Question

The slurry used in precision investment casting is made up of

• A. Zinc stearate
• B. Stearic acid
• C. Both Ethyl and Sodium silicate
• D. Neither Ethyl nor Sodium silicate

Hint:

Investment casting slurry uses {silicate binders} to provide strength and heat resistance to the ceramic shell.

Answer 1

The Correct Option is C

Step 1: Understanding precision investment casting.
Precision investment casting, also known as lost wax casting, involves coating a wax pattern with a refractory slurry to form a ceramic shell. This slurry must provide strength and thermal resistance.
Step 2: Composition of slurry.
The slurry typically consists of fine refractory materials mixed with binders. Ethyl silicate and sodium silicate are commonly used binders because they provide good bonding and strength to the ceramic shell.
Step 3: Eliminating other options.
(A) Zinc stearate: Used as a lubricant or release agent, not as slurry material.
(B) Stearic acid: Used in wax patterns, not in ceramic slurry.
(D) Neither Ethyl nor Sodium silicate: Incorrect, as silicates are essential binders.
Step 4: Conclusion.
Therefore, the slurry used in precision investment casting is made up of both ethyl silicate and sodium silicate.

Answer 2

In precision investment (lost wax) casting, the slurry has one specific job: to coat the wax pattern and, together with a refractory stucco, build up a strong ceramic shell mould. Checking what each option's actual chemical role is clarifies which belongs in the slurry.

1. Zinc stearate: This is commonly used as a mould-release or lubricant agent, for instance to help wax patterns release cleanly from injection dies, not as a binder within the refractory slurry itself.
2. Stearic acid: This is typically used in formulating the wax pattern material itself (to control its properties), not as an ingredient of the ceramic slurry coating.
3. Both Ethyl and Sodium silicate: These are the actual binder chemicals used in the refractory slurry. Ethyl silicate, hydrolyzed in an alcohol-acid solution, gives a high-quality silica gel binder favoured for fine, dimensionally accurate shells, while sodium silicate offers a cheaper, water-based binder used especially for the outer, less critical shell layers. Foundries commonly use both, often in combination across different shell layers, to balance cost and precision.
4. Neither Ethyl nor Sodium silicate: This directly contradicts standard investment-casting practice, where these silicates are the industry-standard binders for the ceramic slurry.

Since both ethyl silicate and sodium silicate serve as real, industrially used binders in investment casting slurry, this combined option correctly reflects the process.

Therefore, the correct answer is Both Ethyl and Sodium silicate.