Gemstone Coloration and Dyeing – Introduction
The prime requisite for coloration of gemstone is that it be porous . That is to say there must be spaces or vacancies into which the coloring chemical, in solution, can penetrate deeply in order to impart the desired color. These spaces are usually spoken of as "pores", but from what I have been able to observe, they are not pores in the sense of parallel tubes. Rather, they are.
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I make a sharp, if academic, distinction between chemical coloration of gemstone and dyeing. While, in the dictionary sense, a dye is any coloring material, in the practical sense, dyes are organic, highly complex, coal tar derivatives.
Chemical Coloration vs. Dyeing
In my experience, at least, they will color gemstone only very superficially, if at all and, therefore, cannot be used satisfactorily to color slabs from which cabochons are to be made. Thus, they can be used only on finished cabochons and baroques and, as will be brought out later, it usually is not feasible to make cabochons first and then color them. Furthermore, and again in my experience, even if some superficial coloration is imparted to gemstone by an organic dye, the color is not "fast", i.e., it will fade in sunlight and/or during tumbling. This prejudice against dyes is based on personal trials with well over thirty organic dyes.In contrast, with a few noted exceptions, aqueous (water) chemicals solutions will penetrate slabs or even chunks of gemstone, and the resulting colors are light-fast and tumble-fast. Also, the colors imparted by these chemicals are much like those in naturally colored gemstone whereas, what color may be induced by dyes tends to be unnaturally garish or gaudy. This distinction between dyes and chemicals as coloring agents for gemstone may be "splitting hairs", but nevertheless, the distinction exists, and it is practical.
Why Color Gemstone?
Once in a while, I encounter someone who (at first) looks askance at chemical coloration of gemstone, on the grounds that it is "unnatural" or "artificial". These allegations do not appear valid to me.
In the first place, color is an important attribute in gemstone, and to old chromophile like myself, it is most important. Surely the supreme popularity of the more colorful gemstone varieties would bear this out. And, if a particular gemstone has all the other desirable attributes (hardness, design, pattern, polishability, etc.) but lacks color, then why not impart the color, if possible? The intrinsic beauty of such gemstone can only be enhanced by so doing.
I fail to see how chemical coloration of gemstone can be considered "unnatural". Such coloration follows certain natural laws of chemistry and physics, and very similar chemicals are used as give "natural" color to gemstone. Even only a cursory study of the chemical composition of recognized gemstone materials quickly reveals the presence of such metals as iron, manganese, copper, nickel, cobalt, aluminum, chromium, lithium, etc. These are not in the metallic state, but occur as more or less complex inorganic compounds, as "impurities" in the gemstone. Thus, jade (nephrite) apparently gets its color from iron in its composition; chrysoprase from nickel; amethyst, rhodonite, and rhodochrosite from manganese; chrysocolla, turquoise, azurite and malachite from copper; ruby from chromium; emerald from beryllium; and so on.
Finally, with the "naturally" colored gemstone becoming ever more scarce and expensive, it seems logical to me to try to simulate them by inducing appropriate coloration in the gemstone materials that are still relatively abundant and inexpensive, but more or less lacking in color. If the end product is as beautiful and durable as apple-green jade, for example, and at a very small fraction of the cost, then the simulated gemstone is surely justified.
What gemstone varieties lend themselves to coloration?
The prime requisite for coloration of gemstone is that it be porous . That is to say there must be spaces or vacancies into which the coloring chemical, in solution, can penetrate deeply in order to impart the desired color. These spaces are usually spoken of as "pores", but from what I have been able to observe, they are not pores in the sense of parallel tubes. Rather, they are extremely minute connected spaces of irregular shape and orientation. Indeed, the fact that in a given gemstone material some coloring agents can penetrate with comparative ease, while others penetrate very, very slowly, if at all, suggests that these pores or spaces are so minute as to accommodate only substances with small molecules. This would explain, at least in theory, why, in general, the inorganic salts of the metals penetrate readily while the complex organic dyes with large molecules, are excluded and, therefore, give only superficial coloration. If complete penetration is effected in a 3/16 inch thick slab in two to four weeks, this is considered "readily" coloration. As will be brought out time and time aqain, this differential coloration often works to the distinct advantage of the lapidary artist.
The porosity of a given gemstone can vary tremendously. This might be expected in heterogeneous materials, wherein the different components might be so physically different that some will color nicely, while others will not color at all. Agatized bog, granite, and other conglomerates, and many of the "fortification" types of agate are examples of such differential.
