Re: (Gum) Tonal scale

From: Tom Sobota ^lt;>
Date: 12/06/05-05:40:04 PM Z
Message-id: <>


Will you allow an 'independent' opinion in this matter?

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.

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.

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.

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.

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."

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.

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.

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.

"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.

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.

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.

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.

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


At 21:39 06/12/2005, you wrote:
>Is it that we' ve simply misunderstood each other about the way
>we're using the word "density"? When you spoke of reading tone from
>a graph relating response to exposure, (density) to amount of
>exposure, I assumed you were using the word "density" to mean
>"density of reaction product," since when I think of "response to
>exposure" I think of the chemical reaction that is the direct
>response to exposure. In silver, exposure causes the production of
>a silver compound; the density of that compound is directly related
>to tone. The tone IS the compound, in a directly quantifiable sense.
>I was simply making the point that it's not that simple for gum.; in
>gum the tone is not made up of the material that results from
>exposure, and the relationship is not so well known.
> From this post (below) that I got just this morning, it seems that
> when you used the word "density" you simply meant "tone" not
> density of reaction products. You just meant that the tonal scale
> means the difference between the lightest and darkest tones, which
> is what I mean by tonal scale. But your use of the word "density" threw me off.
>P.S. I sent a post to the list this morning that came back to me in
>2 minutes, so I thought maybe the problem with the server was fixed,
>and I wrote two or three more posts. But now it's five hours later
>and none of those have come back, so it's fairly obvious that it's
>still not behaving properly.
>On Dec 6, 2005, at 10:46 AM, Katharine Thayer wrote:
>>On Dec 3, 2005, at 12:16 PM, Yves Gauvreau wrote:
>>>You know the difficulties english gives me and I'd like to know if I read
>>>you correctly this time.
>>>The way I read it, you basically say that because the stuff we put on the
>>>paper in gum prints is different then what is used in carbon, cyanotype,
>>>kalitype, platinum, silver, tempera and probably most other types of prints
>>>that we can't establish a quantitative or even a qualitative relation
>>>between a certain given amount of light and a tone on the paper using a
>>>measure like the optical density of this tone.
>>>Is what I said here essentially the same as what you have been saying all
>>Hi Yves,
>>No, that's not what I'm saying at all. I'm not saying that we
>>can't establish a relationship, (for any specific
>>pigment/gum/dichromate mix under conditions where all other
>>variables are held constant) between a given amount of light and a
>>tone on the paper. I've said again and again that certainly we can do that.
>>What I am saying is that because of the nature of crosslinked gum,
>>and because so little is known about the mechanisms of the process
>>for dichromated gum in general, we can't establish a relationship
>>between exposure and density of crosslinked gum nor can we
>>establish a relationship between density of crosslinked gum and
>>tone. And perhaps I should emphasize here that when I say "density
>>of crosslinked gum" I'm not talking about tone, I'm not talking
>>about optical "density,"
>>because this material has no tone by itself; it's colorless and
>>transparent; there's no tone to read the optical density of. I'm
>>talking about the actual density of the crosslinked gum, the
>>proportion of the layer that's crosslinked. Luckily it doesn't
>>matter that we don't understand that intermediary relationship,
>>because the only relationship we need to understand is the
>>relationship between exposure and tone, which is unique for each
>>pigment/gum/diichromate mix in each unique environment of
>>materials, equipment and atmospheric conditions, and which we can
>>graph without knowing the actual density of the reaction product.
>>>----- Original Message -----
>>>From: "Katharine Thayer" <>
>>>To: <>
>>>Sent: Saturday, December 03, 2005 1:57 PM
>>>Subject: Re: (Gum) Tonal scale
>>>>On Dec 1, 2005, at 11:02 PM, Judy Seigel wrote:
>>>>>On Thu, 1 Dec 2005, Katharine Thayer wrote:
>>>>>>... My point about the unpigmented gum was to emphasize that while
>>>>>>the pigment does provide the tonal scale, it does not participate in
>>>>>>the reactions which constitute the response to exposure, so unlike
>>>>>>silver printing and many other photographic processes, with gum you
>>>>>>cannot draw a curve relating exposure to *density of reaction
>>>>>>product* to tonal scale.
>>>>>Actually this calls to mind an expression I've used in trying to
>>>>>explain the process -- I say the action is the gum, dichromate, etc,
>>>>>and "the pigment is just along for the ride." But in fact, it occurs
>>>>>to me that the character of the passenger can make a large difference
>>>>>also -- if he weighs 300 pounds, if he keeps opening all the windows,
>>>>>if he throws rocks at the bicycles passing by, etc. (Or "she," of
>>>>>course.) That is, we do know that the particular pigment affects
>>>>>behavior -- even if just its opacity.
>>>>Which is what I keep saying. Or maybe you're just writing to say you
>>>>agree with me, but every time I agree with you, you write back to agree
>>>>with me again, which makes me think that you think we are debating
>>>>opposite sides of an issue. My whole point is that tonal scale is a
>>>>function of pigment and pigment concentration, (mainly, but along with
>>>>a host of other things) and so if everything else is held constant,
>>>>every pigment and every concentration of that pigment will give a
>>>>different tonal scale. Yes, pigment affects behavior, very very much,
>>>>that's exactly what I'm saying.
>>>>My whole point, and my only point, throughout this discussion, is that
>>>>the relationship between tonal scale (since it is made of pigment which
>>>>does not participate in the reaction) and response to exposure, (since
>>>>it is the production of transparent crosslinked gum) is an indirect
>>>>and largely unknown (in a quantifiable sense) relationship, and so
>>>>can't be graphed to read tone from density of crosslinked gum, as
>>>>someone wanted it to. How many times would I have to say that, I
>>>>wonder, before people started understanding what I'm saying.
Received on Tue Dec 6 17:41:16 2005

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