In Kesa Kudoku Dogen writes the following:

This being so, we should make [the kaṣāya] properly, according to the method for making the kaṣāyathat has been authentically transmitted by the Buddhist patriarchs. This alone is the authentic tradition, and so it has long been expe- rienced and recognized by all common and sacred beings, human beings and gods, and dragons and spirits. Having been born to meet the spread of this Dharma, if we cover our body with the kaṣāyaonly once, receiving it and retaining it for just a kṣāṇaor a muhūrta,31that [experience] will surely serve as a talisman to protect us32in the realization of the supreme state of bodhi. When we dye the body and mind with a single phrase or a single verse, it becomes a seed of everlasting brightness which finally leads us to the supreme state of bodhi.When we dye the body and mind with one real dharma or one good deed, it may be also like this. Mental images arise and vanish instanta- neously; they are without an abode. The physical body also arises and van- ishes instantaneously; it too is without an abode. Nevertheless, the merit that we practice always has its time of ripening and shedding. The kaṣāya,simi- larly, is beyond elaboration and beyond non-elaboration, it is beyond having an abode and beyond having no abode: it is that which “buddhas alone, together with buddhas, perfectly realize.”(Nishijima Cross)

Let's see what is the kesa made of. Rags. Just rags.