The gum/pigment/dichromate combination is hardened by the action of a strong UV light source. There are several options of UV light suitable for our purposes:

 

The Sun - cheap and plentiful, however its variability, which is influenced by geographic location, time of day and year, can make it difficult to set repeatable exposure times. It is usually the fastest printing light and in my experience provides the most contrast of any printing light source.

for more information on using the sun as a lightsource > >

 

Fluorescent lightboxes - You can purchase these ready-made, such as those those made by Jon Edwards, or you can make one yourself using banks of BL fluorescent tubes.

For more information on building your own UV lightbox visit the article on 'Ultraviolet Light Box' at Unblinking Eye > >

 

Mercury vapour lamps - this is a point light source and an alternative to banks of fluorescent tubes. A common choice in Europe. I have never used them so cannot offer any advice.

 

Metal halide - this is my printing light of choice. I find that it is fast and contrasty and allows repeatable exposures to be maintained. I purchased mine from a supplier of hydroponics equipment which came complete with reflector, lamp and transformer. While this is only a 400W unit I find that it works fine for my purposes giving exposures from 3-10 mins on average (depending on negative density)

Supplier of Metal Halide Lighting systems > >

 

Photofloods - these can also be used - Katharine Thayer mentions their use on her website

More about using photofloods > >

 

Further information on UV light sources
Sandy King provides a thorough description of UV lightsources for alternative photographic process.

Ultraviolet lightsources for printing with the alternative process> >

Exposure is an important control in gum as it allows you to manage both the density of colour and the amount of hardening of the coating. Exposure tests ideally should be based on an automatic development time of 30 mins. This will give you the most tonal range and detail that the gum process can deliver. Of course your print-making desires may be different to mine so an alternative approach may be more appropriate.

 

While it is possible with proper exposure and pigment density to complete a 'free form' gum print in two or three coatings (three or four if you are printing three colour gum), I usually like to add additional coatings of contrasting colours - often lightly exposed.

 

A good strategy with printing multiple gum prints is to aim to print the first two coats fully and reasonably flat to provide detail in the highlights and rich mid tones. Subsequent additions of additional colour can then be used to add further contrast, deepen shadows, and give additional depth to the mid tones.

Additional exposure control tips

• If you want to deepen the shadows without influencing the highlights — increase pigment concentration and reduce exposure.
• If you want to add more colour to the highlights — reduce pigment concentration and increase exposure time.