Question

1. how can the steel maker tell when the steel is the correct temperature?9. after the furnace has been broken up and the steel has been cooled, the steel is broken into smaller pieces. how does that help in the grading of the quality of the steel?10. how can a samurai sword be both hard and tough?11. what is slag and how does continual pounding affect the amount of slag in the steel?12. in addition to changing the amount of slag in the metal, how else does the continual bending and hammering of the steel during the sword making process affect its properties?13. what effect does quenching have on low carbon steel? ...on high carbon steel?14. what effect does adding clay to the surface of the sword have on the final blade?15. how long does it take to polish the blade?

Explanation:

1. Steel makers use the color of heated steel (which correlates to specific temperatures) or pyrometers to measure exact thermal levels.
2. Breaking cooled steel reveals internal flaws, inconsistencies, or slag distribution that are hidden on the surface, allowing graders to assess uniformity and purity.
3. Samurai swords are made using differential heat treatment: the edge is high-carbon steel (hardened via quenching) while the spine is low-carbon steel (kept tough), or the blade is folded to blend hard and tough steel layers, and differential quenching with clay creates a hard edge and flexible spine.
4. Slag is a glassy, impurity-rich byproduct of iron/steel smelting. Continual pounding forces slag out of the steel, reducing its overall amount and improving steel purity.
5. Continual bending and hammering aligns the steel's grain structure, removes internal voids, and homogenizes the carbon distribution, increasing the steel's strength, uniformity, and toughness.
6. Quenching low-carbon steel has little hardening effect because it has insufficient carbon to form hard martensite. Quenching high-carbon steel rapidly cools it, forming hard martensite, which drastically increases the steel's hardness and strength but can make it more brittle.
7. Adding clay to the sword surface during quenching slows cooling in areas with thicker clay (like the spine) while the thin-clay edge cools quickly. This creates a hard, sharp edge and a tough, flexible spine, preventing the sword from breaking during use.
8. Polishing a traditional Samurai sword (katana) is a labor-intensive process that can take several weeks to months, depending on the desired finish, the condition of the blade, and the skill of the polisher (togishi).

Answer:

1. Use steel color matching or pyrometers.
2. Reveals internal flaws/impurities for quality checks.
3. Via differential heat treatment or folded steel layers.
4. Slag is smelting impurity; pounding reduces its amount.
5. Aligns grain, removes voids, homogenizes carbon.
6. Little effect on low-carbon steel; hardens high-carbon steel.
7. Creates hard edge + tough flexible spine via differential cooling.
8. Several weeks to months (traditional polishing).