Sooo, a reduction piece can be Yohen, and a Yohen is a reduction piece, but not all reduction pieces are Yohen?
And the answer is a bit different wherever you go or depending on who you ask?
I guess as a pure technically, every single piece that is fired is "youhen"........ changed by the fire. Because it went into the kiln as clay and emerged as ceramic. So it was changed by the fire.
Yes, a piece that has been reduced CAN exhibit youhen effects.....but it is also true that all reduction effects are not routinely considered youhen. No, all youhen does not only involve reduction.
Youhen in general (fire change) can be looked at as a broad category of kiln induced effects. Some involve reduction and some involve oxidation. Some are induced by fusing flyash. Some are induced by being burried in coals and ash. Some are induced by repetitive changes in atmosphere from strong oxidation to strong reduction. Some involve reduction during the up-cycle. Some involve reduiction during the cooling cycle. And so on.
If you live in a particular place in Japan and there is only ONE type of "fire change" that your kiln/village/region does, then you might refer to that particular effect simply as "youhen". It is "youhen" because that is THE youhen effect with which you are familiar. You don;t need a closer identification... because you are not differentiating more closely than that.
People from another location might have a different effect that they utilize that involves the impact of maipulation of the firing....... and they call THAT effect "youhen".
You can also probably say that using the broad term "youhen" is easier to use to communicate with people who would not be familiar with more subtle terms that people other than local potters might not know. A "catch all" term that is "close enough for government work
", so to speak.
I know Kusakabe-san and Mark (have had the pleasure to work with Kusakabe-san a couple of times in Japan and exhibited in a three person show with Mark). Couple of great guys. That book is absolutely the BEST one stop referenece to Japanese wood firing approaches that is available. It is clearly written for potters.... but if you are serious about Japanese wood fired pottery...... it is a good primer in English for you on "what and how".
The section that breaks down the various sub-categories of youhen effects with photos is supurb. But you have to remember that this can tend to get into the supposed category of Eskimos and words for "snow"
. The average person just wants to know it is snowing, not that currently the type that is falling is grauple.