Determining emulsion ISO

[robopro]

This may have already been discussed, but, when making an emulsion is there a good way to acurately determine the ISO (or ASA) of the emulsion?
Is it possible to 'set' the speed to a predetermined factor -- meaning can one make an ISO 100 B&W emulsion with any fair degree of acuracy?
If not, why?

 

[Ryuji]

This may have already been discussed, but, when making an emulsion is there a good way to acurately determine the ISO (or ASA) of the emulsion?

The emulsion has to be coated and made into the form of use first. Then follow the ISO method for the particular kind of the emulsion. Different kinds of emulsion use different measurement to determine the speed.

Is it possible to 'set' the speed to a predetermined factor -- meaning can one make an ISO 100 B&W emulsion with any fair degree of acuracy?
If not, why?

For most of the time, no. There are many factors that affect speed and you don't know the speed until you make a test batch with exactly the same material, method and technique.

Experienced emulsion makers have accumulation of data from an emulsion made with one or two variables shifted within a reasonable range. In this case, it is possible to aim a specific speed a priori. Even so, you typically make a test batch to confirm that the emulsion is exactly what you need.

In reality, pictorial photographic emulsions are a blend or multilayer coating of 3-5 individually prepared emulsions. (In case of color, for each color layer, there are at least 3 emulsions involved.) The speed of the material is again influenced by the blending of the individual emulsions as well, although it is largely determined by the fastest one.

 

[Photo Engineer]

An easy way to determine the useful ISO of an emulsion is to take a picture of the MacBeth color checker or any other similar step scale and the negative with the scale 'centered' in the range of zones is the proper ISO.

In other words, if you get a full range of steps, you have exposed it correctly, but if you get two steps of the same or approximately the same density, you are too close to either end of the scale. Each step of the neutral scale should show a pronounced difference in density.

As for making emulsions with a given speed, it is quite normal to divide emulsion grain sizes into speed ranges, and emulsion content into speed ranges. Therefore, you can normally construct a nomograph of 'average' speeds for any given set of emulsion size / grain content (Br, Cl, I) and come up with a general range for a given emulsion. Superimposed on this is the crystal habit of the grain, with different shape grains giving different speeds.

It is possible to change the speed range drastically by adding ingredients to either enhance or repress the speed for various reason.

When you blend three emulsions, it is generally the middle component which is of most concern to the user (and the film designer), as that is where the image is normally placed in a correct exposure when using negative color and B&W films. Placing the image on the fast or slow component implies either under or over exposure (toe or shoulder). While this is sometimes to be desired, it is not the optimum.

When you blend emulsions, care must be taken to prevent unwanted changes or avoid unwanted mismatches in the emulsions during keeping and coating which can cause changes in their response to light. This can introduce bumps in the characteristic curve.

PE

 

[robopro]

Interesting. I know that film speed is determined by the size, shape, and density of the silver halide chrystals in the emulsion, but As I'm using an albumen emulsion right now that requires sensitization by soaking in a silver nitrate solution, it isn't really possible to 'force' crystals to grow -- you just get what you get. I was hoping there may be some sort of quick 'litmus test' I don't know about I could do to get an idea of the general speed range.
I'm determining exposure times by trial and error and if I could get a better idea of the true speed (probably around a 6 but I'm just guessing) I could control my exposures better.
I'll give you suggestion a try.

I suppose if I'm going to start taking this hobby seriously I should take some time and actually learn what I'm doing.

 

[Photo Engineer]

You cannot 'force' growth by the soaking method, nor can you chemically sensitize the emulsion with sulfur or gold. You may be able to spectrally sensitize it, but that would give you just speed in the region of spectral sensitization. This would only be important in B&W printing of negatives, not really as much in the camera except to improve the tonal reproduction.

PE

 

[Ryuji]

Interesting. I know that film speed is determined by the size, shape, and density of the silver halide chrystals in the emulsion

That's a VERY crude relation. I can make 0.5 micron film/plate emulsion that varies in terms of photographic speed from single digit to three digits ASA speed. Even if the percentage of iodide is held fixed, the sensitivity varies a LOT depending on where in the grain I place the iodide. (Think of a multi-layer tennis ball and which layer contains iodide.)

As I'm using an albumen emulsion right now that requires sensitization by soaking in a silver nitrate solution, it isn't really possible to 'force' crystals to grow -- you just get what you get.

That method is considerably more limiting compared to modern emulsion technology and can't really control anything important in what I mentioned above...

I was hoping there may be some sort of quick 'litmus test' I don't know about I could do to get an idea of the general speed range.
I'm determining exposure times by trial and error and if I could get a better idea of the true speed (probably around a 6 but I'm just guessing) I could control my exposures better.
I'll give you suggestion a try.

