I would say that most spells should not create a "surface", unless they are AOE, and even then it should be limited.
A fireball might cause a fire surface if there was an accelerant (like grease), but most of the time it would explode and fizzle out while maybe catching some dry wood or papers on fire. Casting fireball should not ignite a stone surface, and if cast on ice should make a water surface. Acid/poison should only make a surface if it's an AOE... and should disipate within a small handful of turns. AOE ice spells could create an ice surface on stone or water surfaces (probably not on dirt), but should quickly change to a water unless the environment is cold.
