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The “celestial fabric”: Koishimaru silk

Koishimaru, which literally means “round pebblestone,” is the cute name for this silkworm that has been used in Japan since ancient times. It is the silkworm that has the thinnest thread, while also having a glossy texture and a high level of tension, without being shaggy. All of these qualities make it an outstanding material. But since the silkworms were so small and the quantity of cocoons few, the silkworms disappeared from general production for economic reasons, so that they became a legendary figure that was rarely seen. Koishimaru, with its light, soft texture, that is supple and thin yet warm, combined with its beautiful gloss, has earned it the name of being a “celestial” fabric. This is a fabric that brings out the best qualities inherit to silk.

Most of the cross-bred silkworm varieties used for silk production today were developed for the purpose of increasing yield, so the cocoons tend to be large and the thread thick. Since silk has been traded by weight, most cultivators of silkworms chose that direction. But the fabrics created through such silk are heavy, easily frayed, poor in texture, and not that comfortable, so that they are lacking in many of the qualities that makes silk so wonderful. Even if such silk makes more sense economically, it is much like the approach of a farmer who sacrifices flavor and texture in pursuit of varities that produce the greatest yield. The garments made today with imported silk from China or the typical silk produced domestically have little in common with the clothes that people once wore. Our fabrics are of good quality not just because they are made of silk but because they are made with the outstanding Koishimaru silk.

In pursuit of the ideal fabric

Koishimaru is a silkworm variety used for sericulture in the Imperial Household, known for being very fine and having outstanding quality. At one point during the Showa period the discontinuation of breeding this silkworm had been considered, but Empress Michiko insisted that Koishimaru continue to be used. The Ministry of Agriculture, Forestry and Fisheries bred the silkworm to preserve the species. Following a subsequent revision of the law, the silkworm began to be raised all over Japan, but even before the revision of the law our studio was striving to make the best fabric possible with Koishimaru silkworms through a process of experimenting with all sorts of methods until we succeeded in perfecting a commercially valid method. From 1988, in cooperation with the Miyazaki Prefectural Agricultural Experiment Station, we began experiments in breeding Koishimaru silkworms, and this led to the completion of the first produced fabrics in 1990. At the time, sericulture and silk spinning were subject to strict laws in Japan, so it was quite difficult to obtain a special variety of silkworm like Koishimaru.

Silk reeling in the traditional way

We have to make sure that all the effort made to raise the Koishimaru silkworms does not go to waste in the fabric-making process. To bring out the remarkable qualities of Koishimaru, the cocoons are frozen for storage (rather than dried), and the traditional hand-reeling method is employed. Today most silk is reeled using high-efficiency, high-speed methods. But when this is done, silk threads are dried in a strongly taut condition, much like an overstretched rubber band. When such silk is used to make a fabric, it easily wears out and loses its suppleness. Within the textile industry, such silk is referred to as “wire thread,” which gives you some idea of the great difference between the Koishimaru silk reeled in the past and the type of silk that is generally used today.

Below is an English translation of an article by Masakazu Akiyama that appeared in the August 1991 issue of Gekkan senshoku (Textile Arts Monthly).

My struggle to revive the celestial Koishimaru silk

Masakazu Akiyama

Koishimaru silkworms eating mulberry leaves.

Doubts about "wire thread" silk

Silkworms can be broadly categorized into three types depending on their place of origin: Japanese, Chinese, and European. The Japanese type includes silkworms native to Japan as well as the Korean Peninsula.

Koishimaru (literally “small round stone”) is the cute name given to a type of pure white silkworm that is said to have inhabited Japan since ancient times.

The outstanding qualities of thread from this silkworm include its extreme fineness and very limited shagginess, as well as its ability to maintain sericin in the degumming process and strong elasticity. Despite such qualities, however, Koishimaru silk has not been economically viable because the small size of the silkworms yields very little thread. For this reason, this type of silkworm has long been excluded from commercial production.

For generations up to now, members of the Imperial Household have bred Koishimaru silkworms at the Empress Momijiyama Imperial Cocoonery to maintain this important Japanese tradition. And the silkworms are considered such a precious resource that a pure breed of the silkworm is maintained at a silkworm research facility run by the Ministry of Agriculture, Forestry, and Fisheries in Yamanashi Prefecture.

Silkworms generally used today for silk production have been crossbred to make them more resistant to disease, larger in size, more resilient, and able to produce thicker thread. But the economic advantages thus gained have come at the expense of quality, as is reflected in the term “wire thread” used to describe such silk.

Japan had a booming silk industry in the period prior to World War Two, when silk was one of Japan’s key exports and an item that was essential to women’s clothing in Europe and America. This led the government to focus on promoting mass production by switching to silkworm breeds that could produce the most silk possible.

This effort led to great advances in biology and genetics, but at the same time quite a few things were lost as a result. The mass-produced silk easily wore out and became heavier (since heavier fabric brought a higher price since silk was sold according to weight), and the aspect of comfortability was neglected. In short, the silk became bland and lacking in flavor.

