Gemstone Coloration and Dyeing – Browns and Yellows

At first consideration, it might seem poor organization to include browns and yellows in the same chapter. However, I do this because in some instances the same process imparts brown hues to some gemstone varieties and yellow to others.

Admittedly, brown may not be a favorite gemstone color in nature among rockhounds and lapidaries, but the browns imparted by some of these processes to at least some of the gemstone varieties I have been using are attractive in cabochons made from them.

17. Chromium Trioxide

Materials needed:

  • Chromium trioxide (“chromic acid”), Cr03

Procedure:

This is a one solution, one step process. Prepare a strong solution of the chromium trioxide. This chemical is very soluble, so go sparingly with the water. A pint of water will dissolve almost two pounds of the chemical. It is not expensive. Be careful not to get any of the solution on your skin, nails or clothing. It will quickly produce a stain that is very persistent and can also be irritating to the skin.Immerse clean, dried slabs in the chromium trioxide solution one at a time and carefully so as not to splash any of it out. Allow the slabs to soak in this solution at least two weeks, then pour it of f and store for re-use, indefinitely. Soak the slabs in plenty of water for two or three days to remove excess of the chemical. They are then ready for use.

The response of the eighteen gemstone varieties to this process varies widely. It is questionable whether some were actually improved. However, the fortification types showed up well such as coconut, crazy lace and Mexican fortification agates; also the banded or layered agate sometimes found in Priday Ranch Thundereggs. Some slabs of Mexican dendritic are attractive when colored with chromium trioxide and granite, oolite and stinking water plume usually respond well. Snakeskin agate may color too deeply with this strong solution to suit some tastes. If so, a weaker solution can be used.

This process is not suited to agatized bog, Brazil carnelian, panguitchite, polka dot, troyite and wascoite.

18. Sodium Chromate

Materials needed:

  • Sodium chromate, Na2CrO4 10H20

Procedure:

This is another one solution, one step process and very simple. Prepare approximately a saturated solution of the sodium chromate. It is an inexpensive chemical. One pound of the crystals dissolved in about a cup and a half of water should approximate saturation. Immerse the clean, dried slabs in this solution one at a time, and let them soak for at least two weeks. Then pour of f the solution and store for re-use, indefinitely. If crystals form in the solution, it is saturated. If not, add a little more of the sodium chromate or until no more will go into solution.

You can also leave the lid of f and allow the solution to evaporate until crystals begin to appear. This same solution may also be used for Process No. 19 next.Sodium chromate imparts a nice lemon-yellow to some gemstone varieties but only a yellowish-tan or light brown to others. The yellow that develops on snakeskin agate is especially nice and also that on Mexican dendritic. Polka dot agate and troyite color somewhat less nicely. In general, I prefer the next process, No. 19, although it is not as simple as just plain sodium chromate.

19. Sodium Chromate – Lead Acetate

Materials needed:

  • Sodium chromate, Na2CrO4 10H20
  • Lead acetate (sugar of lead), Pb(C2H302)2 3H20

Procedure:

  1. Prepare a saturated solution of sodium chromate as directed for the preceding process. Immerse the slabs one at a time, and allow them to soak for at least three weeks. Then drain of f the solution, and store for re-use. Rinse and dry the slabs, and they are ready for use.
  2. Prepare a strong, if not saturated, solution of the lead acetate at the rate of about a pound of the chemical to a quart of water. Soak slabs from procedure A in this solution for at least three weeks. Remove the slabs, and store the solution for re-use. It can be re-used indefinitely. Rinse and dry the slabs in the oven at lowest heat.This sodium chromate – lead acetate process imparts a more brilliant yellow than does sodium chromate alone. Furthermore, all eighteen of the gemstone varieties respond, more or less, to the process. Outstandingly, in my opinion, are crazy lace, coconut, Mexican dendritic, moon, polka dot, snakeskin, Priday Ranch thundereggs, troyite and oolite. Even the granite is pleasing; the feldspar takes the yellow color and makes an attractive contrast with the black hornblende. The response of moon agate is also interesting. The “moons” take a deep yellow in pleasing contrast to the amethyst colored agate. Oolite is similar. Some crazy lace agate contains native red coloration. Such agate gives colorful yellow and red designs and patterns with this process.The vivid yellow produced by this process is due to the well known pigment, chrome-yellow (lead chromate) resulting from the interaction between sodium chromate and lead acetate. It is an insoluble compound and forms rather slowly in the slabs. Hence, unless the slabs are given at least three weeks soaking in procedure B, you may find that penetration is not complete.Theoretically, potassium chromate should serve as well as sodium chromate in this process, but I do not find this to be the case. Neither do the dichromates seem suitable.

