Re: coming to terms with gum

Joe et all,

I think this is a great idea, I think it should be expanded to become
somekind of glossary of terms we could refer people to when there is
confusion about the meaning of terms and it could become part of the faq
maybe.

If I'm not the only one who think it may be a good thing to agree as a group
on the meaning of many terms we use all the time in our post. Currently I
think most of us are assuming everyone else thinks the same about them but I
learned the hard way it is not always the case and it can cause useless
conflicts, frustration and whatever other negative effect you can imagine.

I volonteer, not to do it myself and by myself but I offer my help and time
to build this glossary if any one wants it.

Regards
Yves

PS The FAQ doesn't have an "official" glossary at this time.

----- Original Message -----
From: "Joe Smigiel" <jsmigiel@kvcc.edu>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, December 07, 2005 10:16 AM
Subject: coming to terms with gum

> FWIW, below are definitions for terms as I use them in reference to gum
> printing. Hopefully this will help eliminate any confusion for someone
> reading my posts on the matter of gum printing. I've also made a couple
> suggestions for using specific terms when referring to this or that.
> Hopefully these suggestions make sense.
> ---
>
> Density: not a very useful term in itself since it can refer to optical
> transmissive density, reflective density, mass/unit volume, and
> decreased mental capacity of gum workers suffering from long-term
> exposure to various chemicals and life. :)
>
> Transmissive Density: optical density as measured with a transmission
> densitometer. The transmissive density is the log of opacity. While
> this term is useful in silver printing or when measuring the opacity of
> the light attenuator, it probably has little practical value in gum
> printing other than identifying the value of optical transmission of the
> step wedge or the negative being used for printing. Since we generally
> print on a paper substrate and not glass or some other transparent
> medium, referring to the "density" of a gum print is not referring to
> the opacity/transmission of the printed image. In my opinion, the term
> should only be used to in reference to the negative or stepwedge, and
> not the print.
>
> Reflective Density: a logarithmic measure of the reflectance of a
> surface. This measure might have some utility in gum printing except
> for the fact that no gum printers I know of use a reflection
> densitometer when printing. I believe this term is being confused with
> "tone", "print value", "step", etc., in the current onlist discussion.
> IMO, should not be used except when stating relative observations
> regarding printed maximum and minimum densities which are readily
> observable even though their actual reflectance may not be known.
>
> Transparency & Opacity: Confusing in reference to gum printing. Some
> pigments are opaque (e.g., titanium white), yet high in value. Others
> are dark and opaque (e.g., lampblack). Then there are transparent
> pigments (e.g., phthalocyanine blue). IMO, for gum printing, we should
> restrict the terms opacity and transparency to the relative appearance
> of pigments in terms of how well the colors block underlying
> layers/colors or when discussing the specific optical density of the
> light attenuator. And, we should be careful to specify in which context
> we are using the terms.
>
> Tone: The relative value (light vs. dark) of the image deposit which is
> visually observed in the print or the appearance of a pigment in terms
> of relative reflective value. For example, light tones vs. dark tones,
> light vs. dark pigments.
>
> Exposure Scale (ES): in absolute terms I take this to be the
> transmissive density difference between the maximum and minimum density
> values printed using a transmission density step wedge which result in
> distinct print values and are not blocked compared to both adjacent
> tones. For example, if the stepwedge is printed so that only steps 3
> (transmission density = 0.35) through step 15 (transmission density =
> 2.15) are distinct, the exposure scale would be the inclusive difference
> between steps 15 and 3 resulting in a relative 12-step exposure scale
> having a specific density range of 1.80 density units.
>
> Negative Density Range (DR): the total range of key transmissive
> densities of a negative from most to least dense. ("Key transmissive
> densities" meaning here the negative densities associated with desired
> textures and tones of the subject which are to be reproduced as specific
> visualized values reproduced in the print.) This term is often confused
> with exposure scale. However, they are not the same thing. To optimize
> printing, the density range present in the negative should be matched to
> the exposure scale of the print medium. Negative densities which fall
> outside the range of the print exposure scale will not be reproduced
> without further manipulation of the printing process (burning-in,
> dodging, print overexposure, curve manipulation, long or short soak,
> etc.). Additionally, depending on who you read, the optimum density
> range of the negative to match a specific print process may be reported
> with wide variation. (Some authors take the negative density range to
> be from maximum to minimum tones, others from zone II to zone VII,
> others from zone III to zone VII, and still others from 0.10 above fb-f
> density to some highlight negative density, etc.)
>
> Thickness of reaction product: Is anyone actually measuring this? I
> take it to refer to the physical relief of the gum image, pigmented or
> not. Doesn't make much practical sense to talk about it, IMO.
>
> Dichromate "Image": The physical deposit of reacted dichromate present
> in a gum print, pigmented or otherwise. The dichromate image is usually
> tan or light green and has printed out beneath the light attenuator in
> response to exposure.
>
> Dichromate "Fog": A term I use to refer to random deposit of tan or
> green reacted dichromate akin to the familiar term used in silver
> printing. Heat fog, chemical fog, random exposure fog, veiling, etc.,
> in gum printing are non-image reacted dichromate. I believe some gum
> printers refer to this deposit as "dichromate stain" which IMO is
> confusing and innaccurrate.
>
> Dichromate "Stain": Another confusing term which I take to refer to the
> yellow-red dichromate chemical which might remain in a gum print given
> extremely brief washing. I've never observed this type of staining in a
> fully processed print.
>
> Gum Image: the printed image resulting from trapping pigment in an
> exposed and fully processed gum print.
>
> Pigment Concentration: How much pigment is in the gum bichromate
> emulsion. Ideally it would be expressed in grams per solution volume.
> An example would be 1 gm pigment in a combined 10 ml 14B gum arabic +5
> ml saturated potassium dichromate solution. However, other units of
> measurement are frequently used (e.g., an inch of pigment, a pea-sized
> drop, etc.,) although not with the same degree of accuracy implied by a
> mass/volume measurement (i.e., "density" in the strict sense of the
> term).
>
> ---
>
> Joe
Received on Wed Dec 7 12:51:34 2005