There are only two ways that I know of to induce black into gemstone. One of these involves carbon while the other is silver. Theoretically, there should be several other methods possible. I have tested these out however, and they have proved impractical. But black is black, so multiple means of inducing it are not needed. The two presented here are different in that one, the silver, is black with a metallic sheen or luster.
- Honey (any kind, any grade)
- sulfuric acid (concentrated), H2S04
- Dilute the honey with an equal amount of warm water. Add copperchloride (the salt, not the solution) at the rate of one level tablespoon per quart of honey solution.This is to inhibit the development of molds on the surface of the solution. There are some molds that are capable of growing on such a high sugar concentration as this, and they will gradually deteriorate the solution. However, copper (as from copper chloride) will inhibit them and will not interfere with this color process.Immerse the clean, dried slabs (in the oven, lowest heat) in this solution and allow to soak for at least four weeks. Drain of f the solution, and store for re-use. Rinse and dry the slabs in the oven at lowest heat for several hours or overnight.
- The slabs from procedure A above must now be boiled in the sulfuric acid. Use the concentrated acid. Do not dilute . The cheaper, technical grade of sulfuric acid in entirely adequate. Sulfuric acid is powerful and will eat holes in clothing, make nasty burns on the skin, etc., so it must be handled with great respect and caution. The slabs must be immersed in the acid carefully, sliding them in one at a time. Use a Pyrex glass vessel with a handle such as is used in making coffee (I use a two quart Pyrex coffee maker) because the slabs will be boiled in the acid in this vessel and it must be able to take the heat. Do not attempt to use any type of metal vessel .The boiling must be done outdoors because of irritating fumes that are released. I use an old electric hotplate for this in a location that provide an electric outlet, is under cover (roof) and yet is ventilated. It is advisable to put up some kind of barricade around it and also a sign (such as ” HOT ACID!” ) indicating what is going on behind it. Only a low, simmering heat is desirable; just enough to keep the acid boiling hot, but not actively boiling. Leave this eighteen to twenty-four hours, then turn of f the heat and allow the acid to cool thoroughly before attempting to move it. Cold sulfuric acid is bad enough; hot is much worse in case of accidental spillage. After is has cooled, carefully pour all of the acid into a glass jar or jug, and store it for limited re-use. As it is re-used, the acid will gradually turn black itself and become rather watery as contrasted with fresh sulfuric acid which is syrupy. It is best to discard the old acid when it has so deteriorated.After the pot of acid and slabs have thoroughly cooled and all of the acid has been poured off, slowly and cautiously add cold water to the slabs until the slabs are covered. The reasons for the caution is that there will be a little residual acid on the slabs and thus, in effect, you will be adding water to acid. This in principle is a cardinal sin in the chemistry laboratory and with good reason. When water is added acid, there is danger of violent , effervescent reaction, thus spraying acid over everything and everybody nearby. In this case however, the amount of acid remaining on the slabs is comparatively minute and cold rinsing water can be added to them if this is done very slowly.
I have done it many times. Another slower but non-hazardous method is to pick the slabs out of the pyrex pot one at a time using tongs of some sort, and slide them into a pan of cold water. Allow the slabs to soak in water for several days to soak out the acid from the pores of the slabs. If this is not done, the acid remaining in the slabs will draw water from the atmosphere and the slabs will “sweat”. So will the cabochons if made from slabs that have not had residual acid soaked out of them. During the water soaking, change the water every day or two. After six or seven days of this soaking, the slabs are ready to use.
This honey-sulfuric acid process produces some striking effects on some of the gemstone varieties and is totally unsuited to others. Mexican dendritic and moon agate, for example, become totally black and, of course, the inclusions are lost. Wascoite loses its identity likewise and so does panguitchite. Granite and moabite become splotchy at best. The most attractive response is seen in coconut agate. Here, some of the fortification layers (frequently and fortunately alternate layers) absorb the honey and turn black and some do not, thus beautifully accentuating the design and pattern in the agate, especially when the non-coloring layers are white. Some components of agatized bog take on the black while some do not resulting in interesting contrasts. The oo’s of the oolite remain unaltered while the matrical agate turns black again providing interesting contrasts. Crazy lace agate looks like the “black lace” that used to be on the market. Finally, snakeskin agate becomes beautiful, yet black, and will rival black jade anytime.
Sugar can be used in place of honey at the rate of about a pound to a quart of water, but after using both extensively, I think I get better, more consistent results with honey. That there should be any difference defies theory, of course, because either way, the boiling in sulfuric acid reduces the sugar or honey (which is several sugars) to carbon, which is black. There is no way of impregnating gemstone with carbon directly. Carbon is insoluble in almost everything. The use of honey or sugar as in this process is a sneaky way around this insolubility.
This method of coloring gemstone black is an old one. Agate so colored has long been on the market as “black onyx”.
35. Silver Nitrate-Potassium Chloride
- Silver nitrate, AgNO3
- Potassium chloride, KCl
- Photographic developer
- Prepare a solution of the silver nitrate at the rate of one-fourth ounce dissolved in one and a half cups water. Soak clean, dried slabs in this solution for at least two weeks. Drain of f the solution, and store in a brown bottle or in a completely dark place.
- Prepare a solution of the potassium chloride at the rate of about six ounces dissolved in one pint of water. Soak the slabs from procedure A above in this solution for at least four weeks. Drain of f the solution, and store for re-use. It can be re-used several times after which it is well to discard it. Potassium chloride is a cheap chemical.
- Expose the slabs from procedure B to strong light for several hours (at least five). A 200 watt light will do nicely or sunlight is even better.
- Soak the exposed slabs in photographic developer solution such as is used in black and white photography. Use the most contrasty developer you can get. The slabs should turn black immediately, but leave them in several hours so that they can turn black all the way through. Pour of f the developer solution, and store in a brown bottle or in a dark place.