Gemstone Coloration and Dyeing – Cobalt Series

The use of cobalt compounds to impart blue hues to glass, glazes and enamels has been known for centuries. “Cobalt blue” glass is familiar to just about everyone. It seemed logical that cobalt compounds could be useful to impart color to gemstone and this has proven to be true. Several cobalt processes follow.

The Blues: Cobalt Series – Cobalt Compounds

7. Copper Nitrate

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl2 · 6H20
  • Household aqua ammonia as available at the grocery stores is entirely satisfactory.

Procedure:

  1. Prepare a strong solution of the cobalt chloride. It need not be a saturated solution. A pound of cobalt chloride crystals dissolved in a quart of water is sufficient. Strength of solution is not critical in this process and others calling for cobalt chloride. The same solution can be used for all such processes.Soak clean, dried slabs in this solution for at least two weeks at ordinary “room temperature”. Pour of f the solution and store for future use; it can be re-used indefinitely. Rinse the slabs well and dry them in the oven for several hours or overnight at the lowest temperature setting.
  2. Use an ammonia solution, precisely as proscribed for the copper chloride-aqua ammonia process (No. 6) above. Immerse the cobalt colored slabs in the ammonia solution for four weeks or longer. Remove the slabs; rinse and dry. They are ready for use.

Fantastic coloration can be obtained with this process with different gemstone varieties or even within a variety, especially if it happens to be heterogeneous. Blues, purples and amethyst abound. Some material like snakeskin agate may color too deeply to suit some tastes. In such cases, the strength of the cobalt chloride solution should be reduced to one-half or one-fourth saturated. In the agatized bog, the different components tend to react differently to the process, giving blues, pinks and occasionally green hues. Coconut agate with fortifications responds well since there is a differential intensity of color among the agate layers. The same is true with crazy lace agate although the color lacks intensity sometimes. In oolite, the agate colors fairly well, but for some reason it tends to be drab in contrast to the brightness in most of the gemstone varieties I have colored by this process.

Polka dot agate responds well to this process. The pure white, opaque form of polka dot agate tends toward a turquoise blue, with the brown polka dots in pleasing contrast. The harder, translucent polka dot agate takes on lighter shades of lavender-blue. The stinking water plume tends more toward pinkish lavender. It is interesting to note that in troyite the agate portions take on blue to lavender shades, while the opalite portions tend to be pink.

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I have not been able to impart any color to granite with this process. Moon agate is not suitable because it is already amethyst colored.

When the slabs are taken out of the cobalt chloride soak you will notice that they are more or less pink, and you may be tempted to omit the aqua ammonia soak. However, the straight cobalt chloride treatment is another process and will be discussed later as Process No. 12.

When you remove the slabs that have been soaked in the cobalt chloride from the drying oven (procedure A above), you may be startled to observe that although they were pink when you put them in the oven, they are more or less a robin’s egg blue when you take them out. And if they are allowed to remain in the open air for a few days, they will gradually return to the original pink. This is especially true in a humid climate. The reason for this is that cobalt chloride in the hydrated state (i.e. as it is in the crystals as you purchase it) is red . This is why slabs taken from the anhydrous cobalt chloride soak are light to deep pink. But dehydrated cobalt chloride is blue , hence the blue slabs when they have been dried in the oven.

Other cobalt salts can be used with this process. I have used cobalt sulfate, nitrate and acetate. However, I have consistently had better results with the chlorides . The same is true with the next three processes.

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8. Cobalt Chloride – Sodium Carbonate

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl2 · 6H20
  • Sodium carbonate (“washing soda”), Na2CO3 (Laundry or household grade, available at grocery stores, is entirely satisfactory.)

Procedure:

  1. Prepare a strong solution of the cobalt chloride as directed under Process No. 7. If you have already made up a solution of cobalt chloride for this process (or any other calling for cobalt chloride) you can use it interchangeably for Process No. 8. Soak clean, dried slabs in this solution for at least two weeks. Pour of f the solution and store for re-use. Rinse the slabs well and dry in the oven at the lowest possible temperature setting for several hours or overnight.
  2. Prepare approximately a saturated solution of the sodium carbonate; sixteen tablespoons of the soda to a pint of warm water is about right although in this instance, strength of solution is not critical. One at a time, immerse the slabs from part A into this solution and leave them for at least four weeks. Drain of f the solution and store for re-use (indefinitely). This same solution may be used for No. 3 and 5 interchangeably or simultaneously. If used simultaneously for more than one process, label the slabs so as to distinguish those of one process from those of the others. A wax or china-marking pencil is good for this purpose. Rinse and dry the slabs and they are ready for use. This cobalt chloride – sodium carbonate process produces a nice light blue on some gemstone varieties (e.g. snakeskin and polka dot agates) but is pinkish or lavender on others (e.g. crazy lace agate and oolite). On stinking water plume the agate takes on a nice pale blue while the plumes become pink. Likewise, the translucent polka dot agate colors a nice blue with this process while the opaque white of the same agate is an attractive pink.

