Stick incense, or Senko 線香, has its roots in Zen Buddhism imported to Japan from China. Unlike other forms of Buddhism that burned raw materials in granular form, Zen more often utilized incense in stick form in Buddhist rituals and during mediation as a way to measure time. Although Zen Buddhism arrived in Japan during the 12th century, stick incense is believed to have been first introduced in the mid 1300's. During this time, Japan's Muromachi period saw China trade relations resume and Zen Buddhism embraced by Japan's warrior class. Later under the resurgence of the traditional "way of the arts" during the Edo period, the 1700s saw senko popularized as a new way to for the merchant class to enjoy incense and made more readily available.
As the most common form of incense today, the manufacture of stick incense is a blending of art, skill, and technology. Careful selection and closely guarded preparation of increasing rare aromatic materials is as much an art form as it is a science, and requires great skill and dedication of incense artisans who produce senko. Although most Japanese manufactures have centuries of experience making incense traditionally, modern technology, laboratory environments, and automated processes are being carefully integrated with traditional methods.
Each of the steps below require a great skill and attention to detail, undergoing strict quality control every step of the way. In this way, the Japanese manufacturers strive to ensure the art of making senko remains true to the centuries old tradition of Japan's way of fragrance.
With many of the raw materials used in making incense becoming increasing more difficult to obtain, the first step in making stick incense is often the act of acquiring the raw materials. This is done through extensive networks of suppliers developed over decades, and field research throughout the world, as most raw ingredients are imported to Japan.
Many incense manufacturers, understanding the increasingly rare nature of these raw materials, employ researchers who are often increasingly involved in ensuring the continued legacy of these materials. This takes the form of research into cultivation and harvest, as well as conservation and forestation efforts to ensure the continued heritage of these fragrant materials for future generations.
Once acquired, rigorous quality control measures are undertaken. Often conducted now under laboratory conditions, these measures include establishing reproducible measurement methods as well as creating safety standards for those who make and use incense. The development of efficient methods to use raw materials that will extend their use, effectively utilize source materials, and create new product potential is often emphasized. These quality control measures are increasingly being elevated to the highest level of stringency recognized by international standards.
Raw materials can take the form of raw plant based materials, oils derived from woods and plants, and whole wood materials. During inspection, each of these raw materials undergo strict testing for quality, potency, and consistency of fragrance. Whole materials are crushed and ground into powder. This is especially important for fragrant woods such as Aloeswood and Sandalwood, as great care is take not to heat the wood during processing and thereby reduce any of its fragrant properties.
Once the raw ingredients have been selected and reduced to powdered form, weighing and compounding the various ingredients is begun. With a number of different ingredients used in many incense recipes, the ratio of each ingredient is carefully measured out by weight and mixed together with other ingredients.
Many of these recipes are tightly held trade secrets, especially for flagship product lines a manufacturer is well known for. Often, these recipes can be traced back hundreds of years through family or business blending books of incense formulas that often have seen little variation over this time.
Weighing and compounding is an area that is becoming increasingly modernized. Where done by hand traditionally, more often this step is increasingly done by computer controlled fully automatic blending systems. In this way, quality and consistency of fragrance is better assured.
Once all the powdered ingredients are measured, they are mixed together. At this stage binder, colorants, and any remaining materials are added to the raw mix. Once completely mixed, the ingredients are passed through a sieve to remove any impurities and further refine the uniformity of the powdery mixture. The finer and more uniform the mixed powdered ingredients, the better the mixture will blend in the following steps. So once the impurities have been removed removed, the mixture is often sifted to ensure a very fine powder consistency.
After sifting, the powdered mixture is put into a kneading machine and hot water is added. Kneading takes a great deal of skill, as the amount of water added to the mixture will vary based upon the type of mix, the ambient temperature, and the ambient humidity.
After thirty to forty minutes of kneading, the mixture begins to take on a clay-like consistency. Once the mixture solidifies into a single mass of clay, called "tama," it is shaped and readied for the next step. Often this may include putting the tama into a mold and pressing it into a cylindrical mass as a quality preparation for the next step.
