Urushi (Japanese lacquer) absorbs moisture from the air and its main ingredient, urushiol, hardens through hardening and polymerization. This is called “drying” or “hardening” of urushi.
When the temperature is 25~30℃ and the humidity is around 70%, the urushi hardens most quickly, but if it hardens too quickly, the surface hardens first and the inside does not harden, resulting in wrinkles. This is called “shrinkage”. I got something like a good example of shrinkage earlier than I had expected unfortunately.

Time for hardening

In general, urushi that hardens after eight hours is ideal.
When I asked Mr. Kiichiro Masumura[1], a living national treasure, “How many hours do you take to harden urushi?”, he said to me, “I put a piece in a muro with no humidity for 8 hours and let it harden.”
So I try to use urushi that hardens after eight hours. If I want to finish a piece without polishing or if I want to express vivid colors, I adjust urushi so that it hardens in several hours longer.

Boiled urushi

There is a type of urushi sold that hardens slowly, so we can adjust the hardening speed with it. If we do not have that type of urushi, we can adjust the hardening speed by heating urushi and boiling it to make a urushi that does not harden and mixing it with regular one.

The method I learned at the Kagawa Lacquer Art Institute is to put regular urushi directly into a bowl, place it on a net and heat it over an open gas flame, slightly away from the flame. It can also be heated in a microwave oven. The disadvantages are that it cannot be stirred during heating and the smell remains in the microwave oven for several hours. For very small quantities, it is also possible to put it on a small plate covered with plastic wrap and heat it. When the whole mixture boils, the boiled urushi is ready. The boiled one will not harden even after several days.

If the boiled urushi is heated further, it will harden, and this property can be used to apply urushi onto metal. [2]

Mixing and testing

Considering the climate and the condition of urushi, the ratio of the boiled urushi to be mixed is determined.
To check the hardening time, the mixed urushi is placed on a plastic plate or plastic wrap about 3 cm square, with a spatula to harden. If it hardens well, it is used for a piece, and if it shrinks or dries too slowly, the urushi is adjusted again.

If urushi hardens too slowly for fear of shrinkage, the sides of a three-dimensional piece will sag due to gravity. To prevent this, the piece is sometimes turned upside down every few hours, which requires a mechanism to rotate the piece.

This time I spent a week experimenting with the preparation of urushi. I finally used a well-prepared urushi and applied it thinly, but it shrank. Probably because it was old urushi and the ingredients were condensed, even if I applied it thinly, it was as if I had applied it thickly. By diluting the urushi with a solvent, it finally became usable as normal. If I had diluted it from the beginning, it might not have taken a week of trial and error.
If I use diluted urushi when painting bowls or other vessels that directly hold food, the smell of the solvent will remain on the vessel, so I do not dilute urushi when painting these vessels.

Treatment of a shrunken surface

The shrunken surface was peeled off with a spatula by applying acetone, and the hardened areas were removed by grinding. Even though the surface appears to have hardened, the lacquer applied over it will also shrink again, as if pulled, so all the shrunken areas should be removed.

It is a barren process, wasting time, effort, and urushi. A urushi that was applied normally yesterday may shrink today due to climate changes, and there are also areas that are prone to shrinkage, such as the inside corners of the box, even though I am very careful.

I am always nervous when I open the lacquer bath to check the hardening of the lacquer.

References

1.	List of Kiichiro Masumura's Works - Japan Kogei (art crafts) Association
2.	Toshio Kinoshita, Hiroshi Ueno, Toshikatsu Nakazato, Seiko Miyata. (1998). "Study on Traditional Burnt urushi Technique". National Research Institute for Cultural Properties, Tokyo(PDF file)

All of them are in Japanese.

2024.11.28