Class D: Nonfluid Oils. Class D oils are:

• relatively non-toxic,
• do not penetrate porous substrates, and
• are usually black or dark brown in color.

When heated, Class D oils may melt and coat surfaces making cleanup very difficult. Residual oils, heavy crude oils, some high paraffin oils, and some weathered oils fall into this class. These classifications are dynamic for spilled oils. Weather conditions and water temperature greatly influence the behavior of oil and refined petroleum products in the environment. For example, as volatiles evaporate from a Class B oil, it may become a Class C oil. If a significant temperature drop occurs (e.g., at night), a Class C oil may solidify and resemble a Class D oil. Upon warming, the Class D oil may revert back to a Class C oil.