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Etching science: undercutting or lack thereof

I did a trial run of silver etching with ferric nitrate today to test my resists, and while showing my friend the results she asked a question I wasn’t able to answer. I wonder if someone here knows the chemistry/physics behind this.

I know some etching processes are more likely than others to produce undercutting, but how and why is that possible? If the walls of the etched cavity are exposed to the etchant just as the open places not covered by resist are, why doesn’t the etchant eat sideways under the resist on the surface too?

Also, under the “pics or it didn’t happen” doctrine, here are my test pieces. The one with the cursive writing has me most intrigued- I coated it with nail polish, then scratched through it with a scribe. It’s amazing how fine and crisp the lines are! I’m imagining quite a few ways to use that technique. The one at the far right was the sole failure- acrylic paint just was no match for the ferric nitrate.


looks like you had fun today!
those look amazing!
i have never etched…what other resists fid you use?
each piece has different effect…

the round one with the hole etched nice and deep!


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On the round and the nopes I used oil based paint pens, and on the big rectangular piece on the left I used embossing ink and powder from the scrapbooking section of the craft store. You use a rubber stamp to apply the ink, dump the powder on which sticks to the ink, then you melt it with a heat gun or (very carefully) a toaster oven. It makes a plastic coating. You can see that it kind of beads up on the metal and gives a pixelated look.


To answer your question about the sidewalls: it all depends on what kind of etchant you’re using. There are two different types, isotropic and anisotropic. One ‘eats’ in all directions (isotropic), and the other only in a single direction (anisotropic). My guess is that ferric nitrate is anisotropic. If I recall from my chemistry classes, there’s a fair bit more complexity as to the exact reasons for the different behaviors, but I imagine it mostly has to do with the molecule shape, ion charges, etc.

If you were using a different etchant, you could actually control the undercutting (called a ‘bias’) based on a few factors (time, type of etchant, etc). There’s a bit of math involved if you wanted to get a super specific bias, that I won’t go in to.

Hope that answers your question!


That’s very helpful, thanks! Having the right words to look up unlocks so much. :grin:

There’s a science to etching

For every mm down into the surface, the side etch factor is one half of that.

I’ve been fortunate to have commissions that were architectural scale etching (conventional chemical milling). The ratio becomes quite obvious at that scale.



Interesting! So that makes sense why on the scale I’m working at it’s all but invisible.

was this sterling or fine? I have done lots of f.c. etching on copper & copper based metals but not silver. I am anxious to try this. are there other metals that f.c. will etch?

I don’t etch, but I think that you need ferric nitrate for silver…Rob

I have had a lot of fun with home-made electro etchers on stainless and chrome plate. A car battery charger, a dc “wall wart” or even 9 volt batteries have all worked great for me. Usually I use a Q-tip saturated with salt water gripped by the negative alligator clip with the positive clipped to a part of the piece not getting etched (switching polarity seems to cause a dark dirty etch vs clear clean etch, much like liver). The average piece will be something like a knife blade receiving a makers mark, and the etch can be quite deep. Chrome plate disappears quickly revealing nickel then copper then steel if careful. A moist dabbing with a clean-ish Qtip, with light pressure rather than a wiping action, is easier on the mask. Having an “unfinished” part of the piece available for the anode clip that can tolerate some marking is handy. It is as fast as you dump voltage/ current into it, the faster the coarser, the more undercut and the harder on your mask. If you are new electricity stick to a 9v battery, steal a snap connector off of a dead device, attach alligator clips and have fun w/ a q-tip and salt water. Works fast and is safe. Works great on steels and copper, might work great on silver but haven’t tried it. Borax solution is also a great (possibly better) electrolyte but I only use that when I am dipping an entire part, I should try wiping w/ it sometime. I have also made a sock an inch wide to electro-etch larger pieces I couldn’t dip using a battery charger, works great for removing heat tint from tig welds, leaves a light frost, would etch the same as above but will dump too much current for masks with small open area causing undercutting and mask degradation.

Have fun!

I was working on sterling. And yes, there are different chemicals- ferric nitrate for silver, ferric chloride for copper, brass, etc. I’m not sure if FN will etch the base metals, but I do know FC will not etch silver. There are many threads on here with great information on both techniques.

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All good info…the chemistries are pretty logical… primary factors to consider with chemical etching

  1. concentration of reactant…higher concentration = generally means faster reaction
  2. Heat of reactant higher is faster. Professional etching companies all use heat.
  3. Mixing - more mixing means moving reaction products away from the surface to make room for fresh reactant to get in. Professional etchers often spray the surface with etchant but a magnetic stirrer will work nicely.
  4. All the pros add the complimentary acid (HCl or HNO3) to the material as the stability of both ferric chloride and nitrate are maintained in an excess acidic environment. without this excess acid (increased pH) both salts convert to the hydroxide which is unreactive with metal. You can tell as Fe(OH)3 is a milky orange to brown precipitate. However, too much XS acid will lead to undercutting.
    Finally, the logic of chemical reactions is that a fresh solution will allow the reactant to easy find the metal surface. However, as reaction byproducts build up they will physically get in the way of the reaction. So this means that you can freshen up your etchant solution with more salt or acid but eventually you will have to start over. This actually was a problem for one of my customers.