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A practical and educational synthesis of cobalt carbonate: Chris Garcia and David Kammler have experimented with cobalt carbonate and discovered a cost efficient alternative.


EVERY SEMESTER AT MY COLLEGE, my work-study student and I make an inventory of raw materials for clay and glazes. We estimate what is running low and put priorities on what to order first. This past semester, among other things, we were low on cobalt carbonate. I called my clay supply company and asked for an estimate on 4.5 kg (10 lbs). At $27.50 US a pound, I was looking at a necessary material that was going to eventually break the bank. What was driving these prices up year after year? Were there any alternatives that would give me the material I wanted without the heavy cost to my clay program?

This past semester, I was fortunate enough to be co-teaching a class with a chemist. Professor Kammler approached my cobalt predicament and reasoned that while cobalt carbonate may be expensive, cobalt chloride and sodium carbonate were relatively inexpensive materials. Why not synthesise our own colourant in the chemistry lab?

The syntheses presented below may easily be performed with basic lab glassware and are excellent cross-disciplinary activities for both ceramics and chemistry students alike. Many chemistry stockrooms and teaching labs have the necessary chemicals on-hand normally, and since a little cobalt carbonate goes a long way, it doesn't cost much to make enough colourant for a small studio. As student products in many chemistry teaching labs typically end up in waste bins, these exercises are a way not only to obtain expensive material for less money, but also a practical method by which to have students make something useful while simultaneously reducing waste.


One important aspect that would drive any commodity's price upwards is availability. Cobalt is mainly mined in the southern states of Africa and in Canada. While there are other mines in other countries like the US and Australia, Zambia definitely holds first rank in production with Canada following as the second largest competitor for cobalt extraction.

These mines are owned by a handful of large global companies that have repeatedly driven prices up as the demand for cobalt has increased. Like many large, mostly independently owned conglomerates, capitalism and politics have aided in the unregulated monopoly of these companies. One study claimed that price-fixing and government contracts have driven out smaller companies and brought in more and more international corporations to mine national resources. To make things worse, global public lenders, including the World Bank, have helped establish an upsurge of cobalt production despite the damages to the environment and the poor working conditions of the miners. A cobalt miner in Africa earns about two to three US dollars a day. They work in dangerous conditions without protective masks, clothes or helmets. Many of them die each year from accidents that would have been prevented if the companies had invested safety measures to protect their workers.

But it is not the use of cobalt in our glazes and slips that is driving the frantic extraction of cobalt from these mines, it is something quite different. The demand is tied to cell phones. Cobalt is also used to make parts for aircraft engines, magnets, adhesives and other useful products. But the increasing need for more and more rechargeable batteries for mobile phones is driving the cobalt industry into new and profitable financial waters. Considering that the cell phone industry does not seem to be slowing down, we can all expect continued price increases for cobalt.


The syntheses below use cobalt chloride (Co[Cl.sub.2]*6[H.sub.2]O), which is typically the least expensive water-soluble cobalt compound. However, many other cobalt salts should be viable, such as the nitrate (Co(N[O.sub.3])2*6[H.sub.2]O) or sulfate (CoS[O.sub.4]*7[H.sub.2]O).


Dry (or anhydrous) cobalt (II) carbonate is air sensitive and oxidises readily to form cobalt (III) carbonate. As such, this material is difficult to synthesise completely pure without care, and may decompose upon storage. It is for this reason that syntheses of the more stable hydrate (ideally CoC[O.sub.3]*6[H.sub.2]O, but hydration state can vary) or basic carbonate (ideally 2CoC[O.sub.3]*3Co[(OH).sub.2]*[H.sub.2]O, but actual formula varies by lot) are sometimes preferred. Typically, the mauvecoloured basic cobalt (II) carbonate is the colourant obtained from ceramic materials suppliers and may easily be made with basic lab equipment and supplies. The rosy-pink hydrate may also be purchased from ceramics suppliers or chemical companies.

