Why are sand or water used in soft-mud brick molds?

In the soft-mud process of brickmaking, clay is mixed with a significant amount of water to achieve a soft consistency before being placed into wooden molds.1

Sand or water is used to lubricate the interior of these molds to prevent the wet clay from sticking to the wood. When sand is used for this purpose, the resulting product is often referred to as sand-struck or sand-faced brick, while the use of water results in water-struck brick. Once the molds are filled, any excess clay is struck from the top to ensure the unit is properly formed.12345