Hi Mark,
For your
sestertius I would probably use the blade flat and at a 30 degree angle from the vertical, if that makes sense. So kind of lying flat on the coin, but tilted along its length. Then I would make
very small scrapes on the crusts using a circular motion and see what happens, i.e. flaking, powdering, nothing etc. The key is to be patient and do not apply too much pressure through the blade initially. If nothing happens, I would then apply a little more pressure and try again. Depending on what happens as the pressure increases then I may change the angle of the blade, but try to keep the actual blade flat on the coin at all times to reduce the chance of nicking the
patina with the tip of the blade. Obviously around devices and legends it is necessary to use more of the tip of the blade, and this is where you are more likely to catch the
patina, so take these bits even slower. I know this sounds basic, and I don't mean to sound patronising. This approach would allow me to understand how to attack the coin, and whether to continue.
One thing that I have found useful however, and this may go against some other views, is not to use a fresh
sharp blade. Instead I find that using a slightly blunted blade
still allows me to remove encrustation cleanly, but with less chance of damage to the
patina. You can kind of use the blade to "grind" the
patina a little when it is slightly blunter.
Also, I should say that using a blade on this coin for me would depend on the strength of the
patina. If it is a softer
type and easily marked then I may not risk it, however if it is
good and hard, then I would be happy to proceed.
I
hope that helps,
Mark