Lime for whitewashing: what is the maximum combined percentage (by mass) of silica, alumina, and ferric oxide permitted in lime used for whitewashing? (Note: Good whitewashing lime should be very pure, with minimal insoluble impurities.)

Introduction / Context:
Whitewashing requires high-purity lime to achieve good whiteness, adhesion, and durability. Excess insoluble impurities such as silica, alumina, and iron oxides reduce covering capacity and may cause gritty finishes or staining. The question identifies the conventional upper limit for these impurities.

Given Data / Assumptions:

• Whitewashing uses fat lime (high-calcium lime) of high purity.
• Insoluble oxides (SiO2, Al2O3, Fe2O3) are undesirable.
• Specifications prescribe a small maximum combined percentage of these constituents.

Concept / Approach:
Setting the maximum combined content at a low value ensures whiteness and smoothness. Higher levels indicate unburnt stone or contamination that undermines finish quality and can lead to poor binding with the substrate.

Step-by-Step Solution:

Verification / Alternative check:
Conventional practice and textbook specifications for whitewashing lime cite a stringent limit (≈5%) for insoluble oxides, aligning with the need for a smooth, brilliant finish.

Why Other Options Are Wrong:

• 10–20%: too high; would noticeably reduce finish quality.
• 0%: theoretically ideal but impractical; natural limes contain traces of insolubles.

Common Pitfalls:
Confusing “minimum” with “maximum” impurity limits; overlooking that hydraulic limes, though useful structurally, are not preferred for whitewashing because of color and impurities.

Final Answer:
5%