Understanding Temperature and Humidity in Urushi Curing

Introduction

One of the first things anyone working with urushi lacquer encounters is the need for a warm, humid environment in which the lacquer can cure.

Whether you are learning traditional kintsugi yourself or exploring how professional kintsugi repairs are carried out, understanding temperature and humidity is an important part of working with urushi lacquer.

You will often see recommendations such as 20–25°C and 70–80% relative humidity. These figures are useful starting points, but they do not necessarily explain what happens inside a curing box or why its conditions sometimes change unexpectedly.

For beginners, the difficulty is often not creating humidity. A damp towel or shallow container of water can accomplish that relatively easily.

The more interesting challenge is understanding and controlling the relationship between temperature, water evaporation and relative humidity.

Fortunately, this does not require expensive electronic equipment. A basic understanding of the principles, together with a simple homemade curing box and a thermometer/hygrometer, can go a long way.

1. Understanding Relative Humidity

This is where I believe some unnecessary confusion arises.

Relative humidity, usually abbreviated to RH, describes how close the air is to saturation with water vapour at its current temperature.

For example, 75% RH means the air contains about 75% of the water vapour it could hold at that temperature before becoming fully saturated (100% RH).

The important point is that relative humidity depends on temperature, not simply on how much water vapour is present.

As temperature increases, and the amount of water vapour remains unchanged, relative humidity decreases.

Conversely, when temperature falls, relative humidity increases.

This means that a curing box can experience a considerable change in RH without any water actually being added or removed.

Consider a relatively sealed curing box containing air at 22°C and 75% RH (e.g. cardboard box with some plastic liner).

If relative humidity is too high (you want it to be below 75%), there are two possible adjustments: slightly increasing the temperature or reducing the exposed evaporating surface of the water source.

Increasing temperature lowers RH if the amount of water vapour remains unchanged, while reducing the evaporating surface slows the rate at which additional moisture enters the air.

However, in a completely sealed box, continued evaporation may eventually raise relative humidity towards 100%.

In practice, most curing boxes are not perfectly airtight, allowing some exchange of air with their surroundings. Reducing the exposed evaporating surface can therefore help regulate humidity by balancing moisture entering the air through evaporation against moisture escaping from the enclosure.

2. Where Does the Humidity Come From?

In a basic curing box, humidity is usually generated by allowing water to evaporate naturally into the enclosed air.

However, two important factors determine how effectively a water source works:

• The quantity of water available, which influences how long evaporation can continue before the water runs out.

• The exposed evaporating surface, which helps determine how quickly water vapour enters the air.

Consider two examples.

Example A: A shallow tray measuring 20 × 20 cm, containing 200 ml of water.

Example B: A bottle containing 2 litres of water, but with a narrow opening measuring just 2 cm in diameter.

Although the bottle contains ten times more water, its opening exposes only a tiny portion of water surface compared with the much larger surface in the tray.

Under comparable conditions, the shallow tray would generally allow water to evaporate much more quickly.

The bottle contains a greater reserve of water, but that water cannot evaporate as readily through its restricted opening.

Of course, evaporation also depends on water temperature, air movement and how humid the surrounding air already is. As the air approaches saturation, further net evaporation slows.

The essential distinction is that water quantity provides the reserve, while exposed surface area helps control the rate at which water evaporates into the air. By adjusting the size of the exposed surface, we can therefore influence how quickly relative humidity increases inside the curing box.

This is an especially useful principle when designing a simple, inexpensive humidity-control system.

3. Why the Material of Your Curing Box Matters

Another factor that is easily overlooked is the material from which the curing box is constructed.

A plastic storage box and an unlined cardboard box can behave quite differently.

Cardboard is hygroscopic, meaning it can absorb moisture from the surrounding air and release that moisture again as conditions change.

For example, when humidity rises inside an unlined cardboard box, some water vapour may be absorbed into the cardboard rather than remaining in the air.

If the temperature subsequently increases, the cardboard may release some of its stored moisture, potentially increasing RH (instead of decreasing if you were to use a reasonably sealed box).

However, this behaviour is not guaranteed. It depends on how much moisture the cardboard contains, its temperature, the surrounding conditions and the rate at which moisture is exchanged.

In other words, the cardboard introduces an additional source of moisture exchange, making the behaviour more complicated.

