Concentration Spells from Staves

[Fray Fray]

My question is a bit of a puzzler because I have failed to find any rules regarding the situation. The setup goes like this:

My friend has a Clockwork Crab which has INT 1. He uses the item "Dust of Emulation" in order to allow the Crab to trick a Staff of Fire into allowing it to use Wall of Fire. He then leaves the Crab to Concentrate on Wall of Fire indefinitely.

My problem with this is that the Dust of Emulation only lasts for 1 hour, but by his reasoning, the Dust allowed the Crab to trick the Staff into allowing it to use Wall of Fire by emulating a Wizard Class feature which lets them Use Magic Devices, and therefore, since Dust of Emulation gives you an Auto-Succeed on the check to use it, the otherwise Int 1 Clockwork Crab can use the Staff to cast Wall of Fire and since Automatons don't get bored or lose focus, it can concentrate on the spell for as long as the Automaton can function.

I argue against this by stating that if the Crab could only use the magic device for a limited time under the effects of a Dust, it could only concentrate on the spell for as long as the Dust is in effect, which is One Hour. It doesn't make sense to me that an item could be used to grant you a concentration duration effect that you can keep up after the item has worn off.

Since he doesn't need to make any further concentration checks (unless the Crab is disturbed, but for this scenario we assume it is not), and he doesn't need to make any more UMD checks aside from the one he made to USE the Staff in the first place, he states that the Wall of Fire's spell duration is entirely up to however long the caster wants, whether or not if they suddenly lose the ability to do so in the first place.

[dragonhunterq]

A successful UMD check does not have a duration. It will last until you stop using the device you are fooling. In any event in this scenario the UMD is irrelevant. The staff has already done it's job and the crab doesn't even need to keep holding the staff. You are now dealing solely with the spell effect.

While dust of emulation would allow the crab to trigger the staff I'm not entirely convinced I'd allow the int 1 crab to concentrate on maintaining the effect.

If you are allowing the crab to concentrate on the effect then it will last until the crab loses focus or winds down.

[blahpers]

Pretend the crab was a rogue using Use Magic Device directly without any magic dust. The rogue activates the staff, casting wall of fire with a duration of concentration (plus whatever rounds). At what point does the rogue need to make another Use Magic Device check to maintain the spell?

The answer is "at no point"--the spell is cast. All the rogue need do is continue to concentrate on the spell for the spell to last indefinitely.

The same goes for the dusty construct. (This leaves aside whether a clockwork crab can actually concentrate or activate a spell completion item at all.)