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Old Jun 1st 2017, 02:59 AM   #1
cutler
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Crocus powder

I am trying to find out the chemical identity of an abrasive that was commonly used in the cutlery industry. It was called "crocus", and its production is described (in an old book) thus;

"Crocus is an oxide of iron, and it is made by calcining copperas (sulphate of iron; the residue is divided into two portions, a bright red powder known as rouge, and a bluish-red powder known as crocus."

I understand that copperas is Ferric (II) Sulphate ( though I am happy to be wrong about that).

Can someone explain to me the calcining process (temperature, levels of atmospheric oxidation, or more simply, can I do this in my workshop using my propane furnace?)?
Even better, presuming that the first product, named in the above quote as "rouge", is a form of iron oxide, what might the chemical name for the second powder be?

For those helpful sould who might resort to google, "crocus" as it is sold now is either rouge, or fine grit aluminium oxide, or the unrefined emery stone powder known as "emery flour", being a natural mix of both aluminium oxide and iron oxides, inter alia.

Many thanks in advance for your help.

Dan
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Old Jun 1st 2017, 07:31 PM   #2
oz93666
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" copperas. : a green hydrated ferrous sulfate FeSO4·7H2O used especially in making inks and pigments— ."

So this should be easy ...should be .... heating iron sulphate will drive off SO2 and SO3 , leaving iron oxide ... there are a dozen different oxides of iron , each one containing only iron and oxygen , but in different ratios ( FeO....Fe5O7.....Fe25O32 ...etc) ... it appears one of these iron oxides is blue (crocus color) and is sold for colouring concrete etc....



I would imagine the temperature to which you heat could result in different amounts of the many oxides , trial and error .. but how you "divide the residue" into two colour components is not clear ...

Last edited by oz93666; Jun 1st 2017 at 08:32 PM.
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Old Jun 1st 2017, 11:39 PM   #3
cutler
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Originally Posted by oz93666 View Post
" copperas. : a green hydrated ferrous sulfate FeSO4·7H2O used especially in making inks and pigments— ."

So this should be easy ...should be .... heating iron sulphate will drive off SO2 and SO3 , leaving iron oxide ... there are a dozen different oxides of iron , each one containing only iron and oxygen , but in different ratios ( FeO....Fe5O7.....Fe25O32 ...etc) ... it appears one of these iron oxides is blue (crocus color) and is sold for colouring concrete etc....



I would imagine the temperature to which you heat could result in different amounts of the many oxides , trial and error .. but how you "divide the residue" into two colour components is not clear ...
Excellent, thank you very much for your answer!
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