Thermal Modification is a specialized process where wood is heated (using steam) to a very high temperature to transform it on a molecular level into a highly-durable outdoor material.. . Thermal Modification turns the wood into a beautiful chocolate brown color, which indicates that the wood has reached the proper temperature (around 400-410⁰ F). At that point, the wood undergoes a fundamental change where the sugars and starches caramelize, making the wood a non-food source.