Muro (The Curing Box)

– Kintsugi

Muro is a high humidity curing box for kintsugi. The character for muro (室) means greenhouse or cellar. But muro (ムロ) also means a closed room that protects bonsai plants in the winter and prevents their pots from cracking. It’s meant to be ventilated during the daytime, which I’m taking as a cue to ventilate my kintsugi muro daily. Otherwise, it might be susceptible to mildew.

Ideal Conditions

Urushi lacquer won’t cure without the proper temperature and humidity. High temperature and humidity is good—but how warm and how humid? I’ve compiled several temperature & humidity ranges from various sources below.

Temperature Humidity Source
68-77º F 70-85% Starbucks Kenpo
68-86º F 70-85% Kintsugi Library
>82º F 80% Kintugi.com

I’d recommend at least 68º F (20º C) and 70% RH based on these recommendations.

Depending on the season and the humidity in your home, your muro could be quite unique. One article I read recommended working on kintsugi in the wet & rainy season in Japan. At other times, you can construct a space in your bathroom for curing, using the warmth and humidity to your advantage.1 I imagine it’s a bit like caring for orchids or other tropical plants. That is, if your area is humid, you probably don’t need to supplement with a wet towel at all.

Graph of Tokyo humidity from weather-and-climate.com

In Tokyo, the humidity fluctuates from ~45% to ~78%. Look up the humidity in your area and adjust accordingly.

Muro Construction

Cardboard

The most common DIY muro I’ve seen has been a cardboard box lined in plastic wrap, with a loose moist cloth wrapped around the object to be cured. Make sure the cloth doesn’t touch the lacquer, and weigh down the lid with a heavy weight.2 If the box is tall enough, you can also raise the lacquered piece on a tray or colander.

Styrofoam

David Pike of kintugi.com recommends poking a hole in the top of styrofoam cooler to stick a light bulb inside.3 The light bulb serves to warm up the box, which reminds me of a warming light bulb for pet reptiles.

I tried this method and the hole I cut was not precise enough. As a result, the bulb kept falling through the lid. I ended up just leaving the bulb on the bottom of the box, but over time I stopped using the bulb entirely because the temperature in my home was above 68ºF. I also found it bothersome to have a light in there, as it limits the amount of pieces I can put in the box. If you try this, use an LED bulb—when I used a tungsten bulb I smelled something burning.

How long?

For underlayers and steps that will be covered up, it’s safe to scrape it a bit with a sharp tool or a q-tip to test it.2 Give it a couple days before testing.

For Mugi-Urushi and your final coat of urushi, however, this isn’t possible. For these situations, you have two options:

Make Test Strips

Add some of the same lacquer recipe to a throwaway material, such as a plastic tofu box or a piece of paper.4 Leave it in the muro alongside your ceramic piece and check the test strip after a couple days. Make sure the paint on both your kintsugi object and test strip is a similar thickness to ensure they cure in the same amount of time.

You can also try swatch testing with a timer and poking the swatches to test them.

Wait A Long Time

If you don’t want to use test strips, you can also wait 2 weeks5 or longer6 between each layer, just to be sure. For my first project, I only waited 3 days between each layer, and they didn’t cure properly. This caused the whole thing to warp when I attempted to use the bowl for the first time (See: Common Mistakes in Kintsugi 101).

Mold & Mildew

Swap out the damp towel every day! A study on wet fabric mildew observes fungal growth is unavoidable at humidity over 9%.7 Even without your muro, it’s recommended to keep rooms at 30-60% relative humidity, so you can how the problem would be exacerbated inside a sealed box.8 Dark warm places with low air circulation cause fabric mildew within 24-48 hours.9

Check your box frequently for mold/mildew, and give it a good ol’ sniff test to see if it needs cleaning. Ventilate the box every day, just like a bonsai muro. 🌲


Disclaimer

I am not fluent in Japanese. Any and all translations may be flawed. I’m just stumbling through these resources, tracing kanji into Google Translate, and using a dictionary app to supplement my rough understanding of the language. Forgive me for any mistakes. Thank you for reading!

Sources

  1. Precious.jp – Urushi-san (JP)

  2. Kintsugi Library Youtube #1 (JP)

  3. Kintugi.com by David Pike (EN)

  4. Tokyu Hands (JP)

  5. Starbucks Kenpo – My Wellness (JP)

  6. WorthPoint – Article by David Pike (EN)

  7. Marsh, Paul B. “Mildew-and rot-resistance of textiles.” Textile Research Journal 17, no. 11 (1947): 597-615.

  8. CDC: Indoor Environmental Quality

  9. Quora

Here are all the notes in this garden, along with their links, visualized as a graph.