Those same rules also state that the DM is in his or her right to change the damage dealt due to any tyoe of attack based on particular vulnaribilities. The example they give is the rope somebody is trying to cut with a mace, but the same would go for energy attacks. Metal might hardly be affected by alchemist fire, but dry wood certainly is. Even if the hardness dictates that the table would not start burning (which is the fire burns for only 1 round is not strange, but prolonged exposure definitely will lit the table).