Let’s look at Lev 15 in regard to that which is sat upon. It says repeatedly that in the case of a person with a discharge or a woman on her period that the bed or seat shall defile. However, at what point does it no longer defile? I might guess that it would be washed, but there is nothing that actually says that. Only wooden articles are explicitly stated as needing to be washed (aside from the clothes of the people becoming defiled).
At one point I thought it meant the seat/bed defiled while the person was on it, but I don't think that's the only case, because it refers to someone bearing that which was sat upon becoming unclean as well.
I misread a verse and thought that it would continue to defile so long as the person is still defiled, but I don't think that's necessarily textual either. Would it just become clean at evening with no interference? Surely it's not unclean forever.
I think that articles or bed remain unclean until it is either washed or purified. I think purification(i.e Numbers 19:14) perhaps is most logical step, although it is not explicitly stated in the Torah.
But I do not think that unclean bedding/articles become clean by themselves. This would not make sense.
I do think that washing the seats/beds in water (like the articles of wood described rather than in terms of purification of the dead) is the next logical step, but it's not explicitly said.
Here is the discussion with Karaites about Lev 15. I asked the question about articles that are unclean.
“given the source of the issue, there is a difference between objects that are touched and objects sat upon in lev. 15. the objects touched are only unclean if the person didn't wash their hands (וְיָדָיו לֹא-שָׁטַף בַּמָּיִם, v. 11), and the objects sat upon are made unclean just by sitting on them (v. 4). uncleanness is transmitted (by touching), whereas holiness is not - haggai makes this point rhetorically with the priests (hag. 2:10-14).
my answer to your question is found in v. 12. so, you wash yourself and your clothes if you're unclean, but the objects that are touched are to be broken if they're permeable (i.e., earthen), and washed with water if they're not (i.e., wooden). from the uncleanness of death, we can see the requirement to clean an entire space, as everything in the space in which a person dies becomes unclean, and purified with water and fire (num. 19:18, 31:23).
when sitting in the chair of a niddah: if you draped a cloth over the chair, then you'd remove and wash the cloth; if not, then you'd wash down the chair.”
One thing in particular I have been thinking is that the permeability distinction is both non-textual and invalid. While I might understand how someone could come to that conclusion when looking at how metal vs pottery is treated in Lev 11, for example, it simply isn't valid to say that wood and fabric are impermeable. That is obviously incorrect. Certain kinds of wood can be less permeable than others, but they certainly are permeable, and fabric is most certainly permeable. So, for that person to make these distinctions as such and speak as if this is a reality and a fact of what the text says would be incorrect in both senses.
Also, I'm skeptical regarding this "draping a cloth on the chair" thing. For example, if a woman sits on a chair while she has clothes on, she has a cloth barrier between her and the chair, but I do not think that means that she is not sitting on it. However, a person's clothes are a person's clothes, and those are treated as a distinct set of entities in the text, so it is certainly arguable. I would just be hesitant.
Also, I do not think the hand-washing matter extends beyond its context.
However, my understanding is that earthen object (i.e ceramic plates or cups) can't be cleansed due to the way liquid permeates it, as opposed to wood and fabric. But both wood and fabric does not permeates "bad" enough so as not to be able to be cleansed in water.
Also, I wanted to note that even though beginning of Lev 15 talks about a man and not a woman it still talks about "discharge" (zobe) be that semen or menstruation or something more serious. It is all called "discharge". So it seems that what is written in the beginning of the chapter applies in all circumstances of "discharge".
See Lev 15:23. It seems that it talks about "it" (discharge) (if "its" on bed or article) so I think the point is the discharge itself. In case of the draped chair, it would only work if her discharge did not get through the draped cloth. If it did the chair would obviously be unclean. At the same time if a woman is wearing clothes and uses proper hygiene and if she sits on a draped chair (meaning that her discharge did not get on the drape) I think such workaround is acceptable.
Concerning permeability, it is very easy to demonstrate in a physical sense that these claims of permeability are invalid. It is true that earthen objects are permeable, but basically anything can be considered permeable for that matter.
Stoneware, ceramic, and fired clay are actually considered "impermeable" materials.
You mentioned ceramic: take a ceramic cup and fill it with water and leave it that way for many days and there will be no apparent absorption into the object. Dump it out and wipe it off and it will practically be the same as it was before. However, take a cloth and try to fill it with water, and the water will soak through, saturate it, then drip out almost immediately. Can a fired earthen object really be said to be more permeable than that cloth? The cloth becomes soaked! Wood is a slightly different issue because there is variance depending upon the type of wood and how resinous it is, etc, but if you had a wooden cup and left it soaking in water for a time, it would likely absorb some of the water and possibly deform slightly. All of these objects are permeable, and of the three mentioned, ceramic is the least.
However, the issue here is of objects becoming unclean, not about permeability. Apparently Torah claims that earthen vessels (this includes ceramics regardless that it has been through fire) become unclean once touched by unclean things and should be destroyed. I do believe in what Torah says because I do not have a good scientific explanation for it because my knowledge in these areas is limited. However, it does not mean there is not one. Torah talks about pretty advanced things that our society have not discovered yet so I just choose to accept it as truth at this moment.
In case of "zoob"(discharge) the rules seem to be the same concerning the objects. There are different rules when it comes to exposure to such discharge. This seems to be the difference.
Lev 15:23 clearly talks about discharge. The issue is not touching the actual blood (it is mentioned as well btw - Lev 15:24) but touching something that blood came in the contact with. Directly or indirectly - that's debatable. I.e a chair on which a menstruating woman sat…
My personal inclination is to err on the side of caution, especially since it doesn't say that "nidathah" is on the chair/bed/etc. Also, it would seem that "nidathah" is distinct from general discharge, although some of the rules are similar.