Raymond,
I have been cleaning several of these recently.
It looks like you have both
types of grey on your coin.
There are some areas which appear dark grey in your photo which are likely where the
silvering has worn off (on the
obverse - Constantine's cheek,
part of
his neck, likely the highlights on
his ear, dots on collar of
his cuirass and highlights of some letters; on
reverse - highlights of some letters, especially on the vexilatio/banner,
head of captive to left, body of captive to right, thighs of both
captives). These areas are problematic. This dark grey
patina will start to wear through to copper colour if you brush or scrape too much. Some soaks can make it discolor as well.
The lighter grey is very different, it should appear thick and crusty under magnification. It is around all the details (ie letters, helmet and
cuirass details,
captives,
exergue marks, etc.). This is a form of encrustation which lies over top of the
silvering and it is very likely that the
silvering is absolutely
fine under this stuff. Therefore you can attempt to remove it.
Unlike Kevin I
work these coins dry. First I try the toothpick or bamboo skewer on the light grey stuff. It rarely works at this stage but it does sometimes remove bits of the encrsutation. It also lets you test the
silvering. Usually
silvered coins circa AD 317-330
had a nice thick
silvering so, depending on soil conditions, it is usually quite stable.
After this initial probing I take my
tools - 1) a steel sewing needle in a pin-vise, and 2) an exacto-knife with curved-tip blade. These need to be very, very
sharp! (I keep a sharpening
stone handy and sharpen both
tools every few minutes.) Then, working with either a magnifying visor or a binocular microscope I gently probe away at the light grey encrustations. You obviously need gentle touch and the right angle (very shallow, almost parallel to the surface).
When the grey is crusty (like it appears to be in many places in your photo) I find it will flake away quite easily. The pin/needle works all around the letters and other details, the exact-knife or scalpel works in the fields and can be used to scrape the top
part of the encrustation off detailed areas.
The pin/needle is my favourite tool. If you do it properly it does not leave minute scratches through the encrustation, instead it usually pops the grey stuff off in small chunks. You have to probe gently from all angles. The exacto-kife/scalpel can cause the grey to flake away when used on the top layers. It also cleans the fields nicely.
I usually go around the entire coin, from a variety of angles, once and then put it to the
side again. The temptation to keep at it should be avoided at all costs. I have only ever screwed this up when I kept at it in a cleaning session long after the first pass (small scartches through the
silvering between letters). I have never scratched or messed up when being patient.
After going around with the steel
tools once I usually go over the entire coin with toothpicks/bamboo. This rarely removes more stuff but helps shine it up a
bit and shows the results of the cleaning session better.
I must admit that I have never removed 100% of the grey encrustations this way. I only go as far as I feel comfortable with. Sometimes I will go back to a coin which I declared "done" months ago and give it another round. Nevertheless, this route will easily take coins from little or no
silvering to 90% or more showing.
There is another kind of grey encrustation which is much harder to deal with. It is dark and smooth and at first glance looks like where the
silvering has been worn through. Even though it is relatively flat, when seen under mangification it is clearly on top of the
silvering. I think it is the light grey encrustation which has been worn down a
bit. It will also
pop off and reveal nice
silvering underneath but it is much harder to remove than the
standard light grey encrustation because it is too smooth to get
good purchase on and is quite hard. Usuing the pin gently will occasionally scratch through this form of encrustation. One
good scratrch through is often all that is required as you then have a rough edge to catch on to clean the rest. Often though it is the
part I just leave.
I am not brave enough to Kevin's methods of long GG soak or sbbb on these yet.
Shawn