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Kirk Keyes
04-23-2008, 01:18 PM
I know MG papers are typically made by laying down 2 layers of emulsion, one with high contrast and one with low contrast.

Is it possible to mix 2 emulsions, each with differing contrasts, and expect them to function as a MG paper?

(I suspect not as the dyes may interact with one emulsion or the other, and change the original properties of the two individual emulsions.)

PE? Anyone..., Bueller?

Ian Grant
04-23-2008, 01:27 PM
Multigrade emulsions are mixed ! Not two coatings of different emulsions.

Ian

Photo Engineer
04-23-2008, 02:57 PM
I have to agree with Ian. In the early days, they were sometimes coated as two layers. Using properly ballasted or specially compounded chemicals as I mentioned elsewhere prevents crosstalk. There may be a few examples nowdays that are two layer products come to think of it, but I have not investigated this. In one case I know of, the omission of a chemical causes one grade of contrast and inclusion of it gives the other grade emulsion. In another case, the ratio of bromide to chloride did the job.

PE

Dave Miller
04-23-2008, 03:13 PM
When we visited Ilford we enjoyed a demonstration of their coating machine. I thought they explained that although the two emulsions were applied at almost the same time through a special applicator, they did not mix, but stayed quite separate on the paper. They certainly were applied from seperate tanks and not premixed. Maybe I was mistaken?

Photo Engineer
04-23-2008, 03:20 PM
They could be mixed in the lines, but they could be dual coated using a slide or curtain coater Dave.

Kodak has used both mixed emulsions and dual layer (slide) coatings. I was not aware that Ilford used the slide method. I assumed that they mixed them in-line as, I assume, did Ian. All things are possible. I don't know what Polycontrast IV used. The early papers like Varigam and others used 2 layers. I was told that Kodak had begun using 1 layer. All things change. If it is wrong, I am sorry.

I don't know how I would do it yet if I hand coated a VC Azo type paper. I still have that in mind.

PE

Kirk Keyes
04-23-2008, 03:30 PM
OK - I guess I'm living in the past...
I see on Ilford's site, they say "MULTIGRADE papers are coated with an emulsion
which is a mixture of three separate emulsions."

Ryuji
04-24-2008, 02:49 AM
When we visited Ilford we enjoyed a demonstration of their coating machine. I thought they explained that although the two emulsions were applied at almost the same time through a special applicator, they did not mix, but stayed quite separate on the paper. They certainly were applied from seperate tanks and not premixed. Maybe I was mistaken?

Even if the paper is coated with one (mixed) emulsion, which is the case with variable contrast papers (Multigrade is an Ilford trademark), there is a protective supercoat layer and therefore the coater must have at least two solutions (called "melts") applied onto the paper support. In all modern coater designs, ALL layers of the coating materials are applied simultaneously. The commercial emulsions are fairly viscous and taken up by a very fast flowing paper so they maintain the multilayer structure. It is most surprising to cut a section of a color negative film material and look under transmission electron microscope (take a slice perpendicular to the film plane).

Ryuji
04-24-2008, 03:07 AM
I know MG papers are typically made by laying down 2 layers of emulsion, one with high contrast and one with low contrast.

Is it possible to mix 2 emulsions, each with differing contrasts, and expect them to function as a MG paper?


Renwick and his people at Ilford made the first variable contrast paper emulsion. There were a couple of different generations of technologies in this, so you will probably find this subject very interesting to study.

The first generation of Renwick/Ilford used a bromide emulsion without a dye and chlorobromide emulsion with an ortho dye. The bromide emulsion is made about twice as fast as the chlorobromide emulsion. Due to the technical limitation of that time, higher contrast emulsion must be the chloride emulsion, thereby green light giving a higher contrast than blue light. Note that this is OPPOSITE to today's system. Another shortcomming of this system was that, when high contrast is obtained with a yellow filter, silver used in the bromide emulsion is completely wasted, because they are not exposed at all. Usually, higher contrast should utilize more silver content of the emulsion for the best results.

The next generation is a gross simplification from this. and suggested possible operating mechanisms. Potter and Hagaman found that a variable contrast emulsion can be made from one batch of emulsion by using considerably less amount of an ortho dye than needed to cover the surface of all grains. In such cases, the effect of the dye is most pronounced on coarse grains (fast and softer contrast), and thus green light will give softer contrast than blue, the latter which exposes all grains.

