Weird reaction with palm tree sap. Anyone know what's going on?

[White Nomad]

I'm not sure where to post this, but I figured here seeing as it's to do with reactions and such. 
So the story is, I'd made myself a machete from some mild steel flatbar (not sure what SAE exactly, just from the hardware store) and I was using it while gardening just to see how it worked. I was hacking into a palm tree which had to be removes and the sap/tree juice was staining the metal a weird blue purple colour. This coating would really whipe off and I had to take to the blade with some fine grit paper to remove it.

Does anyone know what's going on here? Is something in the tree reacting with the metal? If you know anything do let me know.

[Steve Sells]

sounds like typical staining, FYI your steel wont hold an edge, hardware store metal is mild steel.

[Will-I-am]

Palm sap can be up to 20% sugar.  Can’t explain blue color but friction and sugar equals thick sticky residue.

[White Nomad]

Yeah I know that mild is bad for blades. I used it to test out a design for a blade before I actually made it from the more expensive high carbon or tool steel I have.

 

[Frosty]

2 hours ago, White Nomad said:

Is something in the tree reacting with the metal?

No. If you'd left the sap IN the tree it wouldn't be reacting with the blade. <SHEESH!>

Frosty The Lucky.

[ThomasPowers]

Well tannic acid forms a blue/black metal tannate. Could the palm sap contain that?  I've used black tea for that purpose before.

[teenylittlemetalguy]

17 hours ago, ThomasPowers said:

Well tannic acid forms a blue/black metal tannate. Could the palm sap contain that?  I've used black tea for that purpose before.

I was going to suggest the same thing. I have used tannins to stain steel a dark purple on purpose.  A quick search shows me that Palm trees do contain Tannins. 

I may have to try black tea next time. how was the finish with black tea, did it rub off easily?

[SLAG]

T.L.M.G. and T.P.,

Tannin is found in most tree bark and (less concentrated in ) leaves.

Tanners use, tannins from oak, chestnut, mangrove and the tropical tree quebracho tree. Etc., etc.

Black tea leaves make an excellent tannin-water liquor  that works well with things iron,   like knives.

SLAG.

[ThomasPowers]

I was trying various suggestions in "The Sword in Anglo-Saxon England" as to how pattern welded swords were treated to show the patterns. Immersion in Tannic acid rich peat bogs was one suggestion. I didn't have a peat bog but I could get tea...I used cheap loose leaf black tea and boiled it for a while.   Then dropped in a clean piece of Pattern welded material  and left it overnight. When I examined it in the morning it looked like it had "grown fur" and I thought that that experiment hadn't panned out. However when I took it to the sink to wash it off; the "fur" slipped off leaving the pattern clearly shown in blue/purple black shades that were quite adherent---used it as an example piece for years.  More research showed that Tannic Acid had been used as a rust preventive in the early 1900's too.

Note it did not create any topography though.

[White Nomad]

Thanks for the comments guys! I think the final verdict is that it was the tannic acid in the palm tree. 
I'm thinking about possibly tapping another one of our palm trees and experimenting with using the sap as an etch. I'll post a picture here soon to show what it looks like. 

[ThomasPowers]

Not so much an etchant but a patination mix.