The extraordinary mechanism underlying the ferric gum process

The ferric gum process is like gum bichromate in some respects but it is also radically different. Sensitiser is brushed onto the paper, the paper is dried and exposed under a suitable image, and only at this point would pigmented gum be brushed onto the exposed paper.

The Ferric Gum process was first described in The Photographic Journal in 1983 (ref. 1). It was also described and named for the first time in an article published by AlternativePhotography (ref. 2).

The process works as follows. Ferric chloride solution is brushed onto paper and dried. The paper is then exposed under a positive image. This destroys much of the chemical. Pigmented gum is brushed over the paper and the remaining ferric chloride diffuses through the gum, hardening it as it goes. Finally the print is washed to remove any unhardened gum.

At first sight this description may seem quite mundane; just another photographic process. But look at it more closely.

It says ‘… gum is brushed over the paper and the remaining ferric chloride diffuses through the gum, hardening it as it goes’. One might conclude that the gum is brushed over the paper to form a coating and then the ferric chloride diffuses through this gum after the coating is in place. But this is not what happens. The ferric chloride is actually diffusing through the gum whilst the latter is moving across the paper. So the image is formed whilst everything is in motion. Yet the image is sharp and detailed!

This is surely an extraordinary mechanism. I cannot think of another photographic process which does anything like it. Many processes involve chemicals diffusing in and out of colloid layers but none of them do so whilst the colloids are flowing around.

So how does the mechanism work and in particular why isn’t the image blurred or even swept away in the moving gum? It is clear to me that ferric chloride must harden gum instantly on contact. I am not saying that the whole image is formed instantly. That couldn’t happen because chemicals take time to diffuse. I just mean that when ferric chloride comes into contact with gum it must harden a tiny thickness of the gum instantly.

Now imagine the mechanism happening but imagine it at a microscopic level. Some of the ferric chloride leaves the paper and dissolves in the moving gum. It hardens a tiny thickness of this gum instantly. This hardened gum is fixed immediately above the source of the chemical because there is no time for it to move on. Then some more ferric chloride dissolves and diffuses through this hardened gum. Eventually it reaches the moving gum. This ferric chloride also hardens a tiny thickness of the moving gum instantly. And once again the gum is fixed immediately above the source of the chemical. This process continues until all the ferric chloride is used up. In the end a considerable thickness of gum could be hardened and all of it would be fixed immediately above the source of the chemical. This must be so because each tiny bit of the gum is fixed in this way.

This mechanism reminds me of a crystal growing in a solution.

The crystal grows molecule by molecule so it will grow into the same shape whether the solution around it is moving or not. At least I imagine this is so. In a similar way the gum is hardened bit by bit, so the image will be the same whichever way the gum is moving.

With this similarity in mind I suggest we could call this mechanism ‘crys-gelling’. Of course the term wouldn’t apply just to ferric chloride and gum. It would also apply when any chemical diffuses through any colloid, provided that the colloid is in motion and provided that it forms a hardened image which is attached to the source of the chemical.

So this is the mechanism which underlies the Ferric Gum process and to me it is almost magical. But is the actual process any good? Would people choose to use Ferric Gum instead of other processes? At present they probably wouldn’t. The process is still marred by faults, so it should be thought of as work in progress.

However the crys-gelling mechanism works just fine. In a way the faults are just peripheral. One fault is that ferric hydroxide remains in the finished print and it is difficult to clear without spoiling the image. This fault can be avoided by using gelatine instead of gum. Then the ferric hydroxide can be cleared after the gelatine has set. Unfortunately gelatine introduces new difficulties!

Nevertheless I think it would be possible to develop a really good process based on crys-gelling. This might happen by discovering a new combination of chemical and colloid which worked perfectly, or it might happen by overcoming the current faults in the Ferric Gum process or one of its variations. So how good would this ‘perfect’ process be?

The process would be quick and simple to use. It would be at least as simple as the simplest kind of Gum Bichromate and much simpler than Carbon Printing. It would also be quicker than both these processes.

The image would be sharp and detailed and it would have a good tonal range, as in Carbon Printing.

The image would be as translucent or opaque as one desired. The pigment cannot affect the exposure in this process.

The paper would be as smooth or textured as one desired.

The paper wouldn’t require any extra sizing. The manufacturer’s sizing would be quite sufficient.

The process would have some interesting qualities in colour printing. Several colours could be printed side by side simultaneously and the boundaries between them would be crisp, without any blending.

Colours could also be printed on top of one another, but in this case each layer might affect the subsequent ones. Anyone wanting to do proper ‘three colour printing’ should stick to Carbon Printing!

In conclusion the crys-gelling mechanism could enable a really good photographic process to be developed; possibly one that combined the best attributes of Gum Bichromate and Carbon Printing without any of their shortcomings.

Carbon printing, gum bichromate and crys-gelled printing compared

Gum bichromates, carbon printing and crys gell compared
These diagrams show three different processes in cross-section and very much magnified.

In Carbon printing (fig. 1) the gelatine gets hardened from the top down. This means that after the exposure everything has to be turned over and attached to a new paper support. Then the soft gelatine can be washed away to reveal the image held in the hardened gelatine.

In Gum Bichromate (fig. 2) the underlying mechanism is similar. However the need to transfer everything to a new paper support is avoided in two ways. Firstly a rough textured paper is used; one with so-called ‘tooth’. This allows most of the hardened gum to be attached to the paper. It is attached to the ‘tooth’ where the image is thin or to bulk of the paper where the image is thick. Secondly only thin gum coatings are used in this process.

In Cry-gelled printing (fig. 3) there is no need to do anything special. The gum gets hardened from the paper upwards. So it is naturally well attached to the paper.

N.B. These diagrams show Crys-gelled printing as a negative working process like the other two. This is just a convenience. However this kind of printing can actually be made positive or negative working.

References

  1. The Photographic Journal – February 1983 in an article called My way with gum
  2. AlternativePhotography in an article called Ferric Gum process – a radically different variation on Gum


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