The two sensitisers commonly used, ammonium and potassium dichromate have different characteristics — which one you choose will to a certain extent be determined by your printing lights and your negatives.
Ammonium dichromate prints faster and flatter (prints more steps in one printing) than potassium dichromate. However, with strong printing lights it is easy to overexpose — which can cause dichromate stain. Potassium dichromate prints with more contrast, which can help with flatter negatives, or if you want to print with more contrast.
It is possible to increase printing speed and reduce contrast by changing the standard dilution of sensitiser to gum from 1:1 to 2:1, this is a useful control in multiple printing where you may wish to start with a flat coat to print detail in the highlights followed by subsequent coats of greater contrast.
I currently use both ammonium and potassium dichromate, sometimes using a coat of ammonium dichromate to lay down the first coat to fill the maximum amount of detail in the negative with subsequent coats in potassium to increase contrast. A coat with ammonium dichromate is also useful if you want to reduce exposure times for dense negatives.
For many years I used ammonium dichromate exclusively but switched to potassium on the advice that it was harder to wash the dichromate stain out of the paper. However contrary opinions and further investigation of my earlier prints suggests that the staining was more related to overexposure than ammonium dichromate having a greater tendency to stain.
Development is by water. Slide the exposed print face down into a tray of tepid water. You will note that the water becomes stained with pigment, this is the unexposed areas washing away. Transfer the print after about 5 minutes to another tray of clean water. Make sure that the print is fully immersed and after another 5 mins move to another tray for a further 10 mins. Continue for a total of 30 mins checking periodically how development is progressing. If automatic development does not result in a print with clear highlights after 30 minutes - then you can suspect overexposure, too much pigment to gum ratio, or an unsuitable paper.
A 30 minute development time should be the base for exposure tests and is a good rule of thumb to establish, as it will give you the maximum level of detail and tonality possible with gum. With careful exposure and the appropriate pigment/gum ratio it should be possible to produce prints with full rich tonality in three printings or less.
However you do not need to work this way. Shorter development times can be used to retain pigment in areas that would normally wash out if you developed for the full 30 mins. The guiding principle is that when you are happy with the result — hang it up to dry. If the drips of water off the print still have some colour, then the print will develop more — but it doesn't need to. Alternatively longer times can be used to wash out stubborn highlights. It is also possible to control the speed of development and the pigment washout by increasing the temperature of the development water.
Forced development is the use of physical aids to manipulate the surface of the wet print. When wet the gum print is very fragile - water jets and brushwork can be used to locally manipulate parts of the image. This ability to alter tones, to create additional texture, or to remove details is one of the joys of working in gum bichromate.
Tools for physical manipulation
- Plastic spray bottles — the ones used for watering indoor plants - these provide a fine mist of spray (which can be adjusted) enabling subtle control of selective areas of the print. The effect is very mild and retains most of the tonality and detail of an automatically developed print. It is also a useful control to speed overall development if you have overexposed your print, or you need to clean out stubborn highlights
- Water jet — a more intense manipulation that the spray mist - controlling the temperature of the water will give even more control to wash out highlights or other areas of the print
- Brushes — using hard and soft brushes you can apply local control to specific areas of the print. This is the most extreme form of physical development and will degrade detail and tonality. Brush developing however, used carefully, can add texture and a tactile physical 'feel' to the image. You want to have at hand the following types of brushes — small soft ones for cleaning out highlights, and stiffer, wider brushes for brushing out shadows and larger areas. Start your brushing while the print is still in the water, this helps to cushion the action of the brushing.
The balance between the amount of automatic development (i.e. without any physical manipulation) and the degree of physical manipulation is entirely dependant upon your own preference. Neither approach is more “correct” though there are clear differences.
Automatic Development will create a more 'photographic' image than one which is physically manipulated. Prints that have been heavily manipulated often exhibit sharp changes of tone and contrast. Of course this look can be very attractive. Ultimately how you approach your print-making must be a personal decision. Experiment with both and discover what satisfies your own print-making intentions.