Re: coming to terms with gum

Please, if possible let's proceed with this very hopeful discussion on
making terms more universally agreed.

Jack

> From: Yves Gauvreau <gauvreau-yves@sympatico.ca>
> Reply-To: alt-photo-process-l@sask.usask.ca
> Date: Wed, 07 Dec 2005 13:46:14 -0500
> To: alt-photo-process-l@sask.usask.ca
> Subject: 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 13:05:10 2005