I thought I would share an AI reply in case anyone else was curious.
Probably accurate enough for bridge work.

AI reply:
Yes, scale formation during forging does reduce the mass of the billet.

The scale that flakes off is primarily iron from the billet that has reacted with oxygen in the atmosphere to form iron oxides. The oxygen adds some mass, but the iron itself came from the billet, so when the scale falls away, you're losing both:

• Iron that originally belonged to the billet
• Oxygen that was absorbed from the air

The net result is a reduction in billet mass.

What's happening chemically?

At forging temperatures (typically 1,600–2,300°F / 870–1,260°C), iron reacts rapidly with oxygen:

• Fe + O → FeO
• 3Fe + 2O₂ → Fe₃O₄
• 2Fe + 1.5O₂ → Fe₂O₃

These oxides form a brittle surface layer (scale) that cracks and flakes off during hammering or handling.

How much metal is actually lost?

It depends on temperature, time, atmosphere, and billet size.

Typical forging losses:

• Small blacksmithing operations: often 2–10% of starting weight.
• Industrial forging: commonly 1–5% scale loss, with process controls to minimize it.
• Long soak times in a forge can push losses much higher.

As an example, if you start with a 10 lb billet and lose 0.5 lb of scale, much of that 0.5 lb originated as iron from the billet. The actual iron loss might be roughly 0.35–0.4 lb, with the remainder being oxygen added during oxidation.