For most glaze firings, the exact cobalt carbonate used should not matter. Above 900[degrees]C (1650[degrees]F, Orton Cone 09), the various cobalt carbonates decompose to their corresponding oxides, which then decompose to form cobalt (II) oxide (CoO), specifically, the desired colouring agent.

The purity of the basic carbonate can vary depending upon its method of preparation. The simpler, easier and cheaper method of cobalt carbonate production, mass precipitation at room temperature, can make less pure material due to rapid production of solids that trap impurities. In extreme cases, this can lead to poorer glaze colouration and performance. Typical impurities in the cobalt carbonates produced by standard methods include sodium chloride (NaCl, a byproduct of the synthesis) and sodium carbonate ([Na.sub.2]C[O.sub.3], leftover excess reagent). Both of these impurities decompose in the kiln to produce [Na.sub.2]O, a low-temperature flux, and either HCl or C[O.sub.2] as gaseous byproducts. Neither of these byproducts should pose a significant problem for most colourations, since cobalt colourants are extremely potent and used in relatively small amounts, unless the cobalt colourant is extremely impure. For glaze firings requiring high precision, the more rigorous method for producing purer basic cobalt (II) carbonate given below, high-temperature synthesis is recommended. A variant of this procedure may also be used to purify cruder basic cobalt (II) carbonate.

The basic cobalt (II) carbonates synthesised by the procedures presented below have been successfully tested in our studio at Orton Cone 10.


Reasonably pure basic cobalt (II) carbonate may be prepared by the following procedure: Using a 50 mL graduated cylinder, measure out 50.0 mL of 0.5 M Co[Cl.sub.2] (cobalt chloride) and pour this solution into a clean 400 mL beaker. Rinse the graduated cylinder with a little clean distilled water, and then measure out 50.0 mL of 0.65 M Na2CO3. (sodium carbonate). Add this solution to the same beaker with the cobalt chloride, and stir the resulting reaction with a stir rod for a few minutes. A mauve precipitate will appear.

If you have a vacuum filtration setup, vacuum filter your precipitate, and wash the crystals well with distilled water. If not, gravity filter your precipitate through a folded cone of filter paper, and rinse with distilled water. The precipitate produced by this method can be fine, and in this case vacuum filtration is much faster and much more practical than gravity filtration. Additionally, you may need to stir the filtrate (wet pigment) gently with a glass stirring rod, taking care not to tear the filter paper, to speed filtration (the precipitate forms a sticky paste-like material as it dries). Larger Buchner funnels give faster filtration; a minimum 7 cm diameter is recommended.



After filtration, carefully remove the filter paper (which has your glaze colourant on it), place it on a paper towel to absorb moisture, and let it sit in the air to dry for a few hours. Scraping and grinding will facilitate the drying process.

After the colourant has dried, grind it to a fine powder with a mortar and pestle, and store it in a tightly sealed jar. This procedure yields approximately 3.5-4.0 g of basic cobalt carbonate, typically of a slightly duller mauve colour than the heated method presented below


Pure cobalt (II) carbonate may be prepared according to the procedure below, which is a modification of Schlessinger's original procedure in Inorganic Synthesis (Schlessinger, G. 1963; 6: 189--191). The procedure uses the chloride (Co[Cl.sub.2]*6[H.sub.2]O) because this is typically the least expensive water-soluble cobalt salt but should alsowork with the sulfate (CoS[O.sub.4]*7[H.sub.2]O) or the nitrate (Co[(N[O.sub.3]).sub.2]*6[H.sub.2]O).

Put 50 ml of 0.5 M Co[Cl.sub.2] in a 100 mL beaker, and 50 mL of 0.65 M [Na.sub.2]C[O.sub.3] in a 400 ml beaker, and heat both solutions until they are just below the boiling point. Slowly and carefully add the Co[Cl.sub.2] solution to the [Na.sub.2]C[O.sub.3] solution, a little at a time. Vigorous bubbling will occur, and constant stirring with a glass stirring rod will help prevent the solution from foaming over. The order of addition is important: to ensure production of purer cobalt carbonate, the cobalt solution must be added to the carbonate solution. A mauve precipitate will appear.

