I'm sure we have all used the spine of our knives as a ferro rod striker but for a while it's been puzzling me why I have to re-square the spine every now and again and why the square edge doesn't last long after continual use, I put it down to steel wear, just like the edge of your knife needs to be re-sharpened then so would the spine, but recently I started thinking about this and it hit me.. The reason the spine wears and gets less effective over time is simply that we are damaging the temper of the blade each time we strike the ferro rod. Lets face it, a ferro rod burns at around 3000 degrees Celsius, although it only burns for a very short time that initial burst of heat is enough to damage that area of the knife, I've found that stainless gets brittle and chips and carbon gets soft and wears or rolls,
this is a carbon bladed knife , if you look closely at the spine edge you can see damage to the square edge, every time you strike the ferro rod, the heat damages the temper and causes the edge to get softer
and a stainless blade which is much more noticeable, this is a victorinox farmer, you can see what fewer than a dozen strikes of a ferro rod has done to the spine of the saw blade. Being a thinner blade heat isn't dissipated as much and therefor the damage is more dramatic, but over time all blades will suffer.
Your knife is your most important piece of kit, treat it well, get a proper striker for your ferro rod and use your knife for what it was intended.