For a more predictable low-tech curing environment, I would favour a plastic box or a cardboard box fitted with a moisture-resistant internal lining, such as a plastic liner.

A lining reduces direct moisture absorption by the cardboard and helps protect it from prolonged dampness.

Thermal insulation is a separate consideration. It reduces the speed of temperature changes, whereas a moisture-resistant lining reduces the exchange of water vapour with the box material.

Neither guarantees stable humidity, but both can help make the environment easier to manage.

4. Establishing Your Desired Temperature

Before attempting to fine-tune humidity, I would first establish a reasonably stable temperature.

For conventional Japanese urushi, approximately 22°C is a useful starting point, although suitable conditions vary according to the lacquer and technique.

If the surrounding room is too cold, a low-temperature heating mat controlled by a thermostat may be useful.

Some lighting systems can also produce heat, although their suitability depends on the equipment and enclosure. Any heating arrangement should be appropriate for use around moisture and monitored carefully.

The important point is to avoid uncontrolled heating.

Once the temperature is reasonably stable, humidity becomes easier to investigate.

Otherwise, you may find yourself continually adjusting the moisture source to compensate for temperature fluctuations.

5. Using a Damp Cloth to Control Humidity

One of the simplest and cheapest ways to generate humidity is a damp cloth placed in a shallow tray.

For example, imagine a small, modestly insulated cardboard curing box, preferably lined internally, maintained at approximately 22°C.

Place a damp cloth measuring 20 × 20 cm inside the box, away from the lacquered object, and monitor the hygrometer.

Allow the conditions to settle before making adjustments.

Suppose the reading reaches 90% RH, considerably higher than your intended target.

One possible adjustment is to fold the cloth in half, reducing its exposed wet surface. If necessary, fold it again.

This generally reduces the area from which water can evaporate, potentially slowing further humidification.

Alternatively, imagine the hygrometer reads only 55–60% RH.

You will need to increase the exposed wet surface or ensuring that it remains adequately damp.

The important point is to make one adjustment at a time, allow the environment to respond, and observe the result.

There is one limitation worth recognising.

A damp cloth contains a finite amount of water. Depending on the enclosure and conditions, it may provide relatively rapid evaporation initially, then gradually become less effective as it dries (with the desired RH rage lasting 1 day or 2 days).

A small or insufficiently wetted cloth may therefore require frequent attention.

This is why it can be useful to experiment with other water-retaining materials, particularly when maintaining conditions over longer periods.

6. Using Perlite, Pumice or Akadama Soil

An alternative is to use a water-retaining porous material.

Examples include:

• Perlite

• Pumice

• Akadama (semi-fired red), a granular Japanese soil

These materials contain pores and spaces that can retain water, allowing moisture to remain available for evaporation. Even a small piece of untreated wood that is not prone to developing mould quickly can serve the same purpose, absorbing water and gradually releasing it through evaporation.

They should not automatically be assumed to release water more slowly than a damp cloth. In some arrangements, their large wetted surface area may actually encourage evaporation.

Their practical advantage is that they can provide a reusable moisture reservoir whose exposure can be controlled.

For example, damp perlite could be placed in a shallow tray with a partially adjustable lid.

By adjusting the opening, the exposed evaporating surface can be changed without repeatedly removing the tray or replacing its contents.

A smaller opening generally restricts evaporation; a larger opening allows more moisture exchange.

This gives another simple method of adjustment.

The same principle can be applied using a water tray or damp cloth.

7. Opening the Box Changes the Environment

Another factor worth considering is air exchange.

When a curing box is opened, the humid internal air begins mixing with the surrounding room air.

If the room is considerably drier, the relative humidity inside the enclosure can fall.

How quickly this happens depends on the size of the opening, air movement, differences in temperature and how long the box remains open.

Opening a small section briefly will generally disturb the environment less than removing the entire lid for an extended period.

Fortunately, a suitable moisture source can help restore the humidity once the enclosure is closed.

There is no need to worry over every brief fluctuation. The goal should be reasonably consistent conditions over time, rather than attempting to maintain an absolutely unchanging reading.

8. Experiment Before Putting Urushi Inside

Before using any new curing box for actual lacquerwork, I strongly recommend testing it without an object inside.

