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Cobalt carbonate vs oxide


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Is there a substitution ratio to substitute cobalt carbonate in place of the oxide? I know generally the oxide is a more powerful colorant but I can't remember if its about 2x the carbonate or 1.5X etc. Or is it just not doable?

Another question in trying to decipher a recipe - is magnesium carbonate considered an addition?

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Guest JBaymore

So here is the "background" on this subject that really applies to any substitution of one raw material for another:


In the already fired melted glaze (the only place we really care about the "result") the coloring power of the cobalt is determined by the number of cobalt oxide molecules present in the glaze melt. So that the coloring power involved is all about CoO. CoO is what we need to keep track of.


We can supply cobalt into the raw glaze batch in a number of possible forms. One possibility is Cobalt Carbonate or CoCO3. The other possibility is supplying it using cobalt oxide or CoO.


One molecule of CoO in the raw glaze batch supplies one molecule of CoO in the fired glaze melt. One molecule of CoCO3 in the raw glaze batch supplies one molecule of CoO in the fired glaze melt. So it looks like a one-for-one subsitution from that particular point of vew. And if you could reach into a bag and pick out single molecules with your fingers, then it would actually be that simple. One cobalt carbonate molecule subbed for one cobalt oxide molecule.


Unfortunately, how do we measure the amounts when we are mixing up a raw glaze batch? By weight, of course. So how do the weights of CoO and CoCO3 molecules compare?


Chemical formula are "qualitative" and "quantitative". They specify what is there, and how much there is of that particular material. So if you look at the formula for cobalt oxide, there is one atom of cobalt and one atom of oxygen present in one molecule of cobalt oxide. For cobalt carbonate, there is one atom of cobalt, one atom of carbon, and three atoms of oxygen.


Now we have to get "all scientific" on this. We need to look at the atomic weights for the various atoms involved in this.


Scientists have set up a relative scale that compares the amount of material in one atom to another atom. The mass (and when impacted here on earth by gravity...... the weight) of an atom is determined by the number of protons, neutrons, and electrons of which the atom is composed. The "standard" of reference is Carbon 12....which is a particular isotope (variety) of carbon. This material is used to produce an atomic weight value of "1" by taking 1/12 of it's mass. All of the atoms of the elements are compared to this particular value to determine their weights. This information is summarized on the Periodic Table of the Elements.


As we look at atomic weights here, we are going to look at only one place to the right of the decimal point (called "significant figure"). For what we do with "potter science", this level of accuracy is plenty.


If we look at the atomic weight of Cobalt (Co) we find it is listed as 58.9. The atomic weight of oxygen (O) is 16.0. Carbon is 12.0.


Molecues are composed of groups of atoms "hooked together" and can frequently also be referred to as a "compound". So we can compute the molecular weight of a compound by adding together the atomic weights of the individual components of a compound.


So the compound cobalt oxide is composed of one atom of cobalt (58.9) and one atom of oxygen (16.0). Therefore the molecular weight of cobalt oxide is 58.9 + 16.0 = 74.9. The moecular weight of cobalt carbonate is made up of one atom of cobalt (58.9) and one atom of carbon (12.0) and three atoms of oxygen (16.0). So that comes to 58.9 + 12.0 + 16.0 + 16.0 + 16.0 = 118.9.


By doing this calculation we can see that a single molecule of cobalt carbonate is heavier than one of cobalt oxide. They both supply one molecule of cobalt oxide in the fired glaze though! So we have to account for this fact when we go in the lab to weigh out the cobalt carbonate to substitute of the cobalt oxide we don't have. So how MUCH heavier is the cobalt carbonate raw material when compared to the cobalt oxide raw material?


It is now looking at a ratio of the relative molecular weights of the two raw materials . Using two significant figures here, cobalt carbonate when compared to cobalt oxide is 118.9 / 74.9 = 1.59. Cobalt oxide when compared to cobalt carbonate is 74.9 / 118.9 = 0.64.


If you are substituting cobalt carbonate for cobalt oxide you need 1.59 times the original weight of the raw material. If you are substituting cobalt oxide for cobalt carbonate, you need only 0.64 times the original weight of raw material.


During the earlier stages of the firing of the raw glaze batch, cobalt carbonate actually changes before the melting process fully begins. The carbon and two of the oxygen atoms are given off as carbon dioxide gas, leaving cobalt oxide in the not-yet-melted powdered glaze layer.


__________heat energy__________________

CoCO3 >>>>>>>>> CoO + CO2 (gas)



So when we weighed out cobalt carbonate in the glaze lab, we were weighing out some "extra" stuff that will be given off out of the glaze into the kiln gases. That is the CO2 gas which is produced. That gas has no impact on the color that develops in the fired glaze from CoO.


Hope this helps to understand what is behind the ideas of the weight substitions.





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Thank you, John, for this wonderfully clear explanation. I think it's great that you and others with technical knowledge find the time to impart this wisdom onto the rest of us 'sponges'. For me it was knowledge gained decades ago that has just grown dim with the passage of time, but always grateful and receptive for review. Just possibly keeping Alzheimer's at bay... but ....



is there any quantitative allowance for Higgs boson in these molecular weights....? biggrin.gif

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Thanks Marcia

Just to clarify do you mean that I need 1.5 times as much oxide as carbonate or the other way round?



1.5 more carbonate than the oxide


Cobalt Ox 1% = Cobalt Carbonate 1.5% or using John's measurement Cobalt Carb .64% to Cobalt Carbonate 1.0%



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