Material
A surface that gives you back — used to see with, to send back with, and in a great many houses, to cover up.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Scrying into a reflective surface is old and geographically very widespread: polished metal, dark stone, ink pooled in a palm, a bowl of water in an unlit room. John Dee kept an obsidian speculum in his study in the 1580s, and it is still in the British Museum — a useful object to know about, because it makes the practice concrete rather than atmospheric.
Frazer collected the other half of the tradition. Mirrors were turned to the wall or covered after a death across much of Britain and beyond, on the reasoning that the reflection is a kind of soul and can be carried off with the departing one. That same logic is alive in the modern habit of turning a mirror away from a bed, whether or not anyone doing it would put it in those words.
Practice runs both ways: black mirrors for scrying, small mirrors set facing outward to return ill-will. Broken glass wants the obvious care. Beyond that, long scrying sessions in the dark can be genuinely disorientating — set a timer, put a light on afterwards, and stop the moment it stops feeling like something you are choosing to do.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
James Frazer, The Golden Bough (1890)
Frazer gathers the customs of covering or turning mirrors in a house of the dead, and connects them to a much wider belief that the reflection is the soul stood outside the body. On the same reasoning he explains why the sick were kept from mirrors — a soul already loose being that much easier to lose.
Carried forward
Common in contemporary practice
A black mirror kept covered between uses, uncovered only for the session. The habit is usually explained as keeping the tool from becoming furniture, which is a plainer and more convincing reason than most.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.