I'd contact print a step wedge for a predetermined exposure. But the measurement of speed for print material is done in a different way than negative films (for example, measures the exposure necessary to get midtone, not shadow) and I still recommend to review the ISO method and the idea behing it.

 

[Photo Engineer]

Tennis balls are hollow. A golf ball would be a better analogy.

Placement of the various chemicals within a crystal is very important, but is often very difficult to achieve without extensive equipment. It is difficult to prove as well without very delicate analytical techniques that are able to probe the crystals on the micron or sub micron level.

Besides Iodide, mentioned above, it is possible to include Rhodium, Iridium, Osmium, Lead, Cadmium, Mercury and Copper salts in various locations within a silver halide crystal to control speed and curve shape, as well as latent image keeping and reciprocity. And, just because I list them here does not mean that I advocate their use.

Unless you have a firm grounding in emulsion making chemistry, or you have a fixed formula given to you, you will most likely be groping in the dark. Outside of the patent literature, which is purpously made obscure most of the time, very little of this is published. Mees and James is one of the best textbooks on this subject, but contains little specific information.

PE

 

[robopro]

That's a VERY crude relation.

Yes, it is, I know, but I'm afraid if I start trying to get too technical I'm going to show off my ignorance even more than I am now! . I'm a little too busy to try it right now but I've been thinking I may try making a gelatin emulsion. Of course it won't be on the level you guys are doing, but what the heck? I copied 4 recipes off the web and have read about a dozen patents and I think I'm starting to get at least a basic feel for it. Also think I might try using sodium thiosulphate to try and bump the speed up. One patent I read called for adding 6.7 grams of 1% solution to every 2000 grams gelatin and ripening at 50C for an hour. I managed to break that down to 1 small drop for every 40 grams.
One of the recipes I found calls for 2 ripening periods, 1 for 2 hours and 1 for 1.5 hours, so I'm thinking I should add it during the second phase.
What do you guys think?

 

[Photo Engineer]

I think that you should base the addition of hypo on silver rather than gelatin, and you should also consider the size of the grain. More hypo is needed as the grain size gets smaller.

The first ripening is usually right after the emulsion is made and contains no hypo, and the second ripening which is called 'finishing' or 'chemical sensitization' is carried out in the presence of hypo.

There are few real modern finish formulas published anywhere, but I will tell you that they are as complex as most modern emulsion formuas. The simple ones that use hypo and heat are really old and rather primitive, but they do work.

Some emulsions do not require any finish depending on the speed you require. I have seen excellent camera speed (ISO 6) emulsions with no finish, so it really depends on what you want.

PE

 

[robopro]

Well, you have to remember that in the patent they control the amount of silver halide and the grain size a lot more tightly than I could. I'll just have to guess, but 1 drop of a 1% solution per 40 grams of gelatin doesn't seem like much -- or am I missing something? Anyway, I can always bump the amount up or down and see what happens, right? And it'll be awhile before I can even get started so I have time for more research -- I may even find a better additive than hypo...

I actually want to produce as fast an emulsion as possible in my kitchen without investing in a professional lab. I like the look of albumen and it is cool to see people's eyes pop open when I tell them how I made it, but the exposure times with pinhole are so long I'm really limited in what I can do. It would be cool if I could get the exposure times on an 11X14 format down to a couple of minutes -- or even a few seconds so I could do portraits.

 

[Photo Engineer]

It depends on how much silver is in that 40 grams of gelatin, not on how much gelatin is present.

For example, if you have an emulsion with 10 grams of silver halide / 40 grams of gelatin and another with 20 grams of silver halide in 40 grams of gelatin, if the grain size was the same in both cases, they would take different amounts of hypo. If grain sizes differed in either case, then the amount of hypo would change again.

So, hypo goes up as silver goes up and hypo goes up as grain size goes down.

Since it varies from emulsion to emulsion, this becomes an exercise in experimentation, finding out how much hypo to add, how hot to heat the emulsion, and how long to hold it. For heat and time, I suggest 60 degrees C for 30 - 60 minutes with hypo present, for amount of hypo, I have no exact value.

For starters, you might try 25 - 50 milligrams of sodium thiosulfate pentahydrate for every mole of silver present, and work up or down from there as needed. It should be added as a 0.1% solution in water and should be made fresh.

PE

 

[Ryuji]

Yes, it is, I know, but I'm afraid if I start trying to get too technical I'm going to show off my ignorance even more than I am now! .

Everyone has to start somewhere, so don't be afraid. Exposing ignorance can be embarrasing but you at least learn something and don't do any harm.

