Gemstone Chemical Coloration Techniques

This article is a chapter of the article “Gemstone Coloration and Dyeing” discussing the different techniques used for chemical coloration. Check out the link below this article to view the Table of Contents.

Cabochons. Slabs or Chunks?

Cabochons, slabs or chunks all may be colored. There are advantages and disadvantages to all three, but in my opinion, it is much preferable to use slabs, especially if cabochons or freeforms are to be the ultimate goal.

Cabochons may be made first and then colored. Under certain circumstances this might be good procedure. However, due to the heterogeneity of most gemstone, even in the same slab, it usually cannot be predicted accurately how evenly a given slab will respond to a coloration process. This is especially true of agate with a pattern or design. It is much better to color the slabs first and then scribe the cabochons in such a way as to take advantage of the often unpredictable interaction of color, pattern, design and other built-in variables. If cabochons are made first and then subjected to coloration, the effects of these variables are likely to be unsatisfactorily anticipated. Thus, any way you look at it, there appears to be no advantage in making the cabochons first, even in homogeneous material, where there is no pattern or design, etc. Since cabochons are ordinarily made from slabs, they might as well be made from colored slabs.

Chunks of various sizes can be colored, but here again, what is to be gained if the material is to be slabbed for making cabochons or freeforms as is likely? Whereas slabs may be colored in two to four weeks with complete penetration, it takes three months or more for chunks, and then they still have to be slabbed.

Plaster Casting of Small Gemstone Pieces

Sometimes a given gemstone comes in small nodules or pieces that are a nuisance to saw individually. Especially if you have a fourteen inch saw or larger, it is quite practicable to embed such material in plaster, whereby you can saw several pieces at each cutting. Plaster of Paris works well for this purpose, but so does “casting plaster” or “molding plaster” available where building materials are sold. It is much cheaper than Plaster of Paris and just as satisfactory.

Select a container to match the size of your saw (e.g., a gallon size square milk carton is ideal for an eighteen inch or a twenty inch slab saw) and place the rock pieces in it. It is a good idea to wash the pieces in household detergent and rinse before placing them in the container. The plaster will not adhere well to dirty rock pieces, and these may break out of the plaster block during sawing, quite possibly damaging the blade. Some gemstone with pattern, inclusions, design, etc., may require orientation as the pieces are placed in the container so as to cut them to best advantage. For instance, you would not want to cut moss or plume at right angle to such inclusions.

Do not fill the container with rock pieces clear to the top. Allow two or three inches free of rock. This will be solid plaster and will be the area held in the vise.

Care is necessary in mixing the plaster with water so as to get these two ingredients in the correct proportions. If too much water is used, the plaster block will be soft and crumbly and subject to breaking apart while sawing. If too little water is used, then the mixture “sets” too rapidly, often with poor percolation down amount the rock pieces. Although the resulting plaster block is hard enough, it may not be continuous, i.e., it will have small to large cavities, and this also invites the pieces of rock to break out during sawing. Mix only enough plaster and water at a time to fill not more than three or four containers, because after mixing thoroughly the pouring time is s hort before the plaster begins to set. If you delay pouring for too long, it sets before it has completely percolated down through the rocks.

A procedure I have found to be reliable is to fill a three gallon bucket about one third full of water, then add the casting plaster a double handful at a time, allowing it to sift through the fingers into the water below. Twelve pounds is recommended to a gallon of water. Do this until enough plaster has been added that it no longer sinks into the water, but actually builds up quite a little mound above the water. Then vigorously and quickly mix the plaster and water with your hands and fingers until it is thoroughly blended. When you feel the mixture begin to thicken, stop and pour it into the mold, quickly. It usually requires a little experience to learn when to stop mixing and begin to pour. If the plaster sets in the bucket with your hands in it, you have waited too long. But, be sure to use enough of the dry plaster powder so that a good hard block of the cast plaster is obtained.

