Sodium bicarbonate in fine powder form is often used when patinating silver. flash stain:

After the flash stain, use the neutralizing powder to remove the oxidation from the high spots. Pour about a tablespoon of baking soda into the palm of your hand, then add a few drops of water to make a thick paste. Use your thumbs to rub the metal with the paste. This will not only remove the oxidation from the high spots, it will also help neutralize the oxidizing effect.
It is very important to neutralize the newly blackened pieces. If you don't neutralize the metal, the sulfur liver will continue to react with the metal and your item will need to be repolished. Leave items in the neutralization bath for approximately 1-2 minutes.
For the darkest black, immersing the object several times in a solution of water and flash stain is better than one long soak. Leave the object in the solution until it turns dark gray. Remove the unwanted oxidation and rub it away with a baking soda paste. Rinse and patinate again. Repeat this until you are satisfied with the black color.
+/- 225 gram