If nails left over from burning wood in a stove a retrieved has the metal become brittle or weaker.?

When metal is heated to red hot and then allowed to cool slowly, it softens the metal. When metal is heated to red hot and then cooled quickly by quenching it in water, it becomes harder, but also becomes brittle. When metal is heated, but not to red hot and then allowed to cool slowly, it becomes tempered. Tempering makes the metal stay hard, but does away with the brittleness.

This is the process for making tools such as chisels. Soften the metal and form it, then harden it and then temper it.

Most nails are made of steel in it's original state. Having none of the above done to it, so they are softer than hardened steel, but slightly harder than softened steel and are not tempered.

I wouldn't trust them. It's a completely uncontrolled environment, the heating isn't monitored at all so you have no idea what the nails have been exposed to.

Hardening is unlikely. The mild steel used for making most nails isn't a good candidate for hardening in any case.

Every year I make a new basket for burning pellets in my woodstove. It uses mild steel expanded metal lath. Every year, the basket fails when a hole burns in the bottom, and some of the metal near the top edges becomes so brittle I can snap of bits of it with my fingers. I'd say the nails are ruined and worthless.

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In most cases a nail that has been through a fire is very soft-annealed. Weak enough that you cannot drive it into a piece of softwood. To temper (harden and toughen) steel it needs to be heated, then cooled quickly (quenched). That is not likely to happen in a wood stove.

IF the fire is hot enough (and it's doubtful) it would disassociate the carbon atoms from the steel, reducing the hardness.

Steel when being heat treated to impart hardness is raised to a cherry red, then quenched. This locks the carbon in the steel, forming a matrix, called 'austenite' https://en.wikipedia.org/wiki/Austenite#Behavior_i...

Without this rapid quench, this matrix is not retained, and softness results.

Slow cooling lacks the 'locking' action to "freeze" the austenite.

i think of you will get greater solutions and we would be greatly surprised at how many nonetheless use them. I had kinfolks who cooked and heated with a timber burning range. the appropriate fried hen ever got here off of that range. the living house I stay in had a timber range for the 1st 7 or 8 yrs.

Generally woodstoves do not get hot enough to effect the temper.