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

[Mike and Jan Andrews]

I am working on a printmaking process which involves ferric chloride reacting with gum arabic.  But I am having some difficulty understanding certain things about it.  Can anyone help me with their ideas!
 
The process starts by making an image on paper with ferric chloride.  This can be done photographically or by other means.  Then liquid gum is flowed or brushed across the image.  The ferric chloride is dissolved by the gum and it diffuses upwards, hardening the gum as it goes.  This produces a relief image where the ferric chloride image was.
 
The relief is very robust.  It is probably held together by the ferric ions forming 'bridges' between OH groups in the gum.
 
The first thing I don't understand is the relationship between the amount of ferric chloride and the thickness of the relief. Each amount of ferric chloride produces its own thickness of relief, and these amounts and thicknesses seem to stay constant however much I try to change them!
 
Why does a given amount of ferric chloride diffuse just so far and fix a certain thickness of gum?
 
Can anyone suggest what factors might be at work here?  Alternatively can anyone suggest a way to make the ferric chloride diffuse further into the gum?  It does't matter if the crosslinking is made less 'dense' because the relief is currently much more robust than it needs to be.
 
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The other thing I don't understand concerns the series of events which occur when the ferric chloride diffuses into the gum.
 
First the gum gets fixed and this occurs before any colour change happens.  Then the 'ferric and gum' mixture starts to change colour. It turns light brown, then it darkens and ends up a rich reddish brown. These events appear to happen all at once unless the gum is previously acidified with HCL.
 
Is there any possiblity that the ferric ions crosslinking the gum could also participate in the subsequent colour changes without affecting the crosslinks?
 
If this is impossible then additional ferric ions must be causing the colour changes.  In that case why do these additional ferric ions get stuck instead of diffusing further into the gum, just as the first lot did?
 
Any help with these difficulties would be most appreciated!
 
Michael Andrews