I was reading various questions about whether or not grease is flammable, which got me thinking about other flammable materials, so I'm asking what qualifies a material as flammable or not and how spells that can ignite flammable objects interact like burning hands.
For example if your party is in a forest during the summer and your fighting goblins what happens when you cast burning hands, do you start a forest fire which is likely what would happen in reality if you created a 15ft gout of flames in a dry forest.
Another example would be if your party is fighting cultists who all wear robes do the robes catch fire when hit with burning hands or other spells capable of igniting things, and if the robes do catch fire they begin taking burning damage until the robes are destroyed or extinguished.
One final example I have is what would happen if you casted burning hands on a cask of liquor that would likely be flammable which is normal for liquors that are above 100 proof(50% alcohol content), or if you casted burning hands in a alchemists lab where their would likely be many different flammable liquids.
I'm mostly wondering this to find some new and fun ways to defeat enemies early on such as somehow dousing a group of orcs in a strong liquor and lighting them up with a simple burning hands spell.