After the plaster has set well (allow a day), remove the block from its container, and label as to contents. In the case of milk cartons, you will have to remove the container from the block. You can start sawing the blocks as soon as they are thoroughly set, but I have found it desirable to allow them to “cure” for a week or so. They seem harder, thus reducing to a minimum any tendency for individual pieces of rocks to break out during the cutting, quite possibly damaging the blade.

Finally, be sure to use only good, fresh plaster. Otherwise, the cast plaster will not set well; it will be grainy with little strength, and will break up during cutting. Good casting plaster is soft and powdery. If it feels grainy or lumpy, avoid it.

Preparation of Gemstone For Coloration

Cleaning and Drying

The most important detail in the preparation of gemstone for chemical coloration is that it be clean. Oil and grease, especially, must be entirely removed. I have found it practical to keep a bucket of detergent solution (ordinarily laundry detergent) standing beside the slab saw. Two or three cups full of detergent powder in about four gallons of water (in a five gallon bucket) is adequate. As the slabs come from the saw, they are dropped carefully into the bucket and allowed to accumulate there.Further cleaning of the slabs consists of two additional washings in hot detergent suds, and thorough rinsing with clean water.

After the slabs (or other pieces) have been thoroughly cleaned, they should be dried. This is best accomplished with the slabs spread out on metal trays (such as TV or serving trays) and placed in the oven. Line the trays with paper toweling (not with newspaper because of the possibility of picking up oil from the newsprint). Set the oven thermostat for the lowest possible heat; this usually is about 140°F. A day or two at this temperature seems to be sufficient drying time. It may be that it is not really necessary to dry the slabs in preparation for chemical soaking. There have been a few times when I have omitted the drying and have put the slabs directly from final rinse of the washing process into the chemical baths, and have seemed to get results as good as when the slabs have been dried. However, it seems reasonable, at least in theory, that optimum results can be obtained by thorough drying of the slabs before immersing them in the chemical solutions for two reasons:

  1. The heat drives the moisture out of the pores of the gemstone, thus facilitating at least reasonably rapid penetration of the coloring solutions.
  2. Immersing the (moderately) hot slabs into the (moderately) cold chemical solution creates a vacuum within the pores and this serves to pull the solution into the pores.

Acid Treatment (where necessary)

Some gemstone materials have calcareous (limy) rinds or areas that react unfavorably with some of the chemical solutions, contributing to the decomposition of the latter. The only gemstone material discussed in this book that really presents a problem in this is the “coconut”. The calcareous rind, more or less present on coconuts, reacts with the iron nitrate and copper chloride, for example, with resultant deterioration of the solutions.

This problem can be overcome quite simply by acid treatment of the slabs. Use commercial grade hydrochloric acid (same as muriatic acid) for this purpose. The acid need not be used full strength and can be diluted with water (one part water: three parts acid). Always add the acid to the water, not the water to the acid. Gently drop the washed slabs into the diluted acid, so as not to splash it, do this job outdoors or where there is plenty of ventilation.

The limy areas on the slabs will dissolve away in the acid, “fizzing” in the process. Leave the slabs in the acid until the fizzing and bubbling have completely stopped. This will require several hours or more. Overnight is good. Then carefully pour of f the acid into another container. It can be re-used as long as active “fizzing” results when coconut slabs are immersed in it. With such use, the acid will turn first yellow, then progressively deeper shades of brown. By the time it is “used up”, it will be murky brown.

The acid treated slabs should be soaked in water for at least as long as they were immersed in the acid. Then they should be washed once more in detergent water, rinsed and dried, as before. They are now ready for soaking in chemical baths.

Be sure to use glass or plastic containers for this operation of acid treatment of slabs. Plastic containers (such as a bucket or wastebasket of appropriate size) are really preferable, since glass is easily cracked or broken, spilling the acid. Keep a lid on the container, not only when in use, but at all times and at a place where children or pets cannot get into it.

