What is the purpose of copper naphthenate when applied to treated lumber?

Copper naphthenate serves as a wood preservative when applied to treated lumber. Its primary function is to protect wood from decay, fungi, and insect damage. This is particularly important in construction and outdoor applications where wood is exposed to moisture and pests. By inhibiting the growth of organisms that can cause rot, copper naphthenate helps to extend the lifespan of the wood, making it a valuable treatment option for lumber used in various environments.

Focusing on the other options, it's important to note that while wood can be treated for color, waterproofing, or strength enhancement, copper naphthenate does not fulfill those specific functions. For instance, while it may have some slight impact on color due to its composition, that is not its primary purpose. Similarly, it does not make lumber waterproof, nor does it enhance its structural strength. Instead, its effectiveness lies in preservative qualities that maintain the integrity of the wood over time.