I expect the damascus & wootz experts can make this clear.
I am a metallurgist knowing only what I have read, concerning wootz, but with a couple of years practice in the field of metallurgy.
In a damascus shotgun barrel the high carbon steel - and I would guess this maybe 0.6 - 1.0% carbon - will "etch", or corrode, faster than will low carbon wrought iron, this latter being no more than 0.1% carbon. Bars of high carbon steel and low carbon wrought iron are twisted and forge welded together. As forged and polished, such a barrel would show only the faintest pattern, if any. In manufacture the polished and degreased barrel is coated with a range of corrosive, and amazingly poisonous, chemicals which corrode/rust/whatever the high carbon steel much more than they do the low carbon iron. Hence, a pattern.
Wootz is different. So, well, I know no other way about this than to give a small lesson in metallurgy. At room temperature, annealed steel can have up to about 0.8% carbon in the form of what is called "pearlite". This is a phase consisting of layers of more or less pure iron and iron carbide, Fe3C. (It is called "pearlite" because of an observation made by some guy a century or so past, nothing to do with current subject)
If you make steel with more than 0.8% carbon, all that carbon above 0.8% will be in the form of chunks of iron carbide, Fe3C. The carbide is hard but this hardness does not show up in a conventional Rockwell hardness test, which is a matter of making a dent with a diamond, and a specified load of a couple hundred pounds. Big dent = soft, little dent means hard. When you put wootz on a Rockwell hardness machine, that machine is mostly just measuring the hardness of the pearlite matrix.
Iron carbide is extremely hard, and brittle. It is good to have some in steel for wear resistance.
Wootz can easily be 1.7% carbon, so there is lots of iron carbide around. And yes, iron carbide does not etch as readily as does pearlite.
For reasons best explained by Anne Feurbach, this iron carbide in wootz is present as bands. So, if one polishes and then etches wootz, these bands of very hard iron carbide will be visible as a light pattern within a darker pearlite matrix. If the smith does his job artistically, he can make these bands into various patterns, e.g. Muhamed's Ladder, or the Rose.
I am assuming the wootz has not been hardened.
To make a sword one normally heat treats, or hardens, common steel. Wootz is a bit different, as even if it is not very hard (a modern custom knife might be Rockwell C58, a machete would be closely controlled to be Rockwell C52, and a power lawnmower blade Rockwell C40) wootz can hold an edge because about half of it is made up of extremely hard iron carbide. An un-hardened wootz blade should be able to hold an edge for cutting through armour and flesh, yet still take quite a beating without breaking in half.
I am not a blademaker, just a guy whose job has involved regular steel. Now its time for the wootz guys to chime in.
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