From Project: Redcap
Note that a Momentary spell can have lasting effects even though its duration is very brief. If a magus blasts a pig carcass with a Momentary Creo Ignem spell, then the carcass is going to get roasted and stay roasted after the spell expires. Likewise, if he overcooked the pig and wants to get rid of it with a Momentary Perdo Animal spell, then the spell destroys the carcass but the carcass stays destroyed after the spell wears off.
[anchor:Damage]Momentary Spells and Damage
A Momentary Perdo spell does its damage and then the magic expires, so there is nothing that would prevent natural healing. A Momentary spell that damages a non-living object would do normal damage that could be repaired.
If the spell had some Duration other than Momentary, then the spell would prevent that damage from being healed or repaired while it remained in effect. After the spell wore off, the damage would remain but it could be healed or repaired as normal. That is, the only difference between a Momentary and a non-Momentary Perdo spell is that damage from a non-Momentary Perdo spell can't be repaired until after the spell ends.
There is some confusion arising from older editions, where many Perdo spells were given a Duration category called "Instant" and the text implied in some places that damage from Instant spells could not be naturally healed. The Instant Duration has been eliminated for clarity: Perdo spells that simply do damage in an eye-blink are now categorized as Momentary. Nothing in the ArM5 rules suggests that damage from Momentary-Duration spells can't be healed.
[anchor:Properties]Momentary Spells and Natural Properties
As noted under the description of the Technique Perdo on ArM5 p. 78, Perdo magic can destroy properties of things. So a Perdo spell can destroy an object's weight and make it weightless. The following paragraph goes on to say that items can only permanently lose properties that can be lost naturally, like freshness. Properties that cannot naturally be lost, such as weight, would return after the spell ended.
It is up to the Troupe or storyguide exactly what properties can be lost naturally. Indeed, some decisions may need to be made about what properties Perdo can affect at all - is size a property? Weight is clearly something than an object can't natually lose, but other properties like hardness or flexibility are more of a grey area.
The history of this page before August 6, 2010 is archived at Legacy:momentary_duration