[alt-photo] Re: Trying to understand how ferric chloride reacts with gum arabic

Mike and Jan Andrews botanic88 at hotmail.co.uk
Fri Dec 10 11:15:28 GMT 2010

Hi Kees
Thanks for the link to the patent.  I have tried to fathom what is going on with some success perhaps.
The process may have worked but some of the inventor's reasons are just not true.  For example he says that simple ferric salts like ferric chloride are more photo-sensitive than complex ones like ammonium ferric oxalate.  Not true!
Some of his examples look like Halvor's much more recent Chiba system.  But this involves free-radical initiated polymerisation, which is a completely different mechanism from the one the inventor claimed in this patent.
I am intrigued by the 'prior art' section referring to an Autotype patent.  Apparently  they used gelatine containing ferric ammonium oxalate and after exposure they treated it with an oxidising agent.  This was to oxidise the ferrous salt back to the ferric 'thereby tanning the gelatine'.
So why would the iron salt tan gelatine after it has been reduced and then oxidised, if it doesn't tan it in the first place?
Is it just possible that the oxalates might decompose during the reduction, then the iron may have to become a simple salt rather than a complex one when it is oxidised.  Now that would be interesting!
On the other hand the oxidising agent that Autotype used may have been a peroxide or persulfate, in which case we are back to the Chiba system again!
Anyway it's food for thought.
> From: workshops at polychrome.nl
> Date: Wed, 8 Dec 2010 21:27:15 +0100
> To: alt-photo-process-list at lists.altphotolist.org
> Subject: [alt-photo] Re: Trying to understand how ferric chloride reacts with gum arabic
> On 8 dec 2010, at 15:55, Mike and Jan Andrews wrote:
> > You say that you tried adding ferric chloride to the ferric ammonium citrate in Halvor's Chiba system, and it didn't work well. I'm not sure why it would work. Certainly adding citrates tends to prevent ferric salts hardening gum or gelatine (oxalates and tartarates do so less). But it's not obvious to me how the ferric chloride would help in this case.
> The reason for trying the mix I found in this patent: 
> http://www.google.nl/patents?id=ii4CAAAAEBAJ&zoom=4&dq=RELIEF%20%20IMAGE%20PROCESS%20UTILIZING%20A%20SIMPLE&pg=PA74#v=onepage&q&f=false
> -k
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