Sensitivity spectrum of gum (was: Re: Gum problem(s)

On Nov 19, 2005, at 10:31 PM, MARTINM wrote:

>
> ----- Original Message -----
> From: "Sandy King" <sanking@clemson.edu>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Saturday, November 19, 2005 3:35 PM
> Subject: Re: Gum problem(s)
>
>
>> Dichromated colloid processes, including carbon and gum, have a
>> maximum peak in the near UV at about 360 nm to 370 nm, varying
>> slightly with pH and type of dichromate. From the peak at 360-370 nm,
>> sensitivity falls off sharply to about 320 nm, at which point it
>> starts to rise to another, and much large, peak at around 200 nm,
>> where it has its maximum sensitivity. The peak at 200 nnm is really
>> huge compared to the one at 360-70 nm, like Mt. Everest to the
>> highest peaks in the Appalachians. On the other side sensitivity
>> falls off very gradually in the violet and blue to near zero at
>> around 520 nm.
>
> I don't think that's entirely accurate for dichromated colloid
> systems. I
> guess the data you quote, apply to the absorption spectra of a
> dichromated
> water solution.
> When it comes to dichromated colloids, sensitivity peaks might be
> considerably shifted towards longer wavelengths. By the way, speaking
> of DCG
> holography (dichromated gelatin), there are lots of people to expose
> their
> DCG layer at 532nm...

Browsing through an old chapter on dichromated colloids in the printing
industry this afternoon, I came across something that caught my eye,
that relates back to this discussion of a couple of weeks ago, and
after reading that, then I re-read the section in Kosar on sensitivity
as well. It looks to me from my reading that Martin has an
interesting point here, although the data don't speak entirely with
one voice.

According to Kosar, the dichromates are "efficient absorbers of
ultraviolet, violet and blue radiation, with maximum absorption by
ammonium dichromate at 3670 A and potassium dichromate at 3575 A." It
seems, as Martin says, that most people just measure the sensitivity
spectrum of the dichromate solution rather than the sensitivity of the
dichromated colloid layers. Two different sets of researchers who
measured the dichromated colloid film found, for albumen and for some
unnamed "resins," that the sensitivity of the mixed colloid is the same
as for the dichromate. " But for other colloids, it seems that's not
the case, and here's the part that made me sit up and take notice: A
researcher named Jorgenson found that the spectral sensitivity of
dichromated gum arabic was shifted toward the green or longer wave
lengths compared with albumen. "Spectrophotometric curves of albumen
and gum arabic showed that this shift was probably due to the
difference in the spectral absorption of the colloids. "

  I can't seem to find in Kosar any direct information about dichromated
gelatin, although he cites one study of "dichromated colloid film,"
without specifying the colloid, as finding the sensitivity to be
maximum between 3500 and 4500 A, another study, again with colloid not
specified, that finds a spectrum similar to what Sandy describes,
except it goes to zero at 580 rather than 520.

Just sharing my afternoon's reading, for whatever it's worth to anyone,
Katharine
Received on Wed Nov 30 18:48:55 2005