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Etching vs. Patination.


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I’ve been trying to wrap my head about etching vs patination. So I know that the way an etch is understood to work is that it dissolves away some metal. The different colour that results then, is because when the metal is removed, depending on the identity of the metal, it looks different. But then, I don’t understand why different etches have different aesthetic results. For instance, some etches make certain parts look lighter, and others make certain parts darker. How are we sure that there isn’t also some patination happening?

 

The next part of this is Jim Kelso’s iron patina recipe. Depending on the metal and how it’s applied, it can make the patina look dark brown or black. Is that dark brown iron oxyhydroxide, or is it some other kind of oxidation? If something causes a white patina (white rust) on iron, do we assume it’s iron chloride, or do we not really know what it is? I can’t find formulas for any iron or copper patination reactions anywhere online. Is this just a black box we don’t really understand, or am I just not looking in the right places?

 

The reason I ask is using some Japanese natural stones and traditional, open grain polishing techniques, I’ve been getting a dark brown/auburn patina on the ji sometimes. I actually quite like it, and I want to understand how it is happening so I can replicate it more consistently. I think part of it may be the water I use is pretty basic (at least 7.6 per my preliminary aquarium pH testing kit!), and I think there must be something in some of the stones. I just want to be sure it’s not an etching thing, vs. patination, and whether it’s just caused by acid/base type of stuff (and simple iron oxide oxidation) vs. some kind of complex patination reaction like the Jim Kelso recipe (which also uses clay!).

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41 minutes ago, Carlos Lara said:

How are we sure that there isn’t also some patination happening?

On the contrary, we know for sure that there is patination going on.  We know this because of the dark discoloration.  So when we etch something that has Ni in it with ferric chloride, we know that it resists etching since it stays higher than the non-Ni bearing material (not dissolved), and it also resists the patination (also called staining in an etching context) since it stays bright.  Patinas are often just oxide layers, with varying thicknesses affecting the color, but they are also often chemical reactions besides oxidation.  Many good patinas will rely on a bit of etching to expose fresh metal in order for it to react with the other elements/compounds to form a given patina.  

 

I am not familiar with @Jim Kelso's recipes, so cannot comment on them.  Perhaps he will chime in.  

 

White rust has nothing to do with the iron.  It is zinc oxide on a galvanized steel part (or some other element with a white oxide, but not iron).  

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Here's more than you want to know about patinas...

 

https://www.ganoksin.com/search/patinas/

 

But yes, etches leave a patina. The black goo on carbon steel after etching with ferric chloride is manganous ferrous oxide (Fe3O4 with manganese dioxide). What you're getting is probably a simple patina reaction between your water, the stones, and the steel.

 

Jerrod responded while I was typing, but IIRC Jim Kelso's iron patina is the one with hydrogen peroxide and salt, right?  That's just a way of quickly forming hydrated ferrous oxide, aka magnetite or black rust.  

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Posted (edited)

Thanks! Ah, that helps a lot. I'll take a look at that site!

 

Jim Kelso's recipe is here:

 

https://www.jimkelso.com/tutorials/ironpatina.htm

 

It's a weird one with iron oxide, sulfur, copper II sulfate, potassium nitrate, and iron bearing clay. It's pretty easy to use, mix it up to yoghurt consistency, paint it on, then heat it with a heat gun, wash off in water. It smells strongly of sulfur when you use it. It may well be adding ferrous oxide, but sometimes it's more of a deep brown, which I understand is more likely the oxyhydroxide (both are oxides of iron though). Not sure why this mix sometimes favours the oxyhydroxide, that's part of the mystique I guess.

 

I knew about the zinc, but I thought there was also a white patina formed when steel undergoes alkaline corrosion from chlorine in a solution with high pH? I thought that was FeCl, but I could be wrong. It's hard to find information about that, because as you say, most online info focuses on the alkaline corrosion of zinc. Anyway, there are some etches that make steel white, like polishing with the hazuya. Not sure what that is, but I was assuming FeCl, because it is pretty white as a crystal.

Edited by Carlos Lara
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Hah, hah, you guys clearly know much more about the chemistry than I! Hard to separate etching and patina into distinct, discrete arenas wherein results can be predicted based on anything other than shop experience. We work from the shoulders of generations of artisans having little scientific knowledge but centuries of practical experience and observation. I don't mean to belittle chemistry knowledge. I love the little I do know. But nothing replaces trying stuff out, and letting the wonderment flow as things work as planned, but also not. 

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Haha, thanks Jim! I'm a big admirer of your work and grateful of all the details you posted on your site! I'm a scientist by training, so I'm always trying to figure out what is going on. But I think I also love this work because there's so much mystery!

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