Refractory mixing for hand moulding: up to what maximum water content is commonly allowed in a refractory mix when it is prepared for hand moulding?

Introduction / Context:
Workability of refractory mixes depends on water content, binder type, and grain-size distribution. For hand moulding, higher plasticity is required than for machine-pressed shapes, so allowable water ranges are broader. Excess water, however, raises porosity after drying and firing, reducing strength and slag resistance.

Given Data / Assumptions:

• Plastic fireclay-type mixes for hand ramming or hand moulding.
• No special low-water chemical-bond systems (e.g., castables with deflocculants) are implied.
• Objective: identify the upper typical water range for hand moulding.

Concept / Approach:
Hand moulding requires sufficient water to achieve plasticity for manual shaping and to ensure bonding prior to drying. Standard practice allows higher moisture compared to dry pressing. Typical guidance: machine pressing 3–7%, stiff-mud/extrusion 7–12%, and hand moulding as high as 14–20%, recognizing the trade-off between ease of forming and final porosity after firing.

Step-by-Step Solution:

Verification / Alternative check:
Process manuals and plant experience corroborate that hand-moulded shapes carry higher water content, which must be removed gradually during drying to prevent cracking or lamination defects.

Why Other Options Are Wrong:

• 2–3% and 5–7%: typical for dry pressing and machine pressing; insufficient for hand moulding.
• 7–12%: aligns with stiff-mud/extrusion, not hand moulding maxima.
• 25–30%: excessive, leading to high shrinkage and cracking.

Common Pitfalls:
Using too much water and accelerating drying, causing blowouts; assuming castable practices (with dispersants) apply to plastic hand-mould mixes.

Final Answer:
14–20%