The fading of the Terracotta Warriors’ colors shortly after excavation is due to a combination of factors related to the conservation state of the artifacts and their environment. When the warriors and horses were first buried, around 221 BCE, they were covered in a layer of lime thatprotected them from moisture and microorganisms. This preserved the vibrant colors of the figures for over two millennia.
However, once the artifacts are exposed to the atmosphere after excavation, they are at risk from a host of environmental factors that can cause the colors to degrade. The primary factors contributing to the color fading include:

1. Oxidation: Exposure to air causes the colors to oxidize, which can break down the compounds responsible for the hues. This process is particularly rapid for certain pigments, such as iron-based compounds that turn from a bright red to a brownish color or even black as they oxidize.
2. Moisture: High humidity can cause the lime layer to dissolve, exposing the Terracotta Warriors to water. Water can then interact with the pigments, leading to physical changes, such as flaking or powdering, and chemical reactions that can alter the color and structure of the surfaces.
3. Light degradation: Light, especially ultraviolet (UV) light, can cause the colors to fade. The intense light levels in the environment can break down the chemical bonds within the pigments, leading to a loss of color intensity.
4. Pollution: Air pollution, including particulate matter and gases, can also contribute to the degradation of the colors. These pollutants can adsorb onto the surface of the warriors and horses, causing chemical changes that affect the appearance of the pigments.

To mitigate these effects, conservation efforts focus on controlling the environment in which the Terracotta Warriors are stored and displayed. This includes maintaining low humidity, limiting light exposure, and filtering the air to remove pollutants. Additionally, specific conservation treatments are applied to the artifacts to stabilize the remaining colors and protect them from further degradation.