Re: coming to terms with gum

Joe, this is a great contribution and what's more, I agree with you
almost 100% on your terms here. I have (like, duh!) a couple of
comments.

On Dec 7, 2005, at 7:16 AM, Joe Smigiel wrote:

> 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. :)

Hear hear!!!

>
> 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.

Again, perfect agreement, except that I would say that even when
printing on glass or mylar, the term doesn't apply in the same sense.

>
> 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.

Yes.

>
> 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.

Yes, I've made just that point in a post that's bouncing around in the
random cyberspace we've got going at the moment; who knows whether it
will join us sooner, later, or not at all. I agree with this
completely.

>
> 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.

Yes.

>
> 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.)

I have no particular thoughts on these two one way or another; I'll let
you guys decide that one among you without interference.

>
> 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.

Amen.

>
> 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.

This is the first place I disagree with you so far, and it's a minor
quibble. I would say that the "dichromate" image is not just
dichromate but is crosslinked organic material of some sort that the
dichromate found in the paper to react with. And IME this image can be
colorless as well as one of these colors; when there is a color, it's
what I would call dichromate stain.

>
> 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.

I did make this mistake for a short time in a discussion earlier this
year and I take full responsibility for that confusion, which was
completely cleared up at the time, I thought. It was just that I'd
never seen fog in gum, so I'd forgotten it was possible. But yes, I
agree with this definitiion, except that the phrase "reacted
dichromate" seems odd to me. It's crosslinked gum, crosslinked under
other conditions than exposure under a negative. The dark reaction is
another example of this.

>
> 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.

I would take some issue with this definition, but need more time to
think of an alternate definition. It's not my understanding from
others or my personal experience that dichromate stain, when it occurs,
will wash out easily in water.

> Gum Image: the printed image resulting from trapping pigment in an
> exposed and fully processed gum print.

Mmm. I reserve judgment on this one.

>
> 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).

I don't think this is particularly useful as a way to standardize
terms; unless dry pigment is used, it has no absolute meaning. A gram
of one brand of pthalo paint will give a different amount of PB 15 into
the mix than a gram of a different brand, or even a gram of a different
thalo (with a different name) by the same manufacturer. So a unit of
pigment has no useful meaning, except for the individual printer
working within his individual practice.

Thanks again; I think this was a great service.
Katharine
Received on Wed Dec 7 10:34:31 2005