, stripped of electrons, becomes a radical, and the radicals bond to each other so polymerisation proceeds. After this enzyme stage, the double bonds in urushiol's long side chain are cross-linked by "autoxidation", a slow reaction with oxygen in the air, and the film is thought to harden further. For the evaporation of water, the starting point is Dalton's law of evaporation, which says the rate of evaporation is proportional to the difference in water vapour pressure.
📖 Derivation of the formula, and further reading: Laccase (Wikipedia) ／ Urushiol (Wikipedia)
ResearchWhat is still not clearly understood
- How it keeps hardening for years. After the enzyme stage ends, how far and how fast the cross-linking by autoxidation goes is still being studied in detail.
- The role of the gum. Lacquer also contains a few percent of polysaccharides (gum), thought to keep the water droplets stable. How they affect the toughness of the film is still under study.
- When shrinkage happens. At what humidity and thickness the surface sets first and wrinkles depends largely on the craftsman's experience, and predicting it with numbers is said to be still difficult.
So the content of this article is also "an explanation within what is known today". Chemistry is only just starting to put into words the feel for humidity that craftsmen learned through experience.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science, Year 2: "Weather and its changes" (humidity, saturated vapour amount) | The column in Figure 2, checking with a formula |
| High school | Chemistry: "Oxidation and reduction"; Biology: "Enzymes" | Reason 2, the explanation of oxidation and enzymes |
| High school+ | Chemistry: "Polymer compounds" (network polymers) | The net in Figure 1 |
| University | Polymer chemistry, enzyme chemistry | Two-stage hardening by laccase and autoxidation |
| Research | Materials science of lacquer; conservation science for cultural properties | Long-term hardening, gum, conditions for shrinkage |
| ― | Links to daily life | Caring for lacquerware (keep out of sunlight, don't put it in a dishwasher) |
* This article is a general-audience science explanation. The figures given are rough guides to help you understand the mechanism. The humidity and temperature conditions for lacquer differ by type of lacquer and by craftsman. Raw lacquer can cause a rash, so do not touch it with bare hands.