Later generations are based on the second generation filtering scheme, but may use multiple batches of the starting emulsions, each of which may be dye sensitized with different dyes with absorption peak that are significantly far. The dyes used in this application must be "non-diffusing" and the dye must be used very carefully, in an optimal quantity, which varies with the grain size (and therefore grain surface area per mole of silver).

In short, it is very useful learning process and stimulating mental exercise to understand how multicontrast emulsions work and how they were made, as well as the technical difficulties that the pioneers had to solve, but I don't think it's very fruitful to set a goal to reprecate them in your darkroom. It is FAR easier to make a range of paper emulsions from low contrast to high contrast.

Ryuji
04-24-2008, 03:11 AM
By the way, I hear Denise coming to chase me.

Check out U.S. Patents 2,202,026 (Renwick), 2,280,300 (Potter et al), and, for your reference for the conceptual father of this technology, British Patent 15,054 of 1912 (Fischer).

Ray Rogers
04-24-2008, 04:10 AM
The true date of the early history of variable contrast emulsions has yet to be established.

It is quite astonishingly, very old.

Ray

AgX
04-24-2008, 06:03 AM
The true date of the early history of variable contrast emulsions has yet to be established.

It is quite astonishingly, very old.

Ray

I thought Fischer is believed to be the originator. Am I wrong?

dwross
04-24-2008, 07:44 AM
By the way, I hear Denise coming to chase me.

Check out U.S. Patents 2,202,026 (Renwick), 2,280,300 (Potter et al), and, for your reference for the conceptual father of this technology, British Patent 15,054 of 1912 (Fischer).

:) Thanks! I have been chasing down this info. It's fascinating work with almost limitless potential - just about anywhere along its timeline.

d

Ray Rogers
04-24-2008, 09:37 AM
I thought Fischer is believed to be the originator. Am I wrong?
Not necessarily... It depends on how you define what it is he "originated".

I believe Fischer is credited with the suggestion or invention that color sensitized emulsions could be used to produce images in color,
details of which Ron would be better at describing than I could ever hope to do.

In his paper, I beleive he specificlly mentions the possibility of combining different(ly sensitized) emulsions,
either in layers or as mixtures.

I do not recall any direct mention of this being used to produce black and white images
having contrast differences. (I imagine PE would be quick to point out that color emulsions ARE B/W emulsions... plus alpha!).

My take on the matter is that his role here primarialy is his suggestion that
different(ly sensitized) emulsions could be combined to obtain certain effects,
but this was only part of the picture.

I have not read that much Fischer however, and it could be that I am simply over looking what I am looking for.

In any case, what I have in mind is... well, deals with another aspect of the problem...
as Paul Harvey would say... its "The rest of the story".

Ray

Photo Engineer
04-24-2008, 11:22 AM
The texts I have are uniformly silent on references to Variable or Multicontrast materials. Fischer is indeed credited with early reference to the use of couplers to form dyes in film and control of color image contrast by varying coupler or silver in such coatings. In fact, the first couplers followed his model by being of the structure he proposed and which ended up being called in the industry "Fischer Couplers". They are no longer in use.

The earliest VC paper that I remember was Varigam in the 50s, but there were papers concurrent with it and earlier as well and I have forgotten their names. There is however, a rather extensive thread here on APUG from about 1 - 2 years ago with several references including some to early patents.

The early VC papers were expensive and of poor quality compared to most of today's products. Dickerson and Zawadski published an excellent review of this in Photo Techniques magazine about 2 - 3 years ago. I have referred to it before in posts here as well. They compare the quality of several different brands.

PE

Ian Grant
04-24-2008, 11:41 AM
Ilford were the first to announce and show a commercial variable contrast paper during WWII, releasing samples for trial in 1941. However Dupont put their Varigram variable contrast paper into full scale production first, and it was based on the same technology licensed from Ilford.

Ilford were forced to delay their own full scale production due to demands for other products for the War effort.

Ian

AgX
04-24-2008, 03:33 PM
Well, many know about Rudolf Fischer as the main person behind the idea (sic) of chromogenic development.

