There are lots of websites 1, 2,3 that claim you can sharpen scissors by cutting aluminium foil. Does this work? What would be the cause of this?
Best Answer
It might work to some degree, but not by sharpening.
There has been speculation about a way for the foil to sharpen a blade: The foil is coated by a thin film of aluminum oxide (Al$_2$O$_3$), which happens to be a known abrasive and, thus,
the aluminum oxide layer on the foil acts and an abrasive and there might be some sharpening effect.
The main problem with this idea, besides the tiny amount of oxide found in the nanometer-thick layer, is probably that the oxide is abrasive in its crystalline form, while the thin film of Al$_2$O$_3$ is amorphous (from what I gather from the literature, e.g., papers 1, 2, 3, 4).
Nonetheless, anecdotal evidence is divided on the efficacy of the method (see, e.g., forums 1 and 2). This can be explained by the subjectivity of the reported observations, as well as by confounding factors, such as 1) different types/quality of scissors (e.g., some cut mostly by shearing), 2) initial degree of dullness, and 3) effects other than sharpening that affect how well the scissors cut.
For instance, the perceived improvement might result from:
The multi-layered structure protects against impact fracture.
If you hit an object very hard, you can create a crack; stresses will concentrate at that crack, and make it easier for the crack to propagate (think of the little notch in the ketchup packet: that's where you can tear the plastic...)
Now if you have a solid body (of anything), then that crack can continue to grow. But if you laminate, then the crack will hit the end of one lamina, and stop. That means that a laminated object will be much more impact resistant: it's easy to initiate a crack on the outermost surface (for example with a carbide-tipped object), but it's much harder to do so on an inner surface (which your tool cannot reach).
You get burned because energy is transferred from the hot object to your hand until they are both at the same temperature. The more energy transferred, the more damage done to you.
Aluminium, like most metals, has a lower heat capacity than water (ie you) so transferring a small amount of energy lowers the temperature of aluminium more than it heats you (about 5x as much). Next the mass of the aluminium foil is very low - there isn't much metal to hold the heat, and finally the foil is probably crinkled so although it is a good conductor of heat you are only touching a very small part of the surface area so the heat flow to you is low.
If you put your hand flat on an aluminium engine block at the same temperature you would get burned.
The same thing applies to the sparks from a grinder or firework "sparkler", the sparks are hot enough to be molten iron - but are so small they contain very little energy.
Best Answer
It might work to some degree, but not by sharpening.
There has been speculation about a way for the foil to sharpen a blade: The foil is coated by a thin film of aluminum oxide (Al$_2$O$_3$), which happens to be a known abrasive and, thus,
The main problem with this idea, besides the tiny amount of oxide found in the nanometer-thick layer, is probably that the oxide is abrasive in its crystalline form, while the thin film of Al$_2$O$_3$ is amorphous (from what I gather from the literature, e.g., papers 1, 2, 3, 4).
Nonetheless, anecdotal evidence is divided on the efficacy of the method (see, e.g., forums 1 and 2). This can be explained by the subjectivity of the reported observations, as well as by confounding factors, such as 1) different types/quality of scissors (e.g., some cut mostly by shearing), 2) initial degree of dullness, and 3) effects other than sharpening that affect how well the scissors cut.
For instance, the perceived improvement might result from:
Note: there is an older question on that in the Skeptics SE.