What is the ruling on dead skin (dry, flaky skin) and dandruff on the scalp, and do they constitute a barrier that prevents the validity of ablution, and what is the difference between them?

"Dead skin" (al-qashaf al-mayyit) refers to accumulated dirt on the skin, and it must be removed if it forms a tangible layer that prevents water from reaching the skin, unless it has become an integral part of the body and is difficult to remove. It may also refer to dead skin resulting from a wound or similar, and this does not need to be removed because it is part of the body. As for dandruff, it does not invalidate the ablution (wudu'), because the purification of the head is mitigated (taharat al-ra's mukhaffafah).