Gum bichromate is a contact printing process. A negative the size of your final print is combined in contact with the sensitised paper to produce the finished print. If you are working with 35mm or medium format you will need to produce an enlarged negative.
Enlarged negatives can be created using either traditional film in a darkroom or you can utilise a digital workflow.
Which ever approach you choose, whether wet darkroom or digital, you need to pay attention to the characteristics of the process you are working with. Most historical photographic processes are long-scale, ideally suited to negatives with a consistent range of tones from shadows to highlights. Processes such as platinum and kallitype suit negatives that display a much greater density and range than would be ideal for traditional silver gelatin.
Gum bichromate however is a short-scale process that responds best to the midtones in the negative. Much of the literature suggests that a negative that prints well on a grade 2 paper will work well. However you would be encouraged to explore this in greater detail. If you choose to work digitally, Mark Nelson's guide, 'Precision Digital Negatives for Silver and other Alternative Photographic Processes' is recommended. It provides a systematic method to create negatives that suit each process you work with.
Traditional wet darkroom approach
This involves making a positive copy of the original negative which is then used to produce the enlarged negative.
Suitable films include either continuous tone copy films such as Bergger's BPFB-18, or a range of high contrast lith films such as Aristo APHS. A variety of developers and techniques are suitable. More information on working with lith films is included in the links on the right.
The Bergger film can be purchased in sizes up to 20"x24" and developed in a variety of film and paper developers. The film is orthochromatic so you can work with red safelights. The basic technique involves making a positive copy of the original negative and then either enlarging or contact printing the positive onto another piece of film to produce the final negative. Contrast of both the positive and the negative can be controlled through developer dilution and agitation. A key benefit of the Bergger film is its thick base, which makes it more resistant to physical handling than the thinner bases of lith films.
Digital workflow
Over the past few years I have switched to digital to create enlarged negatives. Digital offers a flexible and efficient way to create negatives for historical processes. In addition if you are working with a variety of processes, digital makes it easy to adjust the scale of your negative to suit the process you are working with.
In summary my workflow is as follows.
- My original medium format camera negatives are scanned and the image is manipulated in photoshop to approximate the desired final print.
- An adjustment curve is applied to the final image (I use one adapted for silver gelatin) and the image inverted to a negative.
- The negative is output onto Pictorico transparency film using the premium glossy paper option with an inkjet printer (the glossy paper option ensures that the printer lays down enough ink to provide the density you want for your negatives).
I recommend that you read both Dan Burkholder's 'Making Digital Negatives for Contact Printing' and Mark Nelson's 'Precision Digital Negatives for Silver and other Alternative Photographic Processes' to understand the techniques and workflow in greater detail.
For scanning I use a variety of tools including an Imacon film scanner and an Epson 2450. The primary difference between these extremes is the dynamic range that can be captured. My printer is an Epson 1290 using Lyson Quad Black inks, and Pictorico OHP film. Unlike many others using inkjet printers I do not create 'colourised' negatives, rather I make B/W negatives using the Lyson inks.
For those making the shift from traditional silver gelatin film negatives to either dye or pigment ink negatives, it is important to highlight the different ways in which these materials convey light.