Coloring during the etch will depend on a couple of things, the composition of the steel, and to a lesser extent the type of acid used to etch. In my experience etching, I have not seen time as being as big a factor of coloration (eg. color vs degree of intensity). The longer the etch, of course the greater the build up of oxides providing a more intense coloration (in a severe case completely blackening a blade), but nothing that will fundamentally change the color/hue, eg. from black to grey to blue, etc... When I etch, I normally do a number of short etching sessions. After the blade is cleaned of grease/dirt buildup, I normally start with a lemon etch, mainly to clean the blade, and get a good idea of what Im working with. I have found lemon juice to be a great way to remove stubborn rust, and generally clean the blade. However, unfortunately for some metals, it sometimes etches rather white, and doesnt give a good color contrast. In those cases I will move the Ferric Chloride. It is much stronger (well at least the blend I use), and tends to give blacker etches. It is a harsher, color than lemon juice, and I now tend to avoid it, unless I just cannot get good color with lemon. Anyways, I will normally do multiple short etches, removing excess oxides between sessions, to adjust color. In the end it is up to your own eye, what you want the color to be, but short etches allow for greater control vs a long etch (though a long etch can have more dramatic effects). You make a mistake in a short etch, its easier to remove oxides and start over. Long etches can cause topographical texture on the blade (which is not suited for many pieces), and will require alot of work to get back to a clean start. But my big recommendation, would be to try a few different acids, short mulitiple sessions, and keep an eye on that blade. Anyways my .002 centavos.
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