Foundry materials — composition of loam sand for large masonry-backed molds In traditional loam molding used for very large castings (e.g., bells, pipes, or architectural components), loam sand is a prepared mixture of sand and clay worked with water and organic binders. Which of the following approximate sand–clay proportions best represents standard loam used in foundries?

Introduction / Context:
Loam molding is a classical foundry method for very large or intricate shapes where the mold is built up on a brick or metal armature and finished by hand. The loam itself must hold shape while wet and resist cracking on drying, hence the balance between sand (for refractoriness and permeability) and clay (for plasticity and strength) is crucial.

Given Data / Assumptions:

• Molding method: loam molding for large castings.
• Loam is made from sand, clay, water, and small organic additives (e.g., horse dung, straw, or molasses wash in older practice).
• Question asks for typical sand–clay proportion only.

Concept / Approach:
The plasticity required for hand troweling and building thick sections demands a high clay fraction, while adequate permeability and high-temperature stability demand significant sand. Standard texts describe loam as roughly equal parts sand and clay by proportion, then tempered to a workable consistency with water and organic binders. Too much clay reduces permeability and increases cracking on drying; too much sand weakens adhesion and makes finishing difficult.

Step-by-Step Solution:

Verification / Alternative check:
Foundry handbooks commonly give loam as an approximately equal mixture, later adjusted with organic tempering agents and water to reach a smooth, trowelable paste.

Why Other Options Are Wrong:

• 30% sand / 70% clay: overly plastic, low permeability, prone to cracking.
• 70% sand / 30% clay or 90% sand / 10% clay: insufficient plasticity, poor cohesion.
• 60% sand / 40% clay: closer, but the widely accepted reference value is a near 50–50 balance for loam.

Common Pitfalls:
Confusing loam with green sand; forgetting that loam is finished on masonry and dried before use, so workability during application is paramount.

Final Answer: