What’s new Gum bichromates Ferric gum process – a radically different variation on gum

Ferric gum process – a radically different variation on gum

Writer and photography / Michael Andrews

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, a photogramThe ferric gum process is an unusual process for making photographic prints.  It could easily have been invented in the 19th century because everything required was available at the time.  However as far as I’m aware the process wasn’t invented until the 1970’s.  You could say that it is a process invented after its time!

Image above: Detail from a photogram made with skeleton leaves.

The process is like gum bichromate in some respects but it is also radically different.  So let me give you an overview by comparing and contrasting the two processes. 

In gum bichromate the sensitiser is first mixed with pigmented gum arabic.  Then the mixture is brushed onto a paper support and dried.  The coated paper is exposed under a suitable negative and finally the print is floated in water to wash away any gum that was not hardened during the exposure.  It is important to note that the hardening begins at the surface of the gum and continues down towards the paper.  Consequently much of the hardened gum is detached from the paper until the print is finally dried out.  This is why a lot of fine detail is lost and indirectly why the prints are tonally flat unless the process is repeated two or three times on the same print.

Now imagine that gum bichromate could be done differently.

Imagine the sensitiser being brushed onto the paper without any gum.  Then the paper would be dried and exposed under a suitable negative, rather like making a cyanotype. Only at this point would pigmented gum be brushed onto the exposed paper.  The sensitiser would rise up through the gum, hardening it as it went.  Finally any soft gum would be washed away.  If the process could be done like this it would be very different.  Most importantly the gum would be hardened from the paper upwards.  The fine details would be retained and a wide tonal range could be achieved in a single stage, provided the gum layer was thick enough.

Okay, so one cannot actually do gum bichromate this way!  However the process that I have just described is not a fantasy.  It is pretty much how the process dealt with in this article works.  The only difference is that ferric chloride is used as a sensitiser and a positive image is required instead of a negative one.

I can imagine skeptics thinking that this new process is impossible.  Firstly, they might point out that ferric chloride is not light-sensitive.  True, it isn’t on its own but it is in a suitable organic medium; it changes from bright yellow to the white ferrous salt.  Back in 1970 any decent paper provided a suitable medium but unfortunately today’s acid-free papers are problematic in this respect.  Secondly, skeptics might argue that the fine details and even the whole image would be swept sideways as the pigmented gum was brushed across the exposed print.  I thought this myself until I discovered otherwise.  Surprisingly the image is both detailed and sharp and the gum layer can be very thick, allowing the print to have a good tonal range.

The tonal rages of a gum ferric print

Sample showing the tones and textural details possible with the process

So this seems like a perfect process; better than gum bichromate and much simpler than making Carbon Prints!  Unfortunately there is a fly in the ointment.  It is difficult to clear the excess iron compounds from the finished print and over time these result in a rusty appearance.  I was unable to solve this problem in the 70’s and 80’s.  But my knowledge of chemistry is patchy and it was difficult for me to find things out then; the web didn’t exist!  I did develop the process in various ways, including making a version with shorter exposure times and negative images instead of positive ones, but no version worked perfectly.

If you are not put of by this imperfection and even better if you see it as a challenging problem to be solved then read on!

If I were writing in the 1970’s I would have given you a recipe at this point.  Then you would be able to make the process work first time.  However that’s no good now because paper has changed so much.  Instead I will describe three separate experiments and if you can make them work you will have no problem with the whole process; indeed you will have a better understanding than following a recipe would allow.

1Put a small amount of liquid gum arabic into a glass container and add some drops of strong ferric chloride solution.  The gum will instantly harden and it may even be possible to lift it out of the container and bounce it on the floor!  This will convince you that ferric chloride can harden gum arabic.

2Find some suitable paper* and make marks on it with ferric chloride solution.  You can dilute the ferric chloride solution to three times its volume or more if you like and it would be a good idea to give your marks some fine detail and tonal variation.  Dry the paper (a fan heater is good for this).  Now use a soft brush to mop pigmented gum arabic across the paper.  Finally wash the paper in cold water.  This will convince you that ferric chloride can rise up from the paper and harden the gum flowing over it.  You will also see that fine details are retained and a good tonal range is possible.

3Find some suitable paper* and brush some diluted ferric chloride solution over it and dry it.  Shade some areas of the paper and expose it to strong light.  The bright yellow ferric chloride will turn pale and eventually white.  This will convince you that ferric chloride is light-sensitive in a suitable medium, whatever the books say!

*Suitable paper

The problem with the second two experiments is to find a suitable paper.  Ferric chloride only exists in a somewhat acidic condition.  If it is brushed onto today’s acid-free papers it will turn brown, which means it has changed to ferric hydroxide.  The ferric chloride must remain yellow on the paper.  I am not certain how to solve this problem.  You could try Buxton Paper, which is designed to work with iron salts.  Alternatively you could try soaking ordinary paper in a weak solution of acid to remove all the buffer from it.  If nothing else works you could coat some glass with a thin layer of gelatine and make it insoluble using dichromate.  You might need to add hydrochloric acid to the ferric chloride to ensure that it remains yellow.  This won’t give excellent results but I know from experience that it works.

I ceased working on this process a long time ago and I don’t intend to work on it again; my motivations have changed.  But I would be happy for  people to use it and even happier if they could develop it further.  If anyone is interested I could write another article to describe why the process works, or rather my best guesses about this.  I could also describe how I tried to develop the process further.  If you would like to know more about the process please don’t hesitate to ask.


A description of the process and its variations was published by The Royal Photograhic Society in The Photographic Journal – February 1983. The article was called My way with gum.

This article was written by Michael Andrews who invented the Ferric Gum process in the 70’s whilst trying to use photograghic methods as a way for artists to make prints, simply as an alternative to etching or lithography.