Such silkworms are quite different from those used in the past much like the watered-down, unappetizing taste of vegetables and fruits grown in the greenhouse compared to those grown naturally, even if at first sight they look the same. The ideal that I have pursued up to now in the creation of silk fabrics has been to weave fabrics using the sort of superior silk thread that was once used in Japan.

In pursuing this dream, I came upon the idea of raising Koishimaru silkworms myself. This led me to undertake the challenge of nurturing this most exquisite breed of silkworm, much like the livestock farmers in Miyazaki who pursue the most delectable breed of poultry.

Koishimaru silkworm cocoons.

Obtaining Koishimaru silkworms

Since I knew that Koishimaru silkworm breeds were only obtainable by research institutions, I founded the Teriha Forest Culture Research Institute in 1985. Fortunately, I was able to obtain the cooperation of the Ministry of Agriculture, Forestry, and Fisheries’ Insect and Agriculture Technology Research Institute in Tsukuba City, Ibaraki Prefecture, thanks to the introduction made by the Ministry’s Silkworm Laboratory that was then located in nearby Miyazaki City. I quickly got in touch with that Institute’s branch in Yamanashi Prefecture, and in March 1989 it sent me three of the Koshimaru silkworm moths that I had cherishing for so long.

Although I had finally obtained the silkworms, there were still many barriers to overcome before being able to actually raise them. A law from the prewar period that was intended to stabilize sericulture in Japan was still firmly in place that strictly regulated the breeding and sale of cocoons, so we had to comply with each of the statutes of the law. There was even a national license required to transport live cocoons, but fortunately I was able to obtain this license necessary for the sale of such cocoons.

In the autumn of 1988, we had already submitted a request to the Miyazaki prefectural government regarding the experimental breeding and propagation of Koishimaru silkworms, and in the spring of the following year we also submitted to the government the plan for a project to develop Koishimaru silkworms into a special local product. On April 20 of the following year, we brushed off the larvae of the Koishimaru moths we had received in March, and in late May we obtained around 400 “grains” (seeds) from each moth, for a total of 1,200. And by autumn of that same year, those 1,200 grains had generated the splendid result of 240,000 grains in total. Still, this was just the first step. In order to begin the full-scale sericulture of the silkworm breed it was necessary to gain the cooperation of other silkworm breeders in Miyazaki Prefecture.

Koishimaru silkworms.
Preparing the cocoon holders.

Raising Koishimaru silkworms

Along with clearing the legal hurdles, it was extremely difficult to select the farmers to help raise the Koishimaru silkworms. Since the number of sericulturists was decreasing year after year, they had become precious resources for the silk mills in Japan. Each of them had close connections to threading factories, to which they distributed their silkworms each season, so there was no way to come into contact with them without gaining the understanding of those factories. My own approach to overcoming this dilemma took the following steps.

  1. Acquire consent of Kanebo Cirque Co., Ltd., which has a network of sericulture farmers in Miyazaki prefecture.
  2. Obtain permission from agricultural cooperatives and prefectural economic associations.
  3. Solve problems related to particulate diseases (through a quarantine system).
  4. Reach an agreement regarding the quantity of cocoons, a guarantee against any decrease in that amount, and a higher price compared to cross-breeds.
  5. Determine the sericulture season.
  6. Select the outstanding sericulturists.

All of the participants were brought together in April 17, 1990 to meet each other and take the first concrete step toward raising the silkworms. The decision was made to raise the three age of silkworms at the Miyazaki Prefectural Agricultural Experiment Station.

The selection of the two sericulturists for the project from within the town of Aya was made in early May. On May 20 they began their cultivation of the silkworms, which are raised by consuming a small amount of mulberry leaves. This effort proceeded well, but some of the problems that arose during the breeding process included concerns about falling temperatures, fast cocoon spinning, and cocoon holders that were too small.

On May 30, the long-awaited day of harvesting, we obtained around 104kg of Koishimaru cocoons, priced at four times the level of cross-bred silkworm cocoons. That same day the cocoons were dried at the Miyazaki Prefectural Cocoon Testing Center, yielding 38.1 kg of dried cocoons. Our project to practically produce Koishimaru silk, which was five years in the making, was finally bearing fruit.

However, just obtaining the cocoons is not enough. Our next challenge was how to use those cocoons in order to create silk thread.

Silkworms begin spinning cocoons.
Cocoon shape.
Harvested cocoons.

Silk reeling

Silk used to be hand-reeled from cocoons to make thread, but these days automatic reeling machines perform this operation at a high level of efficiency and speed.

However, spinning at high speeds is not good for the quality of the thread. When thread is spun at high tension and then dried in a stretched state, the result is thread with no elasticity, much like an over-stretched rubber band, so that fabrics made with it are easily frayed. Unlike this sort of “wire thread” commonly used today, I wanted to create silk that still has elasticity, like a shrunken rubber band. I thought it was necessary to revive some of the traditional methods of silk making, even if it meant sacrificing economic efficiency.

