Why green sand is called green?

The term "green sand" in metal casting does not refer to the color of the sand mixture itself, but rather to its state relative to the casting process. "Green" means the sand is moist and workable, as opposed to "dry sand" which is used in a different casting process.

Here are a few reasons why the term "green sand" is used:

1. Moisture Content: Green sand contains a certain percentage of moisture, typically around 2-3%. This moisture content is crucial because it helps to bind the sand particles together and maintain the mold's shape. The moisture also creates steam when the molten metal comes in contact with it, which helps to prevent defects and improve casting quality.

2. Fresh Appearance: Compared to dry sand, which is often reused multiple times, green sand can take on a dark greenish color. This color comes from the presence of clay and other materials in the sand, which may be more noticeable in the moist state. The contrasting color helps distinguish it from other types of casting sand.

3. Traditional Term: The term "green sand" has been used in metal casting for centuries. While the name might seem counterintuitive, it has become a widely recognized and conventional term within the foundry industry. It's important to note that the color of green sand can vary, and it is not always literally green in appearance.

Therefore, the term "green sand" does not refer to the actual color of the sand but to its state of readiness for casting. It highlights the role of moisture in maintaining the mold's integrity during the casting process.