9. Cobalt Chloride – Ammonium Carbonate

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl2 · 6H20
  • Ammonium carbonate, (NH4)2C03

Procedure:

  1. Soak clean, dried slabs in cobalt chloride solution, as for Process Nos. 7 and 8, for at least two weeks. Pour of f the solution and store for re-use. Rinse the slabs and dry in the oven at the lowest possible heat.
  2. Prepare a strong solution of ammonium carbonate. This will require approximately a pound dissolved in a pint of water. Do not use hot water. Immerse the slabs from part A above in this solution for at least four weeks. Keep a tight lid on the container to minimize escape of ammonia fumes. Pour of f the solution and save for future use. Rinse and dry the slabs at low heat.

As might be expected from the fact that an ammonia compound is used, this process produces coloration somewhat like that resulting from the cobalt chloride – aqua ammonia Process No. 7. However, in some of the gemstone varieties, the color tends more toward lavender and the blues are less intense. Snakeskin agate, troyite,Mexican dendritic agate, stinking water plume, coconut agate and polka dot agate color well by this process. In the latter, the hard translucent type takes on a beautiful sky blue while the opaque white tends more toward pinkish.

10. Cobalt Chloride – Ammoniated Sodium Phosphate

Materials needed:

  • Cobalt chloride (cobaltous chloride), CoCl2 · 6H20
  • Sodium phosphate (sodium orthophosphate), Na2HPO4 · H2O
  • Household aqua ammonia as available at the grocery stores is entirely satisfactory.

Procedure:

  1. Soak clean, dried slabs in cobalt chloride solution (as for Process No. 7) for two weeks or longer. Pour of the solution and store for future re-use. Rinse the slabs well and dry in the oven at lowest possible heat.
  2. Prepare strong solution of ammoniated sodium phosphate. This can be done by either of two methods: prepare a saturated solution of the sodium phosphate and then add enough concentrated aqua ammonia to give the mixture a strong, but not overwhelming, odor of ammonia, or use ordinary household ammonia in which to dissolve the sodium phosphate.
    • i. Prepare a saturated (approximately) solution of sodium phosphate by adding, a little at a time, just enough warm water to a pound (or any given amount) of sodium phosphate crystals to bring them into solution. Add concentrated aqua ammonia (caution-fumes) sufficient to “ammoniate” it (i.e. give the mixture a strong odor of ammonia). The ammonia concentration need not be precise. Approximately one cup of the concentrated aqua ammonia added to a quart of the sodium phosphate solution is sufficient.
    • ii. If you are using ordinary household ammonia to “ammoniate” the sodium phosphate solution, then simply dissolve the sodium phosphate crystals directly into the household ammonia to make approximately a saturated solution of the sodium phosphate.
  3. Immerse the dried cobalt chloride slabs from procedure A above in the ammoniated phosphate solution, one at a time, and leave them for at least three weeks or preferably longer. Pour off the ammoniated sodium phosphate solution and store (in container with tight lid) for re-use. It can be re-used indefinitely. If the ammonia becomes weak (as revealed by weak odor) add more of the aqua ammonia. Rinse and dry the slabs and they are ready for use.

The coloration resulting from this process is similar to that induced by Process No. 7, in that blues predominate from both processes. However, the ammoniated sodium phosphate treatment of the cobalt chloride slabs seems to result in a brighter blue, sometimes with an amethyst tinge. Very pleasing coloration can be obtained with Mexican dendritic agate, fortification agate, panguichite, troyite, stinking water plume and of course, snakeskin agate. Probably most vivid color is produced on the snakeskin agate.

Read also:

Gemstone Coloration and Dyeing – Table of Contents
Gemstone Coloration and Dyeing – The Blues: Iron Process
Gemstone Coloration and Dyeing – The Blues: Copper Series

By George W. Fischer
Copyright © George W. Fischer 1990
1961 Edition, published by Lapidary Journal. Inc. San Diego, California.
All rights reserved internationally. Copyright © George W. Fischer. Users have permission to download the information and share it as long as no money is made. No commercial use of this information is allowed without permission in writing from author.
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