How to Harden Urushi Lacquer.
It is very important to master this key step, too often neglected in all Kintsugi processes.
The essential element to start is the muro necessary to harden the lacquer.
In Japan, Kintsugi craftsmen and lacquerers place their lacquered and moistened objects on large wooden shelves called Muro (piece) to retain all the humidity during hardening. The muro also has a dust cover function.
You should know that lacquer hardens (does not dry) and becomes hard thanks to oxygen, high humidity, and appropriate temperature. It is a natural chemical reaction.
Urushi lacquer hardens when the laccase enzyme in the mugi-urushi and sabi mixture absorbs oxygen from the moisture in the air. It oxidizes and polymerizes the urushiol, thus hardening the lacquer.
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The environment must be appropriate and supervised. The ideal is a temperature between 24°C - 28°C with a humidity of 70% ± for 24 hours. These criteria are essential because if they are not scrupulously respected, the lacquer will not be able to harden, or it will present problems such as poor coating or shrinkage (too high humidity). The space can be a cabinet, a wooden or polystyrene box called Muro, Furo or Urushi buro. For objects that are too large, there are whole pieces adapted as a drying space and called Urushi-Muro.
Of course the muro should be checked or designed first before any kintsugi preparation.
Create your humidified space / simple muro.
1. In a box large enough to hold a bowl, line the bottom and edges with plastic, then put newspaper or a cloth over it.
2. Moisten the newspaper (or cloth, towel) with water using a spray bottle.
3. On the wet newspaper (or linen, towel) put a board or polystyrene to create a support.
4.Place your bowl and the moisture meter next to it. Watch the temperature which should not exceed 24° degrees and the humidity 70%.
5. Close the box and place a relatively light object on top to make a weight. Be careful not to completely close the box, the air must circulate.
6. Place your box in an appropriate place depending on the season. During the winter it is recommended to place the muro in a warm room. The rainy season is more conducive to the hardening of the lacquer. You should also avoid places that are too cool (cellar) or too hot.
Regularly monitor the temperature and humidity of your Muro.
Notes:
For a large number, choose a shape that corresponds to the size and number of pieces to be hardened and to accentuate the hardening I add a slight heat input.
If you want to go further and work several ceramics with different hardening stages, I recommend a polystyrene box with a lid, equipped with a small heat source created by a lamp (no led) with 15w / 25w filament. Depending on the season, it adapts by increasing or decreasing the heat. You can add an adequate bowl filled with water with a thermo-hygrometer. During summer, I recommend turning off the lamp and regularly monitoring the humidity level. It's all about adapting to the seasons.
The classic values for kintsugi are ± 70% relative humidity and between 25°C and 39°C maximum.
WARNING - entry level hygrometers are not very accurate. Expect ±10% for humidity.