Re: (Gum) Tonal scale

Hi All,
I'm responding here to Tom Soboda's post, which I didn't actually
receive but found on the live mirror when I was checking on something
else. Since I've had to cut and paste it over from there, it won't be
designated as quoted text in the usual way, so I'll separate my
comments from his by preceding them with my initials.

Tom: Katherine, Will you allow an 'independent' opinion in this matter?

KT: Certainly, that's what a forum is for, for everyone to weigh in on
everything; the more the merrier.

Tom: After reading your inconclusive interchange of opinions with Yves,
I
  must say that I agree with what you say, but not necessarily with
  your terminology. There seems to be a measurement units discrepancy
here.

Tom: Yves speaks correctly of an "optical density of this tone". Now
  'optical density' is an unequivocal concept related to opacity.
  Actually, the optical density is the logarithm (in base ten) of the
  opacity. Even a completely transparent layer of pure gum has an
  optical density, namely zero. The tonal scale, then, is the gamut of
  optical densities.

KT: No argument there, but as I said in a post earlier today, which
you may or not have received, I didn't understand until today that
"optical density" was what Yves was talking about; I thought he was
talking about density of reaction product.

Tom: Yves, as far as I can see, does not make a distinction between gum
  with pigment and gum without pigment. He assumes presence of pigment
  and this is reasonable, since it is the normal practice.

Tom: You, on the other hand, make such a distinction. But you say that
the
  gum layer without pigment "it's colorless and transparent; there's no
  tone to read the optical density of". This is not so: there's always
  an optical density, even if it is zero, as is the case in a perfectly
  transparent material. Or in a very transparent gum coat without a
pigment load.

KT: Actually I think you don't quite understand the distinction I'm
making. I'm not making a distinction between gum with pigment and gum
without pigment, but between the part of the emulsion that
participates in the reaction (gum and dichromate) and the part that
doesn't (pigment). I seem to have confused everyone by saying that
printing gum without pigment helped me understand that distinction, but
the distinction is simply about what is the reaction product and what
is not. The reaction to exposure, the crosslinking of the gum,
proceeds equally well whether the pigment is in the emulsion or not,
and when I printed without pigment, I saw with my own eyes that that is
the case. That's the whole point there.

KT: I do concede your point that I should have said that the hardened
gum has zero optical density, rather than that it has no tone, although
I'm not sure I see the practical difference between the two.

Tom: However later on you make clear that you are speaking of 'density',
  not 'optical density'. You say "I'm talking about the actual density
  of the crosslinked gum, the proportion of the layer that's
crosslinked."

Tom: This is somewhat puzzling since 'density' is also a well determined
  physical magnitude! Put simply, for a given substance it is the
  weight divided by the volume. A very common example: if something has
  more density than water, it falls to the bottom.

Tom: Now I seriously doubt that you are interested in the actual density
  of the gum coat which, in any case, crosslinked or not, is higher
  than water! The 'density' of the gum is irrelevant in this context,
  as far as I can see.

KT: But that's not the meaning of "density" that's usually used in
photography, and that I am using here. I mean "density" to mean the
distriibution of a product across an area, in the same way that Ansel
Adams means when HE defines density: "Portions of the film which have
been exposed to great amounts of light yield a considerable deposit of
reduced silver upon development, referred to as a higher *density*;
areas of film exposed to less light yield less silver, or lower
*density*.

Tom: Correct me if I'm wrong, but your use of "proportion of the layer
  that's crosslinked" makes me think that you are speaking, not of
  density but of 'thickness'. Indeed, what we are interested in is the
  thickness of the crosslinked layer.

KT: No, I'm speaking of *density* in the exact sense that Ansel Adams
meant it: the amount of reaction product distributed in a particular
area of the hardened colloid film. I suppose I mean the number of
molecules crosslinked per area. Whether that translates into
"thickness" of a layer is a controversial question and I don't care at
this point what the right answer is; at any rate no one knows.

Tom: "the proportion of the layer that's crosslinked" that you speak
about
  is then the thickness of the hardened layer divided by the thickness
  of the brushed on layer. When this ratio is 1, all the gum has been
  hardened and nothing has been washed away.

KT: No, I don't find this a useful distinction at all, sorry.

Tom: The relation between thickness and opacity? Well we know that. A
  thick crosslinked layer will carry more pigment and will be more
  opaque, that is, will have more optical density for any given pigment.

Tom: I agree with you that in the case of gum the hardening reaction is
  independent of any additive. The layer will be thicker or thinner but
  always transparent.

Tom: But in normal operation, there will be a pigment present, and then
we
  can speak of varying levels of optical density (or transparency, or
  opacity) with varying concentrations of pigment. And this can be
  calibrated and so on and so on.

KT: Of course, and no argument there, and I think I've said the same
thing about half a million times in the last few days.

Tom: There is also the possibility that I have misinterpreted _both_ of
  you, in which case I hope to be forgiven :-)

KT: You're forgiven.
Katharine
Received on Tue Dec 6 19:39:48 2005