Any Chemists on forum? Hypo

Sodium Thriosulphate (or hypo) is (was?) used as a photographic 'fixer' for black & white photographs.
It also has a useful reaction with steel producing an intense black colour
on the surface of the steel. Diluted hypo, warmed to 75 deg C, and ultra cleaned steel swirled about in it produce this reaction in about a minute. Useful for blacking tools and the like.
But what is the reaction ? Is this a deposit (so maybe transient) or is there a reaction with the atoms on the surface of the steel (so hopefully reasonably well bonded)?
I know that it works but what is happening, anyone know?
AWEM
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I suspect the black colour is iron sulphide, but I have no idea how well bonded or permanent it is. Incidentally there is no R in thiosulphate.
Cliff Coggin.
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"Cliff Coggin" wrote in message

A typo Cliff !
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On 27/10/2013 16:28, Andrew Mawson wrote:

10 sec Google shoed this: http://www.hubbardhall.com/Collateral/ProductBulletins/English-US/2260013.pdf
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On 13/12/2013 13:56, Rick Hughes wrote:

sent before I was ready ... or this: http://www.smex.net.au/Reference/ColouringMetals02.php
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