Cold Shuts and Laps in a Forged Blade
A hairline seam or crease in a forged blade that won’t sand out is usually a cold shut — steel folded over itself instead of welding together. It looks minor and it is not: a cold shut is a built-in crack, and finding it early decides whether the blade is salvageable.
Symptom
A thin dark line, crease, or seam on the blade surface that survives grinding and shows up starkly after an etch or quench u2014 often with slightly rounded, folded-looking ends rather than sharp crack tips.
Likely Causes
Steel folded over itself during forging: hammering below forging heat (dull red or cooler), sharp bar corners folding over during drawing, over-thinning one facet before rotating the bar, or scale hammered into the surface between heats. The fold traps oxide, so the two surfaces touch but never weld.
Quick Checks
Grind a shallow window across the line: a scratch disappears evenly, a cold shut stays as a defined seam. Check the ends of the line u2014 rounded, folding ends suggest a shut; sharp tips running from the edge or a hole suggest a quench crack instead. An etch (even vinegar) makes the seam jump out.
Fixes
Shallow shut with thickness to spare: grind past it completely and confirm with an etch u2014 on a 2×72 this is a two-minute diagnosis. Deep shut with forging stock left: cut or grind back to sound steel and re-forge. A shut that reaches the final edge or bevel envelope is not fixable u2014 see below.
Prevention
Stop hammering at dull red u2014 cold steel folds instead of flowing. Keep bar corners lightly radiused while drawing. Rotate evenly instead of over-thinning one side (the same discipline as bevel forging). Wire-brush scale between heats, and build hammer accuracy so blows land flat, not on edges.
When to Stop and Get Help
If the seam intersects the final edge or survives into the finished blade envelope, scrap the blade. A cold shut is a stress riser: hardened and put to work, it is where the blade will crack. Never harden or sell a blade with a known shut in the working zone u2014 steel is cheaper than reputation.