question about patinas
Sorry if this has been brought up (likely in that 50 page patina thread which I really can't spend time filtering through) but I'm curious on the affect different foods have on different carbon knives regarding patina.
For instance, I find that my White 2 yusuke suji and masamoto ks take and hold more of a blue patina although the masamoto eventually darkens over time (probably because I use it to cut juat about everything whereas the suji is used for fish) A fujiwara carbon gyuto I lent out to my housemate (also a chef) tends to darken easily and seems to skip the blue phase altogether or is very short lived. I have a clad blue 2 mizuno which takes on a bit of a blue colour on both cladding and core steel and then the cladding seemingly takes an orange colour all of a sudden (which I thought was rust at first) which is pretty ugly. My sugimoto carbon shows a mixture of blues and browns which is okay although I think I prefer keeping it clean. I have a kurochi itinomonn which I scrubbed off and polished up and have started a kind of kasumi finish on. The blade road takes on a brilliant, striking blue and then seems to fade away to grey. Strangely, the cladding (which is supposed to be reactive) shows no discoloration at all. Its almost as though it's stainless (although I dont go chopping onions with it...I use it for cooked meats primarily on the line). Hiromoto as core steel tends to go black relatively quickly as well from experience.
What's going on in these knives that I don't understand? Does polishing iron cladding reduce cladding reactivity extensively? Is brown patina something that turns up in less pure alloys like the cladding on my mizuno or possibly even the carbon alloy sugimoto uses (I'm not sure though as the fujiwara is a pretty impure steel compared to white steel and it doesnt go brown at all)? Does the fujiwara darken quicker due to lower carbon content/lower hardness or perhaps due to the chromium content as the hiromoto tends to darken quickly as well? Do different alloys just react differently to different acidic foods?
Some answers would be great as I obviously have a lot of questions. I know the patina doesn't affect performance but it would be good to know how I can control the colours that appear on my knives so maybe I could possibly even customise it to suit me. I really like the blue colours I get sometimes and would probably prefer to keep that colour over the murky brown colours I get on some of the knives.
Iron reacts with various other things to make insoluble iron salts, which we call patina. Tannins produce various shades of blue and purple to black -- old fashioned ink was made from oak galls and rusty iron, producing a ferrogallate solution that oxidizes to black ferric gallate in the presence of air. Phosphates are usually bluish gray, and you can always soak your knife in soda for a couple hours to force an iron phosphate patina. It's quite non-reactive and protects the iron from rusting fairly well.
I don't know if polishing does much, but if the polish you use has some wax in it (or other substance that coats the iron with a water insoluble film like silicone oil) you will prevent the iron from reacting with food as well as patina formation.
I can't give you much guidance on maintaing colors though. My Korean carbon steel gyuto is sort of a bluish gold color at the moment. I used it to split immature pecans to check on the development stage of the drops from my trees, and rubbed the split nut, full of tannins, on the blade to help force a patina since it stank like crazy cutting onions when it was new. Seems to be very non-reactive now, and since I could care less about what it looks like, will leave it alone unless it gets rusty or something.
My very old Old Hickory butcher knife is nearly black, and it's not going to get shined up either.
After Peter's competent answer, just one remark: yes, the finish matters. Not as much as to know which colour will appear, but rather how fast patina will install. A polished blade is less reactive. On some blades I use coarse ScotchBrite before degreasing and applying the etching agent to fasten the process.
Looks like you have some nice carbon blades. With home use good luck on controlling colors. Cutting some proteins will produce blues on bladeface. At work I used carbons, cut everything with them would take on a dark grey patina with lots of use.
At home my carbons have different hues. Have forced patina with mustard, vinegar, & lemon juice, applied with sponge. Then just cut with them. Wash with warm soapy water dry completely. Let the patina go it will protect from rust & as it gets well established the knife will take on a deeper hue.
I have a solid super blue knife (folder from Spyderco, not a kitchen knife) and I used vinegar to force patina on it. It turned deep grey. It looks good to me and it also seems to be very stable chemically and mechanically:
It is the one on top:
Cut lots of tomatoes, it will turn it black, look awesome and form a protective layer.