20. Nickel Nitrate – Sodium Chromate

Materials needed:

  • Nickel nitrate, Ni(N03)2 · 6H20
  • Sodium chromate, Na2CrO4 . 10H20

Procedure:

  1. Prepare a saturated solution of the nickel nitrate. It is extremely soluble so, while stirring, add warm water very sparingly to whatever amount you are working with until not quite all of it has gone into solution. A pound of the crystals will dissolve in less than one-half cup (4 oz.) of water. This same solution can be used for Process No. 29 in Chapter VIII,The Greens . Soak clean, dried slabs in this solution for at least two weeks after which, drain it of f, and store for further use, for it can be used indefinitely. Rinse and dry the slabs in the oven at the lowest heat for at least twenty four hours.
  2. Prepare a saturated solution of sodium chromate as prescribed for Process No. 17. Immerse the slabs from procedure A above in this solution one at a time. Allow them to soak for at least two weeks; three weeks is better. Pour of f the solution, and store it for re-use in this or the preceding two processes. Rinse and dry the slabs, and they are ready to use.The nickel nitrate – sodium chromate process imparts a beautiful carnelian color to snakeskin and troyite agates and perhaps a less desirable yellowish-brown to warm brown to the others that are responsive to this process. Naturally, those gemstone varieties that already have some native red or brownish hues are not very well adapted to the process, e.g. agatized bog, Brazil carnelian and wascoite.

21. Sodium Dichromate

Materials needed:

  • Sodium dichromate, Na2CrO7-2H2O

Procedure:

  1. This is a one solution, one step process. Prepare a saturated or near saturated solution of the sodium dichromate. It is very soluble, so add water to the crystals very sparingly. One pound will dissolve into less than a cup and a half of water. Fortunately, this chemical is comparatively inexpensive. Immerse the clean, dried slabs in this solution one at a time, and allow them to soak for at least two weeks. Remove the slabs and store the solution for indefinite re-use, either in this process or in No. 23, 31, 32 or 38. Rinse and dry the slabs, and they are ready for use.Nearly all of the eighteen gemstone varieties described in this book respond well to this simple process, some more pleasingly than others. The response is much the same as with sodium chromate (naturally) in Process No. 18, but the coloration is more intense approaching orange in some snakeskin slabs and in the white, opaque polka dot agate.This process is the same as in procedure A of Process No. 25, so slabs soaking in sodium dichromate can be used for the deep yellow of No. 21 or the rich green of No. 25.Other dichromates can be used for this process. Ammonium dichromate colors as attractively as does sodium dichromate but is more expensive. Potassium dichromate however, is paler, more like sodium chromate (Process No. 18).

22. Iron Chloride

Materials needed:

  • Iron chloride, (ferric chloride), FeCl3 6H20

Procedure:

  1. Prepare a saturated solution of the ferric chloride. It is very soluble, so add warm water very sparingly to the crystals in a suitable container until they have almost all gone into solution. Soak clean, dried slabs in this solution for at least two weeks. Pour of f the solution and store for re-use. Rinse and dry, and they are ready for use.This simple, one solution process produces a brownish-yellow. It is especially adapted to highly translucent gemstone such as snakeskin, stinking water plume and troyite agates.

23. Potassium Permanganate

Materials needed:

  • Potassium permanganate, KMnO4

Procedure:

  1. Dissolve two level tablespoons (one oz.) of the potassium permanganate crystals in three and a half cups of water. Take care not to get any of the solution on your clothing or skin.
    Note expression of caution on the bottle of potassium permanganate crystals. Immerse clean, dried slabs in this solution and allow to soak for at least two weeks after which drain of f the solution and store for indefinite re-use. Soak the slabs in water for two or three days to remove excess surface permanganate. The slabs may then be used.

    This is another simple, one step process. It produces a brown to brownish-purple color and works best on the more translucent gemstone varieties. Unfortunately, the ultimate color bears little or no resemblance to the gorgeous amethyst color of the potassium permanganate solution itself.

Read also:

Gemstone Coloration and Dyeing – Table of Contents

By George W. Fischer
Copyright © George W. Fischer 1990
1961 Edition, published by Lapidary Journal. Inc. San Diego, California.
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