Digital BW, The Print

On 09/05/2012 06:57 PM, Paul wrote:
>
>
> Clayton Jones wrote:
>  >
>  > ...
>  > While doing some cleaning I found a stack of old work prints from
> approximately 6 years ago. Some of them were printed on Museo Silver Rag ...
>  > There are 5 SR prints, small prints with about 1/2" to 3/4" borders.
> They were in the middle of about a 5 inch stack of prints on a variety
> of matte papers. All five have yellowed borders. The yellowing extends
> inward from all four edges from 1/8 to 1/4 inch and feathers into white
> (no hard edges), so whatever caused the yellowing appears to have worked
> its way in from the edges while in the stack. The yellowing appears on
> both the front and back. I'm not a smoker and this room is air
> conditioned most of the time (I'm in South Florida, so even with A/C
> there is some humidity). None of the matte prints in the stack had any
> yellowing.
>  > ...
>
> Clayton was good enough to send me a couple of samples of the yellowed
> Museo Silver Rag. I don't have the chemistry background or tools to know
> what caused it, but here are my observations:
>
> The spectro measures the outside 1/4 inch of the front paper margin as
> having a Lab B of from about 4.5 to 5.1. The inside of the 3/4" paper
> margin, closest to the image and furthest away from the edge of the
> paper, has a Lab B of 2.1 to 2.4. The back side of the paper shows a
> similar pattern, although the Lab B values range from 3.3 at the edge
> to 1.2 in the interior. So the effect is on both surfaces, but worst
> on the front.
>
> The yellowing looks a lot like what I saw on the backs of some stored
> silver prints (fiber paper), that, I believe had been attacked by
> airborne acids, possibly put out by other paper in the metal cabinet.
>
> I used my pH test pen on the samples and found that the surfaces of
> the Museo were probably somewhat acidic -- definitely not buffered.
> The interior paper of the Museo did appear to still have a good amount
> of buffering in it. The surfaces of the matte papers were well
> buffered. I then tested the surfaces of newer Museo I had. These
> newer ones did appear to be buffered or otherwise tested as having a
> surface with a relatively high pH.
>
> All of this does not prove that we're dealing with acid attack, but it
> is consistent with that hypothesis.
>
> I'm not sure what would be on the surfaces of the paper that would
> yellow. We usually think of the yellowing being of the paper
> cellulose fibers being attacked by acids. With these glossy "papers,"
> the paper is actually in the interior and separated from the surfaces,
> usually by polyethylene. In the case of the Museo, the interior paper
> appeared to be well buffered and quite different than the surfaces.
>
> It appears polyethylene can also yellow. It looks like inappropriate
> materials combined with oxides of nitrogen in the air are
> possibilities. See
> http://www.tencate.com/8776/TenCate/TenCate-Grass/Region-EMEA/English/en-Grass/Products-amp-Services/Product-advisory/Gas-fading
>
> I suspect these more "advanced" "baryta" glossy papers are still a
> form of RC paper. Something is certainly separating the well-buffered
> interior paper from the front and back surfaces, and I'd guess it is
> the usual polyethylene or a close relative of it.
>
> Again, I can't say what caused the yellowing, but it is clearly there,
> and its pattern suggests an airborne chemical played a part in the process.
>
> Paul
> www.PaulRoark.com

Paul,

True RC papers with PE barriers show their structure when you immerse a 
5mm wide strip in hot water for half a minute and then pull at both 
sides of strip. Typical foil deformation happens then. After that you 
can dissect that paper by rubbing off the coating and paper fibers, 
usually a milky white PE front barrier is left and a transparent PE back 
barrier. Applied in the paper base manufacturing like lamination foils 
are applied.

A cheaper grade has a kind of resin coating as barrier(s), more elastic 
than the polyethylene and it breaks up wire wise like a contact glue 
would when separated. Probably spray coated in manufacturing or rolled on.

I do not see any of that in my sample of Museo Silver Rag. It is paper 
fiber throughout with coating on top and most likely an anti-curl 
coating at the back.

Your acidity theory is a good one but I would suggest; a good buffered 
cotton paper base has been used and that particular batch had coatings 
applied on both sides that were not buffered.


-- 
Met vriendelijke groet, Ernst

Dinkla Grafische Techniek
Quad, pi\ufffdzografie, gicl\ufffde
www.pigment-print.com