I'm a little too busy to try it right now but I've been thinking I may try making a gelatin emulsion. Of course it won't be on the level you guys are doing, but what the heck? I copied 4 recipes off the web and have read about a dozen patents and I think I'm starting to get at least a basic feel for it. Also think I might try using sodium thiosulphate to try and bump the speed up. One patent I read called for adding 6.7 grams of 1% solution to every 2000 grams gelatin and ripening at 50C for an hour. I managed to break that down to 1 small drop for every 40 grams.
One of the recipes I found calls for 2 ripening periods, 1 for 2 hours and 1 for 1.5 hours, so I'm thinking I should add it during the second phase.
What do you guys think?

The real, short answer is that you can't tell from that info. You need to consider a whole procedure to figure out the range of thiosulfate you should use. Even so, it often requires some experimentation to figure out the optimal level. The optimal level can change depending on the gelatin impurity, temperature of precipitation, duration of precipitation, amount of salt used during precipitation (especially at the beginning), and A LOT of factors. Seriously. The detail gets long, but this is because the optimal amount of thiosulfate added for chemical sensitization (digestion) depends strongly on the crystal structure (cube, octahedral, tabular form of octahedral, or tabular form of cube, etc.) and also strongly on the grain size. The detail is always a long story when it comes to the topic of emulsion sensitization, but it doesn't require a lot of work to make a slow emulsion (ASA single digit). (If you want to make ASA three digits emulsion, you better learn all the latest tricks in this.) More of a problem for you is that emulsions that are sensitized with sulfur only typically produce strong highlight contrast and poor shadow details. (Look at photographs taken before gold technique was used and you'll know what I mean. If you make your own emulsions and try you'll actually experience this.) For camera negative emulsions, it is best to use gold plus sulfur technique, which is quite a bit more involved, but will give MUCH better shadow rendition. For faster emulsions, you need reduction, gold plus sulfur technique, which is CONSIDERABLY more involved...)

If you begin with print emulsion, your life will be a lot easier, because you can make quite practical emulsion of good tonality and speed with sulfur only sensitization.

 

[robopro]

Oh -- that's it -- make it as hard on me as you can!



I know I've got a LOT to learn -- that's one reason why I joined this forum. Why try to reinvent the wheel? But, when you're talking about gelatin ASA of 6, then my albumen must be around a 2. ASA 100 would be great, but then I don't really feel like getting a degree in photo chemestry and turning my basement into a laboratory. I can always stick in a piece of ASA 400 sheet film for protrait work. Uh, do they even make 400 sheet film?:confused:

 

[Photo Engineer]

Yes, they make ISO 400 sheet film.

Sulfur sensitization will get you into the ISO 25 - 100 range, and will give good contrast with many emulsion types. The trick is to get the right level of sulfur, and coat the right level of silver. Among other things, making the right type of emulsion helps to start with and exposing in mid scale avoids any sharp toe that may result.

The whole process can be as easy as dump and stir, once you get the initial conditions ironed out. It would be best to use an emulsion formula supplied in a text book such as "Silver Gelatin" rather than doing it all yourself from scratch. After all, why reinvent the wheel.

PE

 

[Ryuji]

Why try to reinvent the wheel?

It is exactly reinventing the wheel. And it won't be anything like what Fujifilm makes and sells for a couple of bucks a piece.

To me the only reason to make emulsion is to make something that's good but unavailable from any manufacturer. When I want results I can get with commercial products, there is absolutely no point of making the material myself, because I will not save time OR money in doing this. If Tri-X in sheet size will do the job, your time and money are better spent with it.

 

[Photo Engineer]

I have to agree with Ryuji. That is why I have concentrated on Azo paper, and a Kodabromide/Brovira paper. But, OTOH, there is such a thing as being prepared.

That is why I am developing a simple film emulision with a 40s look or thereabouts, to be ready for those who want such a film and also to keep the technology alive. I am also doing it because it can be done, and done simply in a darkroom.

PE

 

[robopro]

It is exactly reinventing the wheel. And it won't be anything like what Fujifilm makes and sells for a couple of bucks a piece.

To me the only reason to make emulsion is to make something that's good but unavailable from any manufacturer. When I want results I can get with commercial products, there is absolutely no point of making the material myself, because I will not save time OR money in doing this. If Tri-X in sheet size will do the job, your time and money are better spent with it.

By reinventing the wheel I meant why try to invent my own emulsion from scratch without finding out how others have done it first, avoid their pitfalls and benefit from their experience? If I was going to design my own clock, I'd first take a couple of other clocks apart and see how they worked, then start on mine.
And I certainly would never try to build my own clock to save time or money -- but just to point at it and say 'I made that!'
If I ever start doing ULF portait work I probably will use sheet film cuz I'll need an ISO of 400 or so. But then I'll have to design a better shutter...