An update on the Ferric gum process

Michael Andrews describes two approaches to the ferric gum process: Frank Gorga’s and Peter Friedrichsen.

The Ferric gum process was invented in the mid 1970′s and first described in The Photographic Journal in 1983 (ref. 1). It has been described more recently in two articles on the AlternativePhotography website (ref. 2,3). Since then a handful of people have experimented with this process and discussed their ideas and results on the AlternativePhotography Forum (ref. 4).

When the last two articles were written it was not certain that the process would actually work with modern papers. Paper has changed considerably since the 1970′s. However it is now clear that the process does still work. Here are two approaches to the process.

Frank Gorga’s approach

Ferric gum print by Frank Gorga

Ferric gum print by Frank Gorga

Frank used Fabriano Acquarello paper ‘straight out of the box’. Using this paper allowed him to use the original process without modification.

Frank prepared the sensitiser for this print by making up a 1 molar solution of ferric chloride. This concentration is about 15% w/v.

He prepared the pigmented gum by taking 14 g Gum Arabic and adding enough water to make 40 ml of liquid gum. Then he added pigment from a (water colour) tube at the rate of 1 cm ‘bead’ to each 5 ml of liquid gum.

He coated the paper with the sensitiser and air dried it. Then he exposed the paper under a positive transparency in bright sunlight until the more exposed parts turned pale yellow. This took about an hour.

Next he brushed the pigmented gum over the exposed paper. Finally he washed off the excess gum with warm running water and dried the print.

Peter Friedrichsen’s approach

Peter used Arches Aquarelle Hot Press Watercolour paper ‘straight out of the box’. He added ferric ammonium oxalate to the sensitiser to make it faster.

Peter prepared the sensitiser for this print using the following formula:

  • 2 ml ferric chloride 42 degree baume
  • 1 ml ferric ammonium oxalate 20% w/v
  • 9 ml distilled water with 2 drops 99% isopropyl alcohol

This sensitiser would be brushed over the paper three times.

Ferric gum print by Peter Friedrichsen

Ferric gum print by Peter Friedrichsen

He prepared the pigmented gum by making a 30% w/v solution of Gum Arabic. Then he added lampblack powder, judging the amount by eye.

He coated the paper with the sensitiser and dried it. Then he exposed the paper under a positive transparency for 60 minutes, using a UVA lamp.

Finally he brushed the pigmented gum over the exposed paper and washed the print in a smooth non-aerated cold water stream for about 6 minutes.

Other papers

It is clear that the process works well with the kinds of paper used by Frank and Peter but does it work with other kinds of paper? The answer is yes but two problems may need to be tackled.

The first problem is that many papers now contain buffers. These spoil the sensitiser turning it from its innate yellow colour to rusty brown. It is still possible to use these papers but the buffers must be removed first. This can be achieved by bathing the paper in dilute hydrochloric acid (about 1%) and then drying it.

The second problem is that the sensitiser may not actually be light sensitive when it is brushed on some papers. This problem is easy to solve. Just add a small amount of an oxalate salt to the sensitiser. I found that adding enough potassium oxalate to achieve a concentration of 1% w/v in the sensitiser was sufficient.

Incidentally it is puzzling how such a small amount of oxalate can make a much larger amount of ferric chloride light sensitive.

Iron stain

Ferric gum prints inevitably have some iron stain on them and this stain appears worse after a few years as it turns to rust.

Removing iron stain is not difficult in itself. It can be removed with dilute acid or with a chelating agent like EDTA. However Ferric gum images are actually held in place with iron. So removing all the iron directly is not a good idea!

We have managed to make some progress with the stain removal. One approach is to fix the image with a secondary mechanism before the stain is removed.

Cleared print by Peter Friedrichsen

Cleared print by Peter Friedrichsen

The best fixing agent we have found so far is glyoxal. The late Katherine Thayer described how glyoxal fixes Gum Arabic in a post to the Alt-Photo list several years ago. Katherine also participated in our experiments with the Ferric gum process.

Peter cleared the stain from some of his prints after fixing them with glyoxal.

This approach to stain removal is not completely reliable yet. The glyoxal may attack the image and the cleared prints can only be washed for a limited period of time before they start to deteriorate. Also on a personal note I cannot buy glyoxal in the UK because I am not a registered business!

Another approach to stain removal is to modify The Ferric gum process by substituting the gum with gelatin. Once the gelatin has set thoroughly it is easy to remove all traces of stain. However the process is tricky to handle with gelatin and the prints tend to be less sharp.

A third approach would be to refine the process so that less stain is produced in the first place. This approach might be viable as I have some thirty year old prints which only have small amounts of stain. Unfortunately I cannot recall what I did to make it so! Katherine thought we might be worrying too much about the stain. So the prints she made may have had less stain than ours.

Exposure times

I was surprised to see how long the exposures took in the prints made by Frank and Peter; they took much longer than mine in the 1970′s. Then it occurred to me that I had probably diluted the sensitiser more and used more pigment in the gum. At the time I saw Ferric gum as a way for artists to make prints rather than as a way to print photographs. So my requirements were different.
However it should be possible to reduce the exposure times by adding an oxalate salt to the sensitiser. Frank has used no oxalate yet so it should be possible to cut his exposure times by half or more.

There is a limit to how much oxalate we can be add because it makes the image physically weaker. However if we could find a way to strengthen the image whilst the print is being washed it might allow us to add even more oxalate and make the exposure times shorter.
If we could somehow pull the oxalate away from the iron as it enters the gum then the improvement in the whole process would indeed be dramatic!

Tonal range

There was much debate about the tonal range of the process and whether it was better than with Gum Bichromate prints or not. There was also a debate about the graininess that occurs in both processes.

Tri-colour Ferric gum print by Peter Friedrichsen

Tri-colour Ferric gum print by Peter Friedrichsen

It seems obvious to me that the main factor controlling the tonal range is the thickness of the gum image. I used to think that the images could be made much thicker but they stubbornly refuse to oblige me!

However a wide tonal range can be achieved by multiple printing and tri-colour prints are also possible. So let me end this article by showing the first Ferric gum tri-colour print ever made. The second image shows the print with its iron stain removed .


  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
  3. AlternativePhotography in an article called The extraordinary mechanism underlying the Ferric gum process
  4. AltenativePhotography forum. Scroll to ‘all other processes’. Then open the ‘Ferric gum process‘ thread.

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