Based on information gathered about hand-reeling machines, I investigated the hand-reeling machine used in Kinomoto Town, Shiga Prefecture, and the various machines at the Okaya Silk Museum in Okaya City, Nagano Prefecture, before finally deciding to order a silk-reeling machine from a maker in Okaya. It was combined with old tool parts gathered from here and there to create my ideal silk-reeling machine. There was no problem at all with regard to the reeling technique, because up to that point I had used a hand reeling machine called “Nijouso-ashifumishiki-zasouki” in my studio to reel silk from various types of cocoons.

However, it turned out to be harder than expected for me to master the technique of reeling silk from the small Koishimaru cocoons. After an entire day of work I would only be able to reel about 300 grams of silk from 10 cocoons.

Although the work was hard, it was a great source of pleasure because of the magnificence of the Koishimaru silk. From those 10 cocoons, I could reel around 19 denier of very fine yet strong and highly elastic silk. I could see that Koishimaru silk was the ideal fabric for a fabric maker. (By way of comparison, an ordinary silkworm cocoon produces around 2.7 to 3 denier of thick silk, so 10 cocoons would be around 27 to 30 denier.)

We asked Mr. Shimomura of Kyoto to consolidate this raw silk, and we made a silk thread of 95 to 100 denier (the twisting degree was 190 right twists per meter).

Silk reeling.
Silk reeling.

Perfect thread for indigo dyeing

Although the twisted thread was soft and could be used as is to weave fabrics, we did enzymatic degumming to examine the degree of “lousiness” when the fabric is dyed. Since Koishimaru silk contains less sericin, the degumming was done for 110 minutes, after which the fabric was dried and then dyed.

Since we use the traditional fermentation method for indigo dyeing, even our ordinary silk thread has little lousiness, but it can occur from time to time depending on the condition of the indigo dye.

Once I dyed the Koishimaru silk thread, I realized immediately that it was completely different from ordinary silk thread, including the following excellent traits:

  • The dye quickly soaks into the thread, but for that reason you need to be careful to avoid uneven dyeing.
  • Almost no problem at all with “loudness.”
  • Thread does not lose its gloss.

The thread also naturally had good stretchability during the reeling after dyeing was complete, and this process could be smoothly conducted even though the thread was thin.

We then warped the thread to make a narrow width kimono indigo fabric with a flower design. The fabric had 140 warp threads and 166 weft threads. The weaving process takes considerable time since the thread is so thin. But the weft thread has strong tension with very few frayed threads. Clearly this thread is of exceptional quality.

The finished woven cloth was 380 grams in total weight—a lightweight cloth with a fine glossy texture.

This kimono cloth, which was delivered to a customer in Tokyo via a wholesaler in Kyoto, was praised for the front side being lighter than the lining. The second indigo-dyed fabric I made also had a Kasuri and flower design (see photo). This fabric was used to make a Western-style scarf that took advantage of the thin, light material.

Besides indigo, I also tried dyeing the Koishimaru silk thread with various plant-based dyes, like dyes made from purple gromwell or kariyasu (Miscanthus tinctorius), and the result was also better than for ordinary unfinished thread. But the best results were for indigo dyeing. Although we had not yet conducted dyeing tests using chemical dyes, good results could be expected for the dyeing properties.

With all of these advantages to Koishimaru, it was hard to see the shortcomings. The main drawback is the high price of the yarn. Each cocoon cost around four times the price of an ordinary one, and yielded less silk, plus a more inefficient method of reeling was used, so the cost of the thread is eight times the normal price. But since the end result was a wonderfully light thread, I also felt around 6.5-times more comfortable using it.

Since the market value of the Koishimaru cocoons is high, however, it offers sericulturists a chance to do work with high added value.

Indigo-dyed Koishimaru silk garment.

Reviving the original splendor of silk

Just as the importance of natural growth forests has come into public awareness, as compared to artificially planted monocultures, it may be possible for a new future to be opened up for Japan’s silkworm industry by returning to the use of traditional types of silkworms.

Even though the ultimate point of silk is to be a material for creating comfortable clothing, the way of producing thread today has lost sight of that aim. The effort to raise Koishimaru silkworms and return silk to its original splendor is my own little protest against that situation.

Research conducted in recent years has shown that the thread remnants found in the Fujinoki Tomb in Nara Prefecture and in the ruins at the Yoshinogari site were Koishimaru silk. In light of those findings, there is also great value in using such silk in terms of serving as a revival of a historically important material.

The fairy tales I heard as a child also had angels and fairies that were clothed in Koishimaru silk, so this is a material that spun dreams in my mind.

In pursuing my dream for Koishimaru silk I have relied on the kind assistance of many people and organizations, including:

Kobuchizawa Branch Office of the Ministry of Agriculture, Forestry and Fisheries’s National Institute of Sericultural and Entomological Science; National Institute of Agrobiological Science’s Miyazaki Research Center; Miyazaki Prefectural Agricultural Experiment Station Silkworm Division; Miyazaki Prefecture Silkworm Guidance Center, Kanebo Silk Co., Ltd., Miyazaki Prefecture Economic Association; Aya Town Agricultural Cooperative, Mr. Hirofumi Miyasaka, Mr. Akira Shimomura.

Koishimaru silk thread made from 10 grains (right);
composite; 100 denier of twisted thread.
Weaving machine.