Hi All, as everybody konw there is Aluminium Anodizing, but I wonder
if there is " Silver Anodizing". Could anybody give me the answer?
How many colors can be make? Is it complicate? I hope you can tell
me all about it, thanks!
Thanks & Best,
Frank
Frank, Anodizing in general is applicable to those metals that are
highly reactive with oxygen, allowing them to form a stable and
strong oxide layer. What anodizing does is to accelerate and increase
this capacity to oxidize, so that the resulting oxide layer can be
substantially thicker.
In the case of the “reactive metals”, meaning titanium and niobium
and tantalum, these oxide layers are dense, hard, and
transparent/colorless. However they are capable of reflecting light
from the surface. Because they are transparent, light also
penetrates, and then reflects from the underlying metal surface as
well. The thickness of the oxide layer is close to the wavelengths
of visible light (or some small multiple thereof), so that the light
that reflects off the metal combines and “interferes” with the light
that reflected off the outer surface of the oxide. The result is
“interference colors”, which is the same mechanism that gives oil
slicks (and butterfly wings) their colors. These colors are intense
and iridescent, but your choices of color with anodizing these
metals is then limited to these “colors of the rainbow” found in the
spectrum.
Aluminum is different. In this case, the oxide layer that forms is
aluminum oxide (like sapphire, by the way), but is initially porous.
That means that it can absorb an externally applied dye. So aluminum
is anodized, and then dyed with any number of different types of
dyes, (even permanent magic markers will work), which means that
virtually any color you can dream of, can be applied to the oxide
layer which will then absorb it. The neat part comes next, since
then, with proper treatment (a soak in hot salt water does it), the
structure of that oxide layer changes, sealing it, and locking in the
absorbed dyes, which then makes that color layer durable and
permanent (if the dyes used were stable against fading, at least…)
But note the common thread. In both cases, these are metal types
which form a transparent, colorless, and highly durable oxide layer.
This happens in part because both aluminum and the reactive metals
(it would not be out of place to say that aluminum is very close to
also being a reactive metal) are highly prone to oxidation. In fact,
simple exposure to air cause all of them to instantly form an oxide
layer, even if only a thin one. Heated or as a dust in enough of an
oxygen rich atmosphere, and they can literally be explosive. Think
of them as being highly flammable, even if in normal conditions, the
instant oxidation does not lead to flames, heat, or destruction of
the metal (and that’s because the oxide layers are impervious, and
adhered to the metal. If that weren’t the case, they WOULD burn with
heat and flame, just as metals like phosphorus can do.)
Now to your question. Silver is one of the noble metals. That means
it’s relatively more free from chemical reactivity than most
substances. Although it does form some chemical compounds, in it’s
native state, it’s relatively stable, which is why you can find
silver in it’s native state in the ground, rather than as a compound
of silver. Silver does form an oxide under the right conditions, but
it is not a highly stable oxide, nor does it form an impervious
stable layer on the silver that can be used for a finish. You can put
silver in an electrolytic bath if you like, as the anode (which is
what anodizing is, after all), and it will not form an oxide layer,
even if enough current is flowing to cause oxygen and hydrogen to be
evolved from the liquid. If the liquid happens to be something in
which silver can dissolve, such as silver nitrate or a cyanide
solution, then this will accelerate that dissolution, which in
silver plating is used to keep the silver content of the bath
constant. The silver anode dissolves. But it does not, under these
conditions, form any sort of useful silver oxide layer.
So the answer is no. You cannot anodize silver for any useful effect
beyond using it as the anode in silver electroplating (or a related
method of silver refining). You cannot color it this way.
That doesn’t mean silver cannot be colored at all, however. One of
the compounds other than an oxide that silver can form, is silver
sulphide. This compound is black in color when thick enough. The
common term used of “oxidizing” silver in order to give it a black
“antique” finish is a bit of a misnomer. No oxide is being formed,
but rather a sulphide (this, by the way, is also why dunking silver
that’s been colored this way into the pickle, does not take the
black off. Silver (and copper) sulphides aren’t much affected by
sulphuric acid or it’s salts. When you heat fine silver, it stays
white, as the thin silver oxide layer that may form, is colorless,
and pickles off (or reduces back to silver). If you heat sterling
silver, it turns black, because the copper forms a black oxide. This
black IS soluble in pickle, so it cleans up in the pickle. Don’t
confuse that black with the black from so-called oxidizing agents.
As to colors other than black, if you control the development of a
silver/copper sulphide layer with those coloring agents (like liver
of sulphur) very carefully, you can get a series of iridescent colors
forming when that layer is still too thin to be black. With care, you
can stop the process at those colors, letting you get at least a
little bit of the sort of “oil slick” colors that anodizing reactive
metals produces, though not as intensely colored. But these layers
are thin and fragile, and the metal is going to want to keep
producing these chemicals over time, so it’s a bit hard to get them
to stay even somewhat permanent.
Could anybody give me the answer?
I think I just did. 
How many colors can be make? Is it complicate? I hope you can tell me all about it, thanks!
Again, I just did. Not much to tell. It’s not a process that works
to color the silver.
Peter Rowe
Sorry, no. Silver does not anodize.
Bill, Deborah, Michele & Sharon
Reactive Metals Studio, Inc