Bombing of Silver and Gold
For industrial use only
Equipment and supplies required
- small enamel pot (approximately 1 pint)
- sodium or potassium cyanide
- stainless steel spaghetti strainer with a handle
- industrial strength (approximately 27º Baume or 100 volume concentration) hydrogen peroxide
- distilled water
- tap water
- shot glass
- good ventilation
- Heat one gallon of distilled water almost to boiling. Some shops prefer to use tap water. However, the minerals in tap water may cloud the brightening effect of bombing.
- Add 3 balls of sodium cyanide (approximately 6 ounces, avoirdupois) and wait until the cyanide has dissolved. Store this cyanide solution in a plastic jug until ready to use.
- Put 6-10 rings in the strainer and place the strainer in the pot. Pour enough cyanide solution in the pot to just barely cover the rings.
- Heat the solution to 160º F, measuring the temperature with a thermometer. An accurate and consistent temperature from one bombing to the next is important for safe and consistent results. Remove the pot from the hotplate.
- While holding the pot over a bucket, pour 2 ounces (1 shot glass full) hydrogen peroxide into the pot. After several seconds or perhaps as much as a minute, there will be a strong bubbling reaction. This is the bombing. The bubbling may overflow. The bucket is to catch any overflow. Bombing will remove anywhere from 1-3% of your gold. Recovery of this gold, of course, is very important. See Shor's Recovery of Gold from Cyanide Waters instruction manual (available online) for the procedure to recover this gold.
Technically speaking, cyanide is not a poison. It is however, potentially, very hazardous. Cyanide is what is known as an asphyxiate.
The greatest hazard with cyanide is caused when cyanide comes in contact with any acid. The combination of the two causes the creation of hydrocyanic gas. This is the lethal gas used in the California gas chamber for killing condemned prisoners. To avoid even the possibility of such danger, do not allow both acid and cyanide to be in the same room at any time.
In addition, take the obvious precautions. Avoid swallowing cyanide or inhaling particles of cyanide. The asphyxiation danger of cyanide (but not hydrocyanic gas) may be eliminated by oxidizing the cyanide.
The very act of bombing oxidizes most of the cyanide in your solution. The extra oxygen element in hydrogen peroxide (H2O2) provides for the oxidation of the cyanide. Additional oxidation may be accomplished by the addition of household bleach or swimming pool chlorinator.
Recovery of Gold from Bombing Solutions
- Collect your spent solution in a plastic drum or bucket. Stir the water automatically using a pump or, if that is not available, by bubbling air in the container. Hang strips of zinc foil in the container. In about 24 hours, all of the metal in solution should either be loosely clinging to the zinc foil as a brown film or will have settled to the bottom of the container as a brown mud.
- Test the solution using Shor Gold Detection Pills. To test, scrape a little of one of the pills on a paper towel. Put a drop of the bombing solution on the piece of the pill. If you see a color change, the recovery process is not complete. If there is no color change, then recovery is total.
- When the test shows no gold left dissolved in the solution, pour the solution into another container. Be careful not to pour off any of the brown particles which may have settled to the bottom of the solution. These particles are gold. Using a paper towel, wipe the brown film off the zinc foil. You may note that some of the zinc foil has dissolved into the solution. This is a normal part of the ion exchange process that you are doing with the zinc foil. Any zinc foil that has not dissolved may be reused in the next batch that you recover.
- Neutralize and destroy the cyanide and then dispose of it in accordance with your local regulations.