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Another way to get a "black" fill on your etched design, is to go from a DC voltage to an AC voltage.
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I don't know this for a fact, but I would expect if you just reversed the leads on the DC source you would get the same result. i.e. You go from etching (the work is the anode) to plating (the work is the cathode). An AC current is positive for half a cylce, and then negative for the other half. AC means alternating current.
I blackened the etched image in these experiments only as an aid to accessing the etch quality. In my real application, the work will be hot blued after etching. This means that the surface, including the etched image, will be an even, matte black. Depth of etch will be the only thing supplying contrast.
As for the Etch-o-Matic:
An Etch-o-matic can't supply the mandatory 10 mils of etch depth I require, I would like a deeper etch than 10 mils if I can get it. If you look at the Martronics website (
www.etch-o-matic.com) you will see that they consider a "deep etch" to be 3 mils deep. 3 mils would literally disappear on a blued surface. That includes their "heavy duty indusrtial unit".
Martronics' somewhat more industrial oriented competitor, Marking Methods, Inc., claims "up to 15 mils" etch depth. Whether they can do it or not, I don't know.
Both of them use a stencil approach, I'm using a mask approach. Since I'm only interested in a single image, the multiple image capability of a stencil is of no value to me.
The mask method has the advantage of a liquid tight seal between the image mask and the work. This means it will be capable of much higher image resolution with less undercutting than a stencil.
None of this is intended to bad mouth anyone. I just need a process that meets the requirements for the job I'm doing.