Next the ball of tama is placed in a hydraulic extruding machine. With multiple tons of force, the incense in clay form is forced through a stainless steel plate with rows of small holes, much like dough through a pasta press. The small holes can vary in size and shape, depending upon the product being produced, but most often the holes is sized to achieve 1.8 - 2.0mm in diameter in the dried stick. Over time, these holes do wear and change diameter from the friction of extruding the damp tama through them, requiring the replacement of the stainless steel plate. However, this does not create a quality defect, as the finished product is packaged by weight rather than the number of sticks.
As the wet strings of incense are forced from the extruder, a skilled artisan collects them on a board called a "bonita." The strings of incense are allowed to hang over both sides of the bonita like wet pasta, as the incense is carefully captured as straightly as possible upon the board's surface. The overhanging ends are then trimmed to the bonita's length using a bamboo spatula immediately after it is captured at the extruder. The trimmed off wet incense is collected and can be recycled.
Once trimmed, the bonita is set aside, and another is used to continue the extrusion of incense. This is done many times until a stack of bonita with wet strings of incense is collected.
Once the tama has been extruded and collected on multiple bonita, it is considered raw incense. At this point, a skilled craftsman transfers the raw wet incense, combining the contents of several bonita into one long board, called a model board. This board can be made of either wood, metal, or cardboard.
This is a very important step and one of great craftsmanship. The incense is lifted from the bonita using a bamboo spatula in one swift motion. Great skill and training is required as the incense is quite soft and easily marred at this stage. As the incense is transferred to the model board, it is inspected for quality and straightness. Wet sticks that are marred, broken, or not straight are discarded. These are collected and can be recycled.
Great care is taken to ensure the wet incense sticks are lined up straightly with no gaps between them. They are also smoothed at this stage to ensure uniform consistency. Once the contents of multiple bonita have filled the model board, additional sticks are removed from each end of the row to ensure a uniform length of raw incense on each model board. Again, these unused wet sticks are collected and recycled.
Once the raw incense has been inspected and prepared on the model board, it is cut to size using a bamboo spatula or rotary cutter like a pizza cutter. Depending upon the size of the finished product, sticks may be cut end to end, or into multiple rows.
Once cut, the incense may be transferred from the model board to a drying board made of cardboard if not already on such. Again done by hand, this process takes great skill to move the wet incense from the model board to thick cardboard used to in the drying process.
The individual cardboard drying boards are then stacked in what is called the "laminated drying method" in which they are stacked one atop another. This allows air to pass through the corrugation of the cardboard to evenly dry the incense and prevent the stick warping or bending. Once multiple drying boards have been stacked together, these laminated drying stacks are transferred to a drying room.
Traditionally, natural drying has been used, and some manufacturers consider this the most appropriate way to dry raw incense. In this method, the raw incense is left to dry naturally in the drying room. This usually takes several days in the hotter summer months or up to two weeks in the cooler winter months.
Other manufacturers are moving toward a modern drying method. In this method, temperature and humidity are carefully monitored and kept constant to ensure even and consistent drying. Blowers are used to circulate air through the corrugated spaces in the cardboard. This method cuts drying time significantly from weeks to two or fewer days.
During drying, the incense sticks will begin to shrink. To ensure the sticks do not warp, another quality check ensures the incense is kept tightly together by removing any gaps between sticks that occur due to shrinkage. This assures that the sticks will dry as straight as possible. Any sticks that are found to be broken or warped are removed at this time.
After the sticks have dried, they go through another inspection and broken or warped sticks are removed from the drying boards. The unbroken sticks are then collected into bundles by weight.
Bundling the sticks together has several advantages. First, it further keeps the sticks from warping or bending. Secondly, it provides rigidity and strength to reduce the risk of breakage.
Once the sticks have been picked up and bundled, they may be aged. This step is more likely to occur with premium incense lines than mainichi-koh (daily incense). Generally, the higher the quality of the end product, the more likely it is to be aged, and the longer it will be allowed to age. This allows the ingredients in the incense to further blend together and the fragrance to stabilize.
The final step in producing stick incense is also the final quality control check. Incense is inspected as it is prepared, wrapped, and packed according to the product line being manufactured. Stick incense is packaged by weight, so the diameter of the stick is less important than the package weight.