Re: (Gum) Tonal scale

From: Yves Gauvreau ^lt;>
Date: 11/30/05-08:39:40 AM Z
Message-id: <02b101c5f5bb$e59e3f50$0100a8c0@BERTHA>


If you expose a mixture of gum arabic and potassium dichromate with no pigment in it and wheter you use or not a negative or a step tablet there will be, after sufficient exposure of course, a build up of a transparent, clear and insoluble gum and the amount of this gum will be proportional to the amount of light received. The actual amount (weight) of this insoluble gum per unit of area or volume to be more precice is called density in every dictionary I've look at. The confusion starts when we use an instrument like a densitometer design for B&W silver prints, which is just a light meter by the way, this instrument will not register much if any density in the above situation and will not be much more useful if we add a transparent yellow pigment to our mixture. A color densitometer might not be fooled by the yellow pigment but will still be by the clear gum alone and yet every one will agree that there is something there on the paper (which I forgot to specify above). With a proper densitometer, design for this purpose, a clear material like gum will register a value proportional to the thickness of the gum (and paper) and if calibrated properly it could give us the density of the gum present. True, to be scientific about it, you will have to recalibrate this instrument each time something change in your mixture. All these densitometers, by the way, all use an indirect scheme to figure out the density and all the numbers you get from these instruments are relative numbers.

In a typical monochrome gum print it is probably rare that someone would use only clear gum or even a transparent yellow pigment alone. I would say most of you would use a relatively dark pigment as I see in gum prints all over the net. Using a B&W densitometer on these print will give "reasonable" measures of densities that can be most useful in many practical application such as the preparation of a digital negative for exemple. For color gum prints a color densitometer should prove just as useful in general.

There may still be another possible source of confusion because of the totally opposite route that makes a silver print compared to a gum print. To a reflected light meter or densitometer though all this is irrelevant, light is light it doesn't change because it bounce back from this or that object, the instrument is just reporting how much light get back to his sensor knowing how much light as been emitted.



  ----- Original Message -----
  Sent: Wednesday, November 30, 2005 12:00 AM
  Subject: Re: (Gum) Tonal scale

  How bout using the term Exposure Scale for the number of steps the process prints?

  Mark Nelson

  In a message dated 11/29/05 1:23:50 PM, writes:

    Hmm, this brings out another source of confusion when talking about
    tonal range.... I thought Yves was talking about density values rather
    than number of steps. You can get the same number of steps with a
    light-valued pigment as with a dark-valued pigment, for example, by
    choosing the pigment concentration and exposure to achieve that result,
    but the density range between the darkest and lightest step, and the
    density of the darkest step, will be considerably different for the
    two pigments.

  Mark Nelson
  Precision Digital Negatives
Received on Wed Nov 30 08:40:28 2005

This archive was generated by hypermail 2.1.8 : 12/01/05-02:04:51 PM Z CST