A few gum things

[Henry Rattle <henry.rattle@ntlworld.com>]

Over the past month or two, I¹ve been working through the PDN process for
tricolour gum (for the second time, but this time doing it properly, and
avoiding ³shortcuts² that turned into dead ends). I really enjoyed the logic
of PDN and the way it makes you look at every step of your working
procedures, and also the fact that it actually works!
 
On the way I learned a few things which most of you probably know, but might
be of some use to someone. Here they are:
 
1. There¹s a use for that long-neglected darkroom masking frame - it¹s
perfect for holding paper flat for brush coating.
 
2. Don¹t develop and clear gum, especially yellow, by the light of a
low-energy compact fluorescent bulb! One evening I ³cleared² a yellow
pigment layer in a room lit by an energy-saving bulb. Next morning, by
daylight, the pigment layer was all still there! I looked up the emission
spectrum of these bulbs. There are spikes and gaps in the spectrum
everywhere - (see for example
http://beale.best.vwh.net/measure/cf-spectrum/index.html, or
http://home.freeuk.com/m.gavin/grism2.htm). These lamps emit blue, green and
red wavelengths, but in particular there is almost no yellow. I should have
known this - I studied physics - but experience is a better teacher...
 
3. The best way for me to clear a gum print in a reasonably repeatable and
controllable way is to use a gardener¹s hand-held spray-mist (thank you,
Christine!). 
 
4. For tricolour prints using gum over cyanotype, I¹ve found that
traditional cyanotype, used at full strength, is just too strong a colour to
balance with watercolour pigments. However it works fine if you dilute it.
Diluting 1 ml of (A+B) with between 5 and 7 ml of deionised water gives a
good medium blue. Once diluted, it needs less exposure than full-strength
(1+7 was 2 stops faster than full-strength A+B) and it also needs a
significantly different PDN curve. (Again, thanks Christine for offline
discussion).
 
With best wishes
 
Henry