blade stains after cutting watermelon

[Gary007]

Just curious, I'm sure there's an obvious explanation that escapes me.
Why, after cutting up watermelon does it stain my blade, and seemingly right after I rinse it off with water.
It comes off easy with Metal GLO, but wondering why it stains?

 

[jefptw]

There was a thread about this about a month or so back. The short answer (for people who like the look) is "acid patina". You can make the whole blade look like that if you soak it in vinegar or lemon juice overnight.

Looks like you split the watermelon. With the thick spine on that FF the edge hardly has a chance to touch the flesh on the lower part of the melon. Not that it made any difference to the melon.

 

[Dave Rishar]

It's a food etch. Let it accumulate or polish it off as you please. Nothing will be harmed either way.

Any food with moisture present (which is almost all food) will eventually cause this. Acidic food will make it appear more quickly. Some claim that the oxidation prevents good hygiene and alters the taste of the food. I generally cook what I eat and haven't noticed a difference in flavor so I stick with low alloy knives in the kitchen. There have been no problems.

Of course, my non-stick cookware is cast iron, so I'm a bit old-fashioned in how I do things. YMMV.

 

[Cliff Stamp]

...wondering why it stains?

It is just rust. Unless the blade is of a high stainless steel they will all eventually discolor. Some just do it almost immediately, O1 and L6 will do it so fast it happens during the cutting in seconds.

-Cliff

 

[munk]

I cut some meat with the cleaver. It etched very nicely, wish I could remember what it was, think it was beef. I was a little surprised as I've heard lemon here most of the time.
It was unexpected, is all.

munk

 

[Fiddleback]

Pineapple works super well. I leave patina. It protects the blade from further rusting. AND, it looks good IMO.

 

[Steely_Gunz]

I etch every blade i use/leave at my office. I've tried and tried to keep them polished. However, when you work around chlorine and have a couple of hot tubs adding humidity to the air...well, steel rusts. However, I found that if I give them a nice deep etch and coat of baby oil or motor protectant, then they don't rust nearly as fast.
I actually use a metal remover for pool water to etch them. It's basically phosphonic (sp?) acid. I just clean the blade with good hot soap and water, wipe it down with alcohol, and just take a paper towel soaked in the metal remover. The blade grays up in a matter of minutes. Then I take a standard alkalinity increasing agent (basically baking soda) and scrub it down to neutralize the acid. Rince it off, dry it, and oil it. Nice dark gray blade that looks really sharp when i repolish the edge to a bright sheen.
This works really well and really fast on Old Hickory blades as well. We have a double edge sticker that we use for various stuff around the office. Generally it gets beat to hell in a month's time. I file it, etch it, and sharpen it. Looks great and it's ready to function again.
Big fan of etching here

jake

 

[Bri in Chi]

There are a number of threads on this subject. Search 'patina' for starters. The gray color of high carbon steel blades is the way they're supposed to look. Polish if you will, the gray will win out in the end. I think it's a complex sulfide coating. It seems to hold oil and prevents the red-oxide rust that is destructive. My venerable Old Hickory and Sabatier carbon steel kitchen knives look fine without any special care other that rinsing and wiping after use. Learn to love it.

 

[Grob]

I found the same thing when dismembering a watermelong with my 12" AK this summer, between that and the pineapple I have a nice fruity patina

Grob