However it seems outside Europe to be lesser known that at about the same time he also patented the idea of combining a blue-sensitive, high-gamma emulsion with a blue-sensitive low-gamma emulsion for coating photographic paper. I first came across him concernig varible contrast papers by reading the the original report of Ilford introduncing their new paper to a RPS gathering in 1940 were the idea is related to Fischer.
In the literatrature to follow over the years he again is credited, but it is also indicated that due to emulsion-technical causes such a paper could not be produced before Ilford’s release.
In a way Fischer had to live through his coupler frustration a second time.

Up to now I could not trace that very patent. As dates are given as well 1911 as 1912.
Neither am I sure about those emulsion problems.

The lecture text can be found here:
http://www.photomemorabilia.co.uk/Ilford/Multigrade.html

Photo Engineer
04-24-2008, 04:03 PM
All early versions were plagued by crosstalk of chemistry and light. That is why I noted above that the idea had to wait for a number of years before being perfected.

Fischer had many novel ideas for photographic science that he never saw realized or fully realized in his time due to the state of the art. Since much of his fundamental science was flawed, not from his own ability, but due to the state of the art being so restrictive in his era, we did not study his work at all. We started with what worked rather than what did not work. The only thing we did to was study why it did not work.

The crosstalk is one reason.

PE

AgX
04-24-2008, 04:29 PM
Could you be more precise?

Actually I don't realize the problem of coating two differently sensitized emulsions on top of each other in those days, nor in blending them.

Photo Engineer
04-24-2008, 05:01 PM
If the emulsions were both AgBr/Cl, then they would both have blue sensitivity, and this would cause blue light crosstalk, and if they were both AgCl with no blue sensitivity they would be too slow for enlarging speed (at that time). Now, an AgCl emulsion can be made to have any sensitivity wished at speeds up to about ISO 25 or so.

That is for starters.

If you placed two emulsions togethe or separate with dyes of the time, the dye(s) would migrate throughout the coating and 'contaminate' both of the emulsions.

Today, it is not uncommon to have an acidic spectral sensitzing dye that is only soluable in something like pyridine. When added to the emulsion, the pH and solvent 'shock' literally forces the dye to undergo a change in hue and a change in adsorption characteristics. It binds very tightly to the grain surface, unlike older dyes. In this one, I have given you a major heads up in modern improved technology that I have not seen posted anywhere before. I hope that if anyone uses it in posts here or elsewhere, that it is not used alone without reference to this post. And, I might add that a lot of this is learned through actual experience in the lab, not in any patent or textbook.

The list is quite long, but I'll end there. Up to the 40s or 50s, ballasted components or very insoluable adsorbed species were not common.

Ryuji
04-24-2008, 05:35 PM
Actually I don't realize the problem of coating two differently sensitized emulsions on top of each other in those days, nor in blending them.

When you mix two emulsions of different halide compositions, you'll face the problem of halides being replaced at the surfaces. This is a very fundamental problem before complicating the discussion with unnecessary technical details. As a result of this, the emulsion's keeping property is poorer and contrast is likely lower. In order to prevent this problem, you would have to make a core-shell emulsions with the shell having halide compositions adjusted so as to minimize this problem, and then use a potent organic ripening restrainer.

Not many ortho dyes from that era were actually usable for printing papers. Well, lousy acid dyes like erythrosin can sensitize the emulsion, but the pink color will never disappear during processing. Then these dyes are diffusible, meaning that they move between emulsions and between layers. You can learn a great deal about how problematic this is by looking at Agfacolor type color couplers. Those water soluble couplers incorporate a bulky substitution to reduce diffusion, but then synthesis of such compounds is inefficient, and purification even more difficult. This eventually lead to oil protected couplers using very fine miscelles containing couplers in oil-in-water emulsion system. Similar diffusion problems apply to many of the sensitizing dyes, especially old ones.

Then there is a coating problem. Multilayer coaters were not used at that time.

Also note that, today we have no problem in obtaining monodisperse emulsion and we can rather easily determine the optimal amounts of sensitizing dye (because grain surface area is rather well defined), but at that time, using very primitive single jet and lack of electron microscopy, a lot of things were done ad hoc. I deeply respect the conceptual originators of virtually anything I value or appreciate, regardless of the degree of perfection of the implementation at that time. Today, almost everything is done very systematically by careful problem analysis and then by careful design based on the wealth of scientific knowledge as well as proprietary know-how, whenever possible (meaning whenever science is available and when we are patient enough---and I do believe this is generally the best first line approach). However, it wasn't necessarily like that before.