Three colour gum Bichromate Printing

Over the years much of what has been published about alternative photographic processes and about gum bichromate in particular, has involved the repetition of wrong information from earlier sources by authors who have little or no experience of these processes. I do not claim that what follows is in any way definitive, it is merely a report of what I do and how I do it complete with images for illustration. I make no comment on the methods of others, but do provide at the end some recommended sources.

Separation negatives

I make my separation negatives in Photoshop from originals shot on colour slide or colour negative film which I have transferred to high res compact disc after processing, or from colour prints scanned in on a flatbed scanner. Using colour slide film also gives the option of making the separation negatives photographically. The negatives used in this illustration are from a colour print scan. To produce the CMY separations I convert to multichannel mode (image>mode>multichannel) then split the channels. A contrast adjustment curve is applied to each of the greyscale separation positives before inverting to the negative. Each of these negatives is then printed on an Epson 660 inkjet printer using all 4 colour inks.

The gum print

The look or colour balance of the final print is greatly influenced by the parameters of the process, for example the water colour pigments used, the choice of paper and even the source of the gum arabic. I like the look of a printed 3 colour separation, I give one exposure through each negative and do not print a final black image. The order of printing I use is cyan, magenta and yellow. Some sources sugest the order yellow, magenta, cyan. However, I find the yellow image to be so lacking in contrast and faint looking as to be nearly impossible to judge when developing. The cyan negative is printed first as a cyanotype using a standard cyanotype sensitiser, I find that this provides more contrast in the final image and gives better blacks. The illustrations below show 3 separation negatives and the resulting print from each stage of the process. The real size of the image is nominally 8"x 6".

I use mainly Saunders waterford Not or rough paper in 300gsm weight and would recommend a not or rough paper when starting out as this provides a key for the pigmented gum. However,some very fine detailed gum prints can be made using smooth surfaces, so do not believe anyone who tells you that gum printing lacks detail. I do not pre-shrink or size with the above papers and have no problems registering visually with the previous image which shows through the gum coating up to a print size of 8x6. This not a recommendation but this method works for me with the above paper.

I use only artists quality water colours, Winsor & Newton or Rowney. The gum arabic is 14' baume lithographers gum which I purchased in a 5 litre can from lawrence art supplies in Hove. When I first started gum printing I was using Winsor & Newton gum arabic without any problems, but at £4.30 for 75ml it is horrendously expensive compared to £28 for 5 litres of the 14 baume stuff. Also the gum arabic can be a major process variable and as it keeps for ever having a large supply of the same batch is a distinct advantage.

As stated above I print the Cyan layer first as a cyanotype using a standard two solution sensitiser. For the Magenta layer I use Alizarin crimson water colour, squeezing a 3/4" worm of paint from the tube and mixing with 3.5ml of gum using a soft round brush and then adding 3.5ml of 10% potassium dichromate solution. For the yellow layer I use a 1/2" worm of Rowney permanent yellow with the same quantities of gum and dichromate. The amounts of pigment used are given only as a guide, your 3/4 or 1/2" worm may be different from mine, but should provide a starting point. Coating is carried out under tungsten lighting using a foam brush, as soon as the surface is coated I smooth it with a hake brush using light strokes, first in one direction and then in the other. The paper is then left to dry in the dark which can be between 20 minutes to 1 hour depending on conditions, but no heat is applied. The coated paper is used as soon as it is dry.

For the light source I use a Phillips facial solarium at a distance of 3", the exposure for the cyanotype is usually between 4 to 7 minutes and for the gum printing between 1 and 2 minutes. In the example above this was 6.5 and 1.5 minutes respectively. development is by floating the paper face down in water at 20' C, this automatic development can take between 2 to 5 hours. I prefer to give more rather than less exposure and if necessary use light brushing to achieve the desired result. If development appears to be complete in much less than one hour, then in my experience exposure is not sufficient and the pigment will tend to run during drying. When complete I drain the print and lay face up on a sheet of blotting paper, the image can be very lightly blotted with kitchen roll but if in doubt leave alone. The print is then transferred to a flat surface ( to avoid hammocking) and left to dry at ambient temperature.

Resources

I was first encouraged to try gum printing by a series of articles by Tony Mclean in Darkroom user magazine, latterly Camera and darkroom, but now unfortunately no longer published. You could try Tony's web site though at http://homepage.ntlworld.com/tony.mclean/ . However, in my opinion the best publication on all alternative photographic processes is 'The World journal of Post-factory Photography'. For further information about subscription and back issues email Editor@post-factory.org or write to Post-Factory Press, 61 Morton street, New York, NY 10014. For 14' baume lithographers gum and foam brushes try T.N. Lawrence & son East Sussex and Redruth Cornwall. www.lawrence.co.uk .