Some agates such as the beach agates of Oregon and Washington appear to be homogeneous and could be expected to color uniformly throughout. Sometimes they do and sometimes they do not. Frequently, the interior of such agates is quite noticeably more porous than the peripheral area, as indicated by differential coloration. In such cases, the peripheral area will color little or not at all, while the much larger interior will color well. I consider this to explain why I have failed frequently to color whole beach agates, but, if they are sawed in two to expose the interior, they then color well. Possibly if such beach agates were tumbled excessively in coarse grit, the impervious peripheral area would be ground off and the agates would then color readily.
The distinction between organic and inorganic substances revolves around the presence or absence of one element carbon. Organic compounds are usually of direct or indirect plant or animal origin, contain carbon, and usually are very complex in their molecular compositions. Inorganic compounds lack carbon and are relatively simple in composition.
In general , chalcedony, agate, and agatized materials (bog, wood, etc.) respond well to chemical coloration. However, there are many exceptions to this generality. I have not been able to discover any means of knowing if any particular gemstone material will respond to chemical coloration other than to try it. Usually, a good test for this is to immerse a few slabs (from different pieces) of the questionable gemstone material in sodium dichromate for a day or two. If it is yellowish at this time, the material in question will respond to at least some of the processes discussed in this book and probably, but not necessarily, all of them. If no yellowishness is evident, the material is, at best, a poor prospect for any process of coloration.
Who can color gemstone?
Anyone who can read, understand, follow a few simple directions, and observe a few simple precautions can participate in the wonderfully fascinating and absorbing art of chemical coloration of gemstone. There is no more need to be apprehensive of it than applies to many things we regularly do. All have their hazards if precautions pertaining to them are not observed.A knowledge of chemistry is not necessary, although, naturally, even a year of high school chemistry would certainly help in understanding the procedures involved in chemical coloration, just as the housewife understands many of her cooking procedures better if she knows the chemistry behind them. And the rockhound who has some knowledge of chemistry is more likely to be innovative, and thus develop new procedures. However, I hope I succeed in outlining the various procedures for chemical coloration in such a way that they can be followed by persons with little or no knowledge of chemistry, with confidence of success. Most of the procedures are very simple indeed.
Rockhounds, lapidaries, dealers, all who are fascinated by the rockhound hobby, should find fulfillment in the art of chemical coloration. The initial cost of the chemicals may at first be discouraging. Cooperative purchasing is suggested. Cooperative groups such as lapidary clubs, etc., could be the answer. Also, school projects should fit in here. Middle and high school science classes should find agate coloration projects a good outlet for their needs in these endeavors. The basic principles of chemistry are illustrated in such projects.
What facilities are required?
Only simple facilities are required for chemical coloration of gemstone. Some sort of sink is very useful, as is running water. Glass jars of various sizes are needed to contain the chemical solutions and the slabs soaking therein. Plastic containers will do, but I much prefer glass, because the contents are visible. Shelving for storage of the jars of solutions is certainly desirable. Two of the processes require an oven, at high heat (475 - 500°F). The kitchen range is quite satisfactory for this, provided, connubial harmony is not thus disturbed, and the Thanksgiving turkey is not preempted.
I do not recommend the household kitchen as a "laboratory" for chemical coloration. The chemicals used will not only color gemstone, but much more quickly they will stain carpet, vinyl, linoleum, formica, woodwork, concrete, and the sinks themselves. A sink or laundry tub in the basement or laundry room is preferable because, no matter how careful you are, sooner or later vari-colored spots will appear, and while they do no harm, they do not fit in with the average kitchen decor.
Does chemical coloration produce "Fast" coloration?
Chemical coloration definitely does produce "fast" color in gemstone, with a few minor exceptions that will be pointed out where applicable. This is the big advantage that chemical coloration has over the use of "dyes". While dyes will give more or less fast colors on fabrics, etc. they fade in gemstone. At least that has been my experience, and others have told me the same. Possibly this is because dyes do not readily penetrate agate and similar gemstone and thus, give only superficial coloration.
Also, in general, gemstone that has been chemically colored is tumble fast. Thus, baroques may be made from chemically colored slabs or other pieces. However, I personally prefer to tumble-polish baroques first and then color them. That way, I do not feel that I have to leave the baroques in the chemical baths long enough to get complete penetration; and I still get plenty of coloration.
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