As an example, when I am coating paper I feel how thin, or thick the
coating
is. When I know that the sensitizer proportion is the same, a thicker, or
thiner coating gives me an idea of relative gum/pigment concentration.
More
gum makes for a thicker coating - a thinner coating more pigment. In my
experience the thinner coating leads more to staining than the thicker
coating. Just to confuse the issue, I often use a 2:1 mix of sensitizer to
gum/pigment, to give less contrast (useful for multiple printing) and this
mix feels thinner when you coat (though in a different way to a 1:1 mix),
though the presence of more sensitizer seems to cause less problems with
staining - but the colour of course - isn't as intense. At the heart of
the
issue I think is that staining is not something necessarily caused by
pigment to gum ratio alone. You also have to take into account how many
printings the length of exposure and so on. What I do know is that
staining
ususally is more a problem after 9 or 10 coatings than the first... Plus
too
much exposure.
Apart from the problem of having no sensitizer, the spot test would also
need to be done exhaustively, probably at least every six months, because
nothing is ever
the same. Manufacturers change the way they make paper, and different
batches can be different also. And of course temperature, humidity
influence the final result also. Plus you would need to test each batch of
gum also. Some are thicker than others, though from what I can ascertain,
a
thinner gum doesn't necessarily lead to more staining.
My preferred method it to print as
heavily (pigment) as I can, or as much as I can get away with. And I stick
to a
fairly limited palette, and mix the colours I want from there. I know for
instance that cad yellow stains easily, and needs less pigment to gum
ratio. cad red and alzarian crimson are well behaved, rarely staining,
even with copious amounts of pigment to gum. Ultramarine and prussian
blue tend to be the worst, as others have indicated. Browns and viridian
on the other hand also allow a strong pigment gum ratio without staining.
In
summary, I try to keep my variables to a minium, I always use winsor and
newton, because I know how their pigments react, and if something changes
(in the manufacture of the pigment for example), I will have a good idea
of what the source of the problem is.
On the question of the printing out image in gum. Browns yellows reds
viridian - you can judge pretty accurately, forget about blue, and after
the second coating.... I read recently that different colours also affect
exposure time - so for example reds are slower, and blues faster (my own
experience would suggest the other way around, with viridian in my
experience taking the longest exposure time)
I have read many articles and books on gum printing - and none ever seem
to agree. Automatic development for instance - half an hour 1 hour, or
more, perhaps 15 minutes... These are just a few examples, there are
others, but I don't have all my sources handy at the moment. But I still
read everything I can get, try it out, or compare it to my own methods -
and
each little extra idea extends what is possible. The definite book on gum
printing would be hard to produce, because different techniques are driven
by the desire to produce a specific finished result. Trying to explain all
of this logically is difficult - its better to show people..
Cheers
Hamish
Hamish Stewart
Sales Department
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Fax: (+33) (0) 1 41 16 49 55
email: hs@europages.com
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