Acid etching? Chemical etching?

[KiddZimaHater]

I was looking on Ebay and Amazon for a pen that will etch metal.
My searches turned up nothing.
What are those chemical/acid pens officially called that etch metal?
I've seen thread gages and personal tools marked with some sort of etching pen, but I cant find any online!
I'm not talking about vibratory markers, I'm looking for the acid kind. (I guess?)
Any ideas?

[RetiredFAE]

Enco carries the SPI Chemical Etching Pen, part number 308-0495, regular price is $11.98, sale price is $7.49, but they really don't work very well. At least, they never have for me.

The electro chemical etchers work much better, but with either method, preparation is everything it seems. Metal should be clean, dry, no grease, etc.

[Jaakko Fagerlund]

I just use electric etching, just needs a 6 V power supply, a "pen" (metal rod or your preferred text as a metallic stamp) and a piece of sponge dipped in brine. Just about 15 seconds will leave a nice text pattern on metal with about 1-2 A current.

However, best result is if you get a stencil that won't conduct until developed, because the sponge leaks current from between the letters and thus ruins the marking if left on for too long.