On drying, film of distemper ___________
🔬 Understanding Distemper and its Drying Process
Distemper is a traditional, water-based paint. Its basic composition is a base (like chalk or lime), a binder (like animal glue), pigments for color, and a large amount of water as the vehicle or solvent.
Drying Mechanism: The drying of distemper is a purely physical process. The water, which makes up a significant portion of the wet paint's volume, evaporates into the air. No chemical reaction like oxidation takes place.
⚖️ Detailed Analysis of the Options
Let's analyze what happens to the film as the water evaporates.
(c) Shrinks
Why it's correct: When the distemper is applied, the solid particles (chalk, pigment) are suspended in water. As the water evaporates and leaves the film, these solid particles are forced to pack closer together to fill the void. This consolidation of solid particles results in a reduction of the overall volume of the film, causing it to shrink. This shrinkage is why distemper can sometimes show fine cracks (crazing) if applied too thickly.
(a) Swells
Why it's incorrect: Swelling is the opposite of drying. A material swells when it absorbs a liquid. Since the distemper film is losing water, it cannot swell.
(b) Disappears
Why it's incorrect: Only the water disappears (evaporates). The solid components—the base, binder, and pigments—are left behind on the surface to form the colored film.
(d) Thickens
Why it's incorrect: The wet paint in the can is thick. As it is applied and the water evaporates, the resulting dry film becomes much *thinner* than the initial wet film. The film itself does not thicken as it dries.
💡 Study Tips for Paint Drying
- Water Leaves, Solids Huddle: This is the key concept. Imagine a crowd of people (solid particles) in a large room (the water). When the room gets smaller (water evaporates), the people are forced closer together. The whole group "shrinks."
- Drying = Losing Volume: For any paint where the liquid part evaporates (like distemper or emulsion), the final dry film will always have less volume than the wet film that was applied. This loss of volume is shrinkage.
- Swell vs. Shrink: Swelling is caused by absorption (taking in liquid). Shrinking is caused by evaporation (losing liquid).