Not all of the chemical processes to which coconuts respond well require prior acid treatment.

The amount of calcareous, or limy, rind on the slabs varies considerably with different specimens of coconuts. The extent of acid treatment necessary can be considerably reduced by first carefully snipping of f the limy rind around the slabs, using pliers, or preferably, tile snippers. This leaves less to be dissolved away by the acid and therefore, makes it last longer.

Soaking in the Chemical Solution

After the slabs have been washed and dried as outlined above, they are ready for soaking in the desired chemical solution. There is nothing mystical about this; it is really quite simple, but a few directions and precautions are in order.

The chemical solutions are best kept, and used, in glass containers. Plastic is alright but I personally prefer glass because I like to be able to see “what’s going on”.

Simply immerse the dried slabs (preferably while still quite warm from the oven) carefully into the desired chemical solution. Avoid splashing, for these chemicals will not only color gemstone, but many other things as well, and much more quickly. Keep your fingers out of the solutions. They may be irritating to the skin; and also, multi-colored fingers and nails may make a negative “conversation piece” at the bridge table.

The duration of the soak varies with the different processes. For some, two weeks (minimum) is sufficient for complete penetration of 3/16″ – 1/4″ thick slabs. Other soaks require four to six weeks, or even more.

Many of the chemical coloration processes (most of the best ones) require separate soaks in two different solutions to achieve the desired color. The second soaking is done the same way as the first. Simply remove the slabs from the first soak, rinse, dry, and then immerse them in the second solution for the required time.

The ambient temperature during soaking does not seem to be critical. Therefore, I have not conducted experiments to determine precisely the optimum temperature for penetration of chemical solutions into gemstone slabs. I have not been able to detect significant differences in penetration between 45 0 -50°F and 80°-85°F although theoretically, the rate of penetration should be influenced by temperature. So, for all practical purposes, “room temperature” (68° – 70°F) is probably sufficient.

Heat Treatment

Some kinds of gemstone (e.g. Brazil carnelian) can be improved in color by heat treatment, without benefit of any chemical soaks. This is possible when the gemstone has some native “impurity” in it, particularly iron compounds that impart improved color changes when heated sufficiently.

Two of the chemical coloration processes described in this book involve the introduction of chemical compounds that result in marked desirable color changes when the soaked slabs are properly heated. This heat treatment must be accomplished so that the slabs (or other pieces) are heated very gradually, and then allowed to cool very gradually. A “sand bath” is ideal for this. This simply means that the slabs are embedded in sand during the heat treatment. This promotes gradual heating and gradual cooling.

Some kind of metal container is necessary such as a bread pan or refrigerator pan. It should be five to seven inches deep. The sand I use is common plaster sand, available where building materials are sold. Pour a layer of sand into the bottom of the pan to a depth of about an inch. Place a layer of the chemically treated slabs to be heat-treated on this layer of sand. Cover with another layer of sand about one-half inch deep. Follow this with alternate layers of sand and slabs until the pan is full. The top layer must be sand, of course, and approximately one inch deep. If the pan has a lid, so much the better.

When the sand bath has been packed, it is ready to be heated. The oven in an electric kitchen range serves nicely for this and is the only kind I have ever used. Probably a gas oven would be just as satisfactory or a kiln. The important thing is temperature control. The following heating schedule is recommended:

  • 175°F Several hours (overnight is fine)
  • 275°F Four hours
  • 375°F Four hours
  • 475°F 500°F Four hours

Allow the pan and contents to cool in the oven overnight or longer, or at least until the pan feels only warm, not hot. If you can stand the suspense of waiting to see the results, allow the pan to cool until it does not even feel warm. If the sand bath is opened prematurely, fracturing of the suddenly cooled slabs is likely to result. Once you have experienced the dismay of having a beautifully colored slab thus fractured, you will become quite patient in waiting until the sand bath has entirely cooled. After the sand bath has properly cooled, remove the slabs, wash and dry them. The sand may be re-used indefinitely. You may notice that even the sand changes color, usually taking on a reddish tinge.