Once the two solutions are completely mixed, heat the mixture, with stirring, for 10-15 minutes either just below the boiling point or at a gentle boil. Boiling the solution too hard may result in the suspension bumping: splattering out of the beaker. (1) Remove the mixture from the heat, cover it, and allow it to cool and the solid to settle (this may take several hours).

Initially wash the precipitate by decanting: pour off most of the liquid through the Buchner funnel (so as to catch small amounts of cobalt carbonate), add 20 mL of distilled water, stir up the suspension, and let the solid settle again. Repeat this washing and decanting process several times before collecting the precipitate by vacuum filtration, and wash the filtered solid several times with 5 ml portions of distilled water. (2)

Follow similar drying procedures as for the room temperature method.

Cobalt carbonates of suitable quality for use in a ceramics studio may easily be made and used by undergraduate students in any field using basic chemical laboratory and ceramics studio equipment.

These experiences gave our students an excellent way to bridge the theory and practice of separate fields, while simultaneously using their new knowledge in a practical way.

For the ceramists in our class, the use of chemical exercises and observation opened up new and exciting ways of viewing common clay materials. It proved to be a successful introduction to different ways of interacting with ceramic compounds in a glaze and has brought the use of cobalt into a whole new level of understanding in the studio.

For the chemists, studio experience gave them a new avenue of chemical exploration and the chance to delve further into chemistry and art. The synthesis and use of a glaze colourant is an excellent method of using an otherwise waste product in a practical arena to create something functional and artistic.



Chemicals for this experiment: Distilled water 50.0 mL of 0.5 M Co[Cl.sub.2] (freshly prepared) 50.0 mL of 0.65 M [Na.sub.2]C[O.sub.3]

50 ml graduated cylinder 100 mL beaker 400 mL beaker Glass stirring rod Hot plate (for heated method)

Vacuum filtration setup (much preferred): Buchner funnel (7 cm or larger) Filter paper (7 cm or larger) Filter flask (125 mL or larger) Vacuum adapter Vacuum tubing Water aspirator or Gravity Filtration setup (possible but not recommended): 7 cm funnel Larger filter paper, folded into a cone (cotton ball inadequate) Erlenmeyer flask (125 mL)

To make 50.0 ml of 0.5 M Co[Cl.sub.2], put 5.948 g of Co[Cl.sub.2]*6[H.sub.2]O in a 50 mL volumetric flask, add about 20 mL of distilled water, swirl until the solid dissolves, and then dilute to 50 mL with distilled water.

To make 50.0 mL of 0.65 M [Na.sub.2]C[O.sub.3], put 3.445 g of [Na.sub.2]C[O.sub.3] in a 50 mL volumetric flask, add about 35 mL of distilled water, swirl until the solid dissolves, and then dilute to 50 mL with distilled water.


(1.) This period of boiling helps purify the carbonate by constant dissolving and reformation of the precipitate. Cruder basic cobalt (II) carbonate may also be purified by this method: suspend about 3.5 g of the carbonate in 100 ml of distilled water, heat to a gentle boil and boil for 10-15 minutes, and proceed with the remainder of the heated procedure.

(2.) More advanced undergraduate chemistry students can help assay the purity of the cobalt carbonate produced by testing the final washing filtrate (water that came through the filtration) for chloride ions ([Cl.sub.-]), or by performing a cobalt analysis of the precipitate.

Chris Garcia is an Associate Professor of Art at Antioch College in Yellow Springs, Ohio, USA. He is also a journalist and studio artist who exhibits his work internationally. David Kammler is an Assistant Professor of Chemistry at Antioch College in Yellow Springs, Ohio, USA. A synthetic organic chemist by training, he enjoys dabbling in other chemical arenas, and is particularly fascinated by the chemistry of art. Acknowledgements: Thanks to all of our students in ABC: Art, Business and Chemistry who helped make and use the cobalt carbonates, and to Mary Ann Willits, our science technician, whose support was invaluable in completing this project.
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Author:Garcia, Chris; Kammler, David
Publication:Ceramics Technical
Geographic Code:1USA
Date:May 1, 2008
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