For example, start with a target temperature of approximately 22°C and a suitable moisture source.

Then observe:

1. What RH does the box reach after the conditions have stabilised?

2. Does the humidity continue rising over several hours?

3. Does it fall as the moisture source dries?

4. What happens when the surrounding room temperature changes?

5. Does opening or closing part of the moisture tray produce a predictable response?

6. How quickly do the conditions recover after the box is opened?

For example, if you achieve 75% RH at 22°C, that is a useful observation.

However, do not assume the RH will remain at 75% throughout the day. Continue monitoring to see whether it rises towards saturation, falls because of air exchange, or fluctuates with temperature.

If RH is too high, reduce the exposed wet surface or adjust ventilation carefully. In some arrangements, leaving a very small opening may be sufficient to allow additional air exchange.

If RH is too low, check that the moisture source has not dried out, increase its exposed surface if appropriate, and consider whether excessive air exchange is preventing the box from maintaining humidity.

Keep in mind that simply adding more water to a tray does not necessarily increase the evaporation rate if its exposed surface remains unchanged.

Also remember that inexpensive hygrometers have measurement tolerances. Minor changes of one or two percentage points may reflect instrument uncertainty rather than meaningful environmental changes. However, for the purposes of this article, they are more than adequate for monitoring temperature and humidity and identifying changes in the conditions inside your curing box.

10. A Practical Starting Point

For someone constructing their first homemade curing box, I would approach it in the following order.

First, prepare the enclosure. A plastic storage box or a cardboard box with a moisture-resistant lining covering at least five of its six interior surfaces (the sixth being the opening, which can be left unlined) is a practical starting point. Ensure there is enough room for the object to sit safely, preferably above and away from the moisture source. Alternatively, if a lined box is not available or possible, an ordinary unlined cardboard box will also do. However, because cardboard absorbs and releases moisture, you may find that RH falls as temperature falls and rises as temperature rises, rather than behaving as described earlier. This is something to be aware of when monitoring your curing box.

Second, establish your temperature. Aim initially for approximately 22°C (or 20°C, 25°C, or whatever temperature your particular application requires). You can use the surrounding room temperature, a suitable thermostatically controlled heating source, or a low-wattage heat source. If your heat source cannot be thermostatically controlled, make sure you test and monitor it to ensure the temperature inside the box does not exceed your desired range.

Third, introduce a modest moisture source. This could be a damp cloth in a tray, a shallow container of water, or a partially covered tray containing a moist material such as perlite. Where possible, use a tray with an adjustable lid or cover so that you can control the exposed evaporating surface and therefore the rate at which moisture enters the air.

Fourth, close the box and monitor the results. Position a thermometer/hygrometer near the work without placing its sensor directly against the wet material or heating source.

Fifth, adjust the exposed evaporating surface. If the humidity rises too high, reduce the exposed wet surface. If it remains too low, increase the surface or check whether too much air is escaping outside the box (aka the box is not insulated enough).

Finally, observe the conditions over time. A reading of 75% RH after thirty minutes tells you very little about what may happen after several hours. Test the box over a longer period to understand how it behaves as temperatures change and moisture evaporates (e.g. overnight).

For many conventional urushi applications, approximately 20–25°C and 70-75% RH provide a reasonable initial range to investigate, although the requirements of particular lacquers and techniques may differ.

You do not need to achieve a perfect reading every minute. The more useful objective is to establish conditions that remain within a suitable range for the intended work.

Final Thoughts

Creating a suitable curing environment for traditional urushi does not need to be complicated or expensive.

A simple box, a moisture source and a thermometer/hygrometer can provide a useful starting point.

The key is understanding that temperature and relative humidity are connected, and that the amount of exposed wet surface influences the rate at which moisture enters the air.

The enclosure itself also matters. Cardboard can absorb and release moisture, insulation can slow temperature changes, and air exchange affects how much humidity the box retains.

My approach is therefore straightforward: establish the temperature first, introduce a controlled moisture source, and then adjust its exposed surface gradually while monitoring the results.

Rather than attempting to reproduce someone else's setup exactly, experiment with your own enclosure until you understand how it behaves.

There is no need to begin with an expensive electronic humidity-control system.

What matters is understanding the basic principles, observing the results and learning how to maintain suitable conditions consistently.

Privacy Policy

Contact