I've been compiling a guide for my guild on siege weapons and we've been having a discussion around what is purgeable, I was under the impression that you could purge the fire DoTs from oils and fire ballistas but that was all. Anyone any further information?

For interest I'll post my guide:

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Types of Siege Weapons

1. Battering Rams can only be deployed near keep doors and cannot be used to attack walls or enemy players. Once deployed, it will move to the nearest door and begin attacking. A ram works faster with allies near it, to a maximum speed with 6 allies nearby. Battering Rams are most vulnerable to flaming oils as there're always spaces to place oil pots on the roof at main doors

2. Trebuchet are best used against keep walls and can even be fired into keep courtyards and on the walls due to their long range and arcing shots.
Each Alliance has five types of Trebuchet available:
Stone Trebuchet - Deals high damage to buildings and enemies.
Firepot Trebuchet - Deals flame damage over time to enemies.
Iceball Trebuchet - Deals frost damage to enemies and reduces their movement speed for a few seconds.
Cold Stone Trebuchet - Deals extremely high damage to buildings, structures and enemies - deals the highest initial damage out of all siege weapons. These weapons can only be obtained from Dark Anchor chests.

Cold Fire Trebuchet - Deals Coldfire damage to enemies and very high damage to structures. These weapons can only be obtained from Dark Anchor chests.
Trebuchets are tough and have a long range. Due to their long radius, they can shoot enemies behind walls easily and can even hit the Inner Postern Walls of keeps if they are placed correctly. Among all the siege weapons, the Cold Stone Trebuchet does the highest damage to walls and other structures for each hit, with around 5,000 damage per hit, but it is somewhat rare. Firepot and Iceball Trebuchets are better for attacking enemy players than keep walls, but they are outnumbered by Ballistae due to their faster reload time. All types of trebuchets have a long reload timer and cannot shoot targets in close range, unlike the Ballista and the Catapult. Trebuchets are ideal for attacking due to their long range and very high damage, but it is recommended for trebuchets to be accompanied by ballistae or catapults when attacking as trebuchets cannot fire in close range.

3. Catapults are a wide area siege weapon that can be combined with effects such as disease. Useful against large groups of players.
Each alliance has three types of catapults:
Meatbag Catapult – This type of Catapult deals disease damage to enemies and reduces their healing taken for a few seconds.
Oil Catapult – This type of Catapult ensnares enemies and drains approx 5000 Stamina.
Scattershot Catapult – This type of Catapult hits a large area for each shot which is useful of detecting hidden enemies and adds 20% damage to all other siege damage..

4. Ballistae are useful to take down walls and doors, destroy other Siege Weapons and against players.
Each alliance has four types of ballista:
Ballista - This type of ballista shoots a normal bolt that deals very high damage to buildings and enemy siege weapons, but less damage to enemy players.
Fire Ballista - This type of ballista shoots a fire bolt that deals fire damage to players plus damage over time.
Lightning Ballista - This type of ballista shoots a lightning bolt that ensnares enemies, reducing their movement speed for a few seconds also drains approx 5000 magika.
Cold Fire Ballista - Deals Coldfire damage. Does a lot of damage to players but no extra damage to walls/doors. These weapons can only be obtained from Dark Anchor chests.

5. Flaming Oil is used to defend against attacks by Battering Rams and breaking up groups of players charging into breaches in the walls and doors.

Flaming oil is not a range siege weapon, but it has a decent area of effect radius. If placed at the edge of walls, the oil will be dropped off the wall. It MUST drop at least 6 metres to be effective. Flaming oil is very effective in battle of defense. It is good at destroying enemy Battering Rams as well as other siege weapons closed enough, dealing high flame damage over time. Other than that, Flaming Oil is also good at dealing flame damage over time to enemy players, especially vampires.

All side effects from siege weapons (snare, healing taken reduction, siege damage increase taken) are no longer purgeable. (Not sure if this includes oils or fire ballistas, can't find any information.)

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I'd be grateful for any corrections etc.