Skyrim:Permanent Items

Most items in Skyrim may be easily lost if placed into a respawning container, left on a body of an NPC or sold to a vendor. Still, some rare items persist in the world of Skyrim, and may even prevent corpses from disappearing. The latter quality is actually useful, as it allows you to accumulate a decent number of corpses for reanimation via Dead Thrall spell.  All persistent items will remain in respawning containers and some of them prevent non-respawning dead bodies from being deleted. 

Type 1 - Permanent (fully)
#Keep their RefID number, when dropped into the world
#Do not lose any of their persistent traits when dropped into the world

Type 2 - Permanent (until dropped)
#May keep or lose their RefID number, when dropped into the world
#Lose any or all of their persistent traits when first dropped into the world.

Sources of permanent items

For some items permanent qualities are caused by those items filling an alias in game's quest system.
Thus, all quest items are permanent, until their quest is finished and they are removed from alias by quest system.
Unfortunately, most of such items can not be placed into most containers (except those related to the same quest), and are of limited use. 

It seems, that permanency is (usually) a property of the reference, not of the base item, so giving yourself an item from the list below via console will not give you a permanent copy of the item. You will need to get the item "legitimately" to benefit from its permanent property.

Identifying permanent items

#Executing "sv" console command with the reference of the item selected. If it shows that it has an alias - the item is permanent (at least until its quest is running);
#Placing an item onto a non-respawning corpse and leaving its area for an extended period of time, if the corpse persists, then the item is probably persistent (unless "sv" command on the corpse reveals the corpse to have an alias, in which case the corpse persists on its own.)
#Selling the item to a vendor, and returning after the merchant chest respawns.  If the item you sold is still there, despite everything else being new, the item is probably permanent.  This is a particularly useful method, since merchant chests respawn much faster than other containers or corpses.
#If there is one permanent item in a stack of like items, it will prevent all items of the stack from refreshing when their container refreshes. This makes finding out what exact item in a stack is permanent somewhat tedious, because you must sell it one-by-one to the vendor and refresh his inventory each time.

List of permanent items

The following items were found to be fully permanent:
#Aetherial Crown 
#Ebony Blade
#The Gauldur Amulet
#Zephyr 

The following items were found to be permanent until dropped (or placed on a weapon plaque):
#Declaration of War 
#Eduj
#Emerald Dragon Claw
#Halldir's Staff - the one you get from Halldir's ash pile, not the one which drops near it
#Ironhand Gauntlets - looted from Hajvarr Iron-Hand
#Iron Dragon Claw
#Journals of Ralys Sedaris  - all volumes
#Klimmek's Supplies
#Okin
#Ruby Dragon Claw
#Saerek's Skull Key - only if you do not use it to awaken Otar the Mad (awaken him with shout or rune spell instead)
#Staff of Hag's Wrath
#The Aetherium Wars  - only the first copy, obtained before or during associated quest
#Torsten's Skull Key - only if you do not use it to awaken Otar the Mad (awaken him with shout or rune spell instead)
#Whiterun guard's armor, shield, helmet, gloves and boots - looted from Mirmulnir


When permanent items do   prevent corpses from disappearing?

#Permanent weapons, despite remaining in respawning containers, do not reliably prevent corpses from disappearing.
#Permanent items prevent corpses from disappearing due to 1-day corpse cleanup. There are other ways for corpses (and actors in general) to disappear, which are   prevented by the actor having a permanent item:
##Being respawned when the associated area/dungeon respawns (Hence, only store corpses from the "never resetting" dungeons);
##Being removed by a script, usually at the end of associated quest (example: Necromancers in the Wolfskull Cave during The Man Who Cried Wolf).

Losing permanent properties

Fully "permanent" items never lose this status. "Permanent until dropped" lose it when dropped into the world, placed on weapon plaque, and sometimes if they are forcibly removed from the player and then returned (such as in Diplomatic Immunity quest).

More suspected items are listed on the  talk page.

Explanation of permanency

 This post by Arthmoor provides insight into the mystery of permanent items and other references.