Re: Fugitive pigments

From: Katharine Thayer ^lt;[email protected]>
Date: 07/25/05-06:34:31 AM Z
Message-id: <>

David & Jan Harris wrote:
> I haven't tried a sun exposure, the sun here isn't reliably bright :-) ,
> my thinking was based on the fact that I'm using a facial sun-lamp as a
> light source, which comes with fairly stringent recommendations as to
> maximum use. Even allowing for over-cautious manufacturers this suggests a
> higher UV exposure (at the distance used) than they expect you to get from
> the sun.

Good point, but still I doubt it's enough more to cause fading. If you
had a sunny day and were curious, you could compare exposures and see
how much faster the sunlamp prints than the sun itself. But my
experience is that the sun itself prints in a very short time, less than
a minute in direct sun even here in the Pacific Northwest.

It would be interesting to know which wavelength of light is most
> active in causing the gum/dichromate hardening reaction - one for the
> chemists to think about! Do we all use UV just because it's the easiest way
> to get a suitable light intensity, or is the UV itself necessary?

UV itself -- my understanding from my reading is that there are peaks at
365 and 440 nm (although 440 is in the visible range) but I think it
depends on the colloid as well; those numbers are probably for PVA.

I'm pretty
> new to alt process so my sorry if I'm asking silly questions!
There's no such thing as a silly question; the only silly question is
one someone is afraid to ask.

> ----- Original Message -----
> From: "Katharine Thayer" <>
> To: <>
> Sent: Monday, July 25, 2005 11:44 AM
> Subject: Re: Fugitive pigments
> > David & Jan Harris wrote:
> > >
> > > Given that watercolour pigments are affected by light, and when we
> expose a
> > > gum print we expose the pigments to a lot of UV; probably more UV light
> than
> > > a watercolour print probably gets over a period of a year; will some of
> the
> > > pigments already have faded by the time we have finished a gum print, or
> > > would more occur later? I know this will vary with different pigments,
> but I
> > > am interested in the general rule. Do manufacturers test the pigment by
> > > intense exposure to light (or UV)?
> >
> > Hi Jan,
> >
> > Interesting thought, but I don't expect the amount of UV a gum print
> > gets in exposure would be very significant with regard to fading. It
> > depends on the light of course, but in my experience most lights we use
> > indoors take longer exposure than direct exposure to the sun does, so I
> > would deduce from that that in general, exposing a gum print with
> > whatever light would give it the same or less UV than exposing it
> > directly to the sun for the same amount of time. Certainly a few
> > minutes exposure to the sun shouldn't cause any noticeable fading.
> >
> > Katharine
> >
Received on Mon Jul 25 13:30:08 2005

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