Japan’s own industrial standard defines washi, and the definition contains the problem: it says that most washi today is machine-made from chemical pulp. The fibre that gives the material its properties has been measured, and it is not present in every sheet the word is applied to.
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The reason is fibre length, and it has been measured. Yamashita, of the Kochi Prefectural Paper Technology Center, gives the figures in Hyomen Kagaku in 2017: kozo fibre runs 6 to 21 mm and averages 9.4; gampi 3 to 5 mm, averaging 3.2; mitsumata 3 to 5 mm, averaging 3.5. Conifer pulp is 2.0 to 3.0 mm and poplar averages 1.5. Widths barely differ — 10 to 30 micrometres for the bast fibres against 22 to 50 for wood — so what changes is the ratio of length to width, and he names that ratio as the major factor behind the bulk and the tensile strength that characterise washi.
Now the definition. JIS describes washi as a general term for paper that developed in Japan, divided into hand-made and machine-made; originally, it says, paper made from bast fibre using neri and the hand-made method — and then adds that at present much of it uses chemical pulp and the machine method. That is the standard saying, in its own words, that the term now covers paper made from neither the fibres above nor the process below. A listing using the word is not misusing it. It is simply not telling you which of the two you are getting.
The process is the other half. Yamashita separates two ways of using the mould: tame-zuki, where the stock is held and the water simply drains, which suits thick papers; and nagashi-zuki, where neri — a viscous liquid extracted from certain plants — is added and the mould is worked back and forth so that the sheet builds under control. Nagashi-zuki is what makes long fibres into a thin sheet rather than a thick one, and he writes that most of the technique developed in Japan. The thin papers the tradition is known for are its products.
The designation people cite follows the process rather than the word. UNESCO’s intangible cultural heritage listing, inscribed in 2014, is titled the craftsmanship of traditional Japanese hand-made paper, and it names three papers: Sekishu-banshi in Shimane, Hon-minoshi in Gifu and Hosokawa-shi in Saitama. It is a listing of specific hand-made traditions in specific places, not a description of everything sold as washi.
What this page does not tell you is how long any of it lasts. The source above measures fibre and describes technique; it does not measure ageing, and the thousand-year figure that circulates with this material is not something this site has a measurement for. Treat the fibre and the method as what is established here, and the longevity claim as a separate question that would need a separate source.
No shop link here. The single question this page says to ask — which fibre, and hand-made or machine — is one that most listings do not answer, and linking a sheet whose fibre is not stated would undo the page in the act of monetising it. If a seller does state the fibre and the method, that is the seller worth reading; if the description stops at the word, the description has told you nothing the standard does not already allow.
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Ground on one side only. The wrong side does not cut badly — it does not work.
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One boils water and gives you iron. Putting the other on heat cracks its lining for good.
The spots appear in the first week. The makers’ association says do not take them off.
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