Chemicals and Chemical Solutions

There are two general grades of chemicals available: “reagent” and “technical” or “purified”. The reagent grade is usually much the more expensive, and is chiefly used for analytical purposes. For our purposes though, the less refined technical or purified grades are entirely satisfactory.

These chemicals can, to some extent, be purchased at or through drugstores, but they are most readily available from scientific supply houses or chemical companies. The chemistry department of your local college, university, or perhaps high school, can suggest sources of supply. Or, “look in the yellow pages” of your phone directory for firms or businesses that deal in. scientific supplies or chemicals.

As with most everything else, chemicals are progressively less expensive in progressively larger quantities. Therefore, I suggest that significant savings may be effected by “cooperative” purchasing by small to large groups of rockhounds or lapidaries such as a rock club.

How much does it cost to chemically color gemstone? Actually, very little because the solutions can be used over and over again. The initial expense is it. Since the solution can be re used indefinitely, it is obvious that the chemical cost of coloring slabs of gemstone is low.

Solutions of the chemicals discussed in this book should be prepared in glass jars or other glass containers. Wide mouth jars should be used, to accommodate larger slabs. The same container in which the solution is to be used is convenient. Most of the chemicals are used in saturated strength, i.e., as much of the chemical as will dissolve in a given amount of water at a given temperature, and this usually is at “room temperature” (68-70F). To make a saturated solution (for our purposes), simply pour a quantity (as prescribed) of the chemical into the jar and, while stirring, add warm water slowly until the chemical is not quite all dissolved. Some chemicals go into solution readily while others do so slowly. Chemicals differ widely in their solubility. In some cases, a pound will dissolve in a pint or less of water. Here, more than ordinary care is necessary to keep from adding too much water.

Treat Chemicals as Poisons

In this section, I do not wish to create fear of chemicals, but rather, respect . If chemicals are used with respect for what they are and can do if misused, there is no reason to fear them, just as the same philosophy applies to medicines, electricity, fire, explosives, automobiles, alcohol, firearms, women, etc.

Some chemicals used in the coloration of gemstone are more or less poisonous and, therefore, it is recommended that all chemicals be treated as poisons. These chemicals come in well labeled bottles or other containers. Likewise, the containers for their solutions and those in which the slabs are soaked, should be well labelled, so that there can be no mistake as to their contents.

Chemicals, like medicines, must be completely inaccessible to children. They should be stored and used on high shelves or behind locked doors.

People differ with respect to their contact sensitivity to chemicals, but it is a good idea to assume sensitivity to all of them and to protect your hands by wearing rubber gloves. If a chemical is accidentally spilled or splashed on the skin, it should be washed of f immediately. Even if it does not feel irritating, it could cause a rash, and it is likely to stain. Protect your clothing by wearing a rubberized or plastic apron. And, obviously, full precautions must be taken to avoid taking even small amounts of these chemicals internally. Some would do little or no harm, but others could be fatal. So, I repeat, treat all chemicals as poisons!

Although I have tried to emphasize the need for caution in the use of chemicals for coloration of gemstone (or purpose really) and shall do so throughout this book, where applicable, I and the publishers of this book must and herewith do disclaim any and all responsibility for any deleterious effects suffered directly or indirectly as a result of the instructions and procedures presented herein. So, you “travel at your own risk”, although the risk is indeed negligible if you observe intelligent precautions. I have been working with these and other chemicals since 1920 and have suffered nothing worse than yellow, brown, blue or green stains on my fingers and nails, simply because I hate to wear rubber gloves. But, I do recommend wearing rubber gloves when working with these chemical solutions, especially for milady.

Read also:

Gemstone Coloration and Dyeing – Table of Contents

By George W. Fischer
Copyright © George W. Fischer, 1990
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