Foundry sands — typical composition of green sand (by mass) Which of the following approximate compositions best represents standard green moulding sand used in foundries?

Introduction / Context:
Green sand moulds are the backbone of sand casting. A correct balance of silica sand, clay binder, and moisture gives adequate strength, permeability, and collapsibility. While full specifications include moisture and additives, exam questions often compare sand-to-clay proportions.

Given Data / Assumptions:

• Only sand and clay percentages are shown in options (moisture typically 3–8% is omitted).
• We seek the most representative sand-to-clay ratio.
• Silica sand is the base; bentonite/Fireclay is the binder.

Concept / Approach:
Standard green sand commonly contains about 85–91% sand, 6–11% clay/binder, and the balance water and small additives (sea coal, wood flour). Among the given choices, 90% sand and 10% clay best matches typical practice and yields adequate strength and permeability for general-purpose moulding.

Step-by-Step Solution:
Recall typical ranges: sand ≈ 85–91%, clay ≈ 6–11%.Compare with options: 90/10 fits within the standard window.Select 90% sand and 10% clay as the closest correct composition.

Verification / Alternative check:
Foundry handbooks list common mixes like 88% sand, 8% clay, 4% water, which corroborate the 90/10 relationship for sand/clay alone.

Why Other Options Are Wrong:
30/70 or 50/50 clay levels are unrealistically high and would destroy permeability.

70/30 still has excessive clay, leading to gas defects and poor collapsibility.

10/90 is obviously non-functional for moulding.

Common Pitfalls:
Ignoring moisture and additive roles; over-bonding which increases gas-related defects; under-bonding causing erosion and sand wash.

Final Answer:
90% sand and 10% clay (water not considered)