Final answer:
The presence of dark streaks on clay shingles indicates excessive porosity due to under firing, which allows soluble salts to surface and be visible after water evaporates.
Step-by-step explanation:
The visibility of soluble salts on the surface of a clay shingle is an indicator of excessive porosity, which is often a result of under firing during its manufacture. The correct answer is A) Dark streaks. These streaks can appear when soluble salts come to the surface and are left behind as any water evaporates. This phenomenon is similar to what scientists identify as recurring slope lineae on Mars, suggesting the presence of water and hydrated salts due to evaporation. Such streaks signify that the shingle material might not have been exposed to enough heat, leaving the clay overly porous and likely more susceptible to water absorption and salt deposition.
Dark streaks of soluble salts on the surface of a clay shingle may indicate that it has excessive porosity, which results from under firing during its manufacture. These dark streaks are caused by water and the presence of hydrated salts produced by the evaporation of salty water. This indicates that the clay shingle has absorbed water and has not been properly fired, leading to its excessive porosity.