Yes, plaster absolutely gets hard.
The Science of Plaster Hardening
Plaster, when mixed with water, undergoes a chemical process called curing. This is not just simple drying, although evaporation of moisture plays a crucial role in the final state.
How Curing Works
During the curing process:
- A chemical reaction occurs between the plaster material (often gypsum or lime) and the water.
- This reaction causes the material to recrystallize or solidify.
- Simultaneously, all the moisture evaporates when plaster cures, leaving a hard, completely dry material.
This process transforms the soft, workable paste into a rigid, durable surface.
What "Dry" Plaster Means
While the reference states that curing leaves a "hard, completely dry material" on the surface, it also provides an important distinction:
- "'Dry' plaster may still contain some moisture beneath the surface."
- "It will look and feel dry, but if you sand the plaster, you may discover it's still wet."
This means that while the outer layer is hard and appears dry to the touch, the core might still be holding residual moisture, especially in thicker applications. Full, through-and-through drying can take much longer than surface hardening.
Why Plaster Hardens
Plaster's ability to harden makes it an invaluable material in construction and decorative applications. Its transformation from a moldable substance to a solid allows it to:
- Create smooth wall and ceiling surfaces.
- Form decorative moldings and architectural elements.
- Provide fire resistance to structures.
- Be used for casting molds and sculptures.
In summary, the curing process involving chemical reaction and moisture evaporation is what causes plaster to become hard, resulting in a solid and functional material for various uses.
