Re: (Gum) Tonal scale

On Dec 3, 2005, at 12:16 PM, Yves Gauvreau wrote:

> Katherine,
>
> 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
> along???

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.
Katharine

> ----- Original Message -----
> From: "Katharine Thayer" <kthayer@pacifier.com>
> To: <alt-photo-process-l@sask.usask.ca>
> 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.
>> Katharine
>
Received on Tue Dec 6 21:43:55 2005