9 thoughts on “Ferric gum process – a radically different variation on gum

  1. I’ve read this contribution concerning a ferric gum process and need to raise some questions.

    Firstly it would be interesting to hear why the author started this work and what kind of problems considered to be solved or improved compared to similar processes.

    Secondly I don’t understand or maybe misunderstand item 1. I interpret this as ferric iron reacts with gum even without light and hardened the gum.

    Thirdly the author says ferric ions turn pale or yellow upon exposure in a coated or soaked paper. If this is a reduction of a ferric ionic state or not is difficult to say. However, what’s worries me is remaining ferric ions will catalyze a degradation of cellulose in paper and make the paper brittle over time, ie reduced the DP (degree of polymerization). There should be some step present to complex remaining ferric ions to be able to dissolve such ions out of the paper.

    Looking forward to hear your comments.

  2. Hi Christer
    Thanks for your comment.

    I started working with old photographic processes after being an art student in the 1960’s. I had had some experience of artists printmaking processes like lino printing, etching and lithography but I felt there had to be a simpler way; something that could be done in a bed-sit! So I started exploring old photographic processes like Carbon and Gum Prints. However I had two problems. I wanted to work with positives rather than negatives and I didn’t like the time-consuming complexities of these traditional processes. Eventually, after making some lucky mistakes, I discovered the two facts described as experiments 2 and 3 in my article. So the process promised to be everything I had wanted. It was simple and quick to use and it had all the subtley of etching and lithography plus a lot more tonality. Unfortunately it came with a ‘ferric stain’ and I could find no easy way to clear this.

    You are correct in your interpretation of experiment 1. Ferric chloride does react with gum arabic to harden it and the reaction requires no light. I am not a chemist but I believe the ferric salt is hydrolysed by the gum and this results in the ferric hydroxide (which is charged but not ionic, I think) binding the gum. This reaction doesn’t happen well with other ferric salts such as oxalate or citrate; I think the organic components are already complexed with the ferric ion and this interferes with the hydrolysis.

    When ferric chloride solution is coated onto the surface of paper and dried it does become light-sensitive. The ferric chloride is bright yellow and the resulting chemical is white (or possibly colourless). It seems to me that extra acid is also generated in the reaction. So I assume the ferric chloride is reduced to the ferrous salt. The print is thoroughly washed at the end; so I believe that the only iron salt remaining is ferric hydroxide, which is not ionic. Eventually this becomes an iron oxide, I think. I have seen no sign of the paper becoming brittle after twenty five years but I would like to remove all the iron salts from the prints, for aesthetic reasons.

  3. It is fascinating how our lives are touched by people we do not know and will perhaps never meet.

    Michael Andrews : Ferric Gum Process.

    Your article opens up a whole range of possibilities. Very much appreciated.

    Thanks Malin for your excellent website.

  4. Thanks for your kind comment Abeed. You may be the first person to see the possibilities inherent in this process.

    I still find it magical that a robust image in gum builds up from the paper, somewhat like crystallisation (call it ‘crys-gelling’ if you like).

    Please keep me in touch with any ideas you have or any developments you make based on this process.

  5. Hi
    very interesting, I make my own ferric chloride for etching PCB’s, its very easy.

    You write

    “If anyone is interested I could write another article to describe why the process works”

    If you have time I would be very interested in any thing else you would like to write about this


  6. Hi Bruce

    You obviously enjoy working with chemicals so you may be just the person to carry this process forward.

    I have been thinking about another article and your comments will spur me on to write one sooner rather than later!

  7. Michael,

    I was recently attracted to experimenting with a positive process similar to yours. The great potential advantage of a long scale (like that of a carbon print) had me very interested.

    I came upon a process called Poitevin’s process (from the mid 1800s by Alphonse Louis Poitevin) which acts positively and uses ferric chloride, tartaric acid and gelatin instead of gum.

    In this process he used the tartaric acid (citric acid also worked) as the organic compound which sensitizes the ferric chloride. In your process, perhaps it is the paper or an additive that supplies the organic component however he coated the paper with the gelatin/pigment, then later bathed it in the ferric chloride/tartaric acid solution, dried it and exposed it for a positive print.I tried to replicate it but results were very poor…it may have been the paper but I do not know. I suspended my experiments for the time being but seeing your work makes me wonder if I should at least try your approach.

  8. Hi Peter

    I have found the Poitevin Process you referred to, on the wonderful Web (if the Web had existed in the ’70s I might have made much faster progress with my process!)

    I can see how that process should work (including a hidden subtlety that the inventor may not have been aware of). I might be able to help if you decide to try that process again. If so please go to the Forum and put something under ‘All Other processes’. It would be useful if you described what aspect doesn’t work well.

    Of course you should try my approach! But seriously Ferric Gum is not a fully fledged process yet; it has faults. So it depends what you want. If you decide to try it then be aware of the following.

    Firstly you need to be certain that the ferric chloride does not get hydrolysed by the acid diffusing away during the exposure. Provided you do this I guarantee you will get a good relief image when you put the gum arabic on. This part of the process is very reliable.

    One way to lessen this hydrolysis is to add potassium oxalate (and perhaps a bit of acid) to the ferric chloride. Be aware that oxalates are poisonous. You can add a fair amount of oxalate but it makes the image less robust, so don’t add too much.

    Secondly you will end up with some ferric hydroxide in the print and this will be annoying. Ultimately this is the problem that really needs to be solved!

  9. Oxalic acid is sold at the hardware store for bleaching wood. This acid removes iron staining from minerals such as quartz crystals and is used by mineral collectors to remove iron staining. It should work on these images as well. The acid is somewhat poisonous so typical precautions should be followed. No contact with skin and wash all surfaces in contact with solution well. No using this inside the house.

    Best Regards


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