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  1. #1
    Photo Engineer's Avatar
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    Beyond T-Grains and 2 electron sensitization

    We have all heard and experienced the T-Grain revolution started in the 80s, and now we are experiencing the 2 electron sensitization revolution.

    Each step in this evolution (revolutions in themselves) improved grain and sharpness. Along with this is the new tellurium sensitization introduced by Fuji this last spring. I am personally torn by this one as it uses a rather toxic ingredient, bypassed by Kodak, but who am I to judge.

    But now we are on the brink of perhaps another evolutionary step in this series of revolutions in analog photography that may or may not be exploited. This is based on discoveries by J. Maskasky at Kodak and several others at Fuji including T. Shiozawa. The work began in the mid 70s, about the same time as the development of T-Grains.

    It is called epitaxy, and it consists of growing small silver halide crystals on each corner of a cube of silver halide.

    This is what is done: basically, you grow a cubic bromide emulsion, the emulsion is then specially treated with silver nitrate and iodide and other chemicals to form additional small crystals at each of the 8 corners of the cube. These sites have the potential to be very light sensitive, and the technology can even be applied to t-grains or any emulsion in which individual grains are regular enough in size and shape. The emulsion is then sensitized by any of a variety of otherwise normal methods, and the result is a high speed, highly developable emulsion.

    Much of the inherent speed comes from the use of an iodide epitaxy, but silver iodide is not very developable by itself, so it pumps the energy into the silver bromide cube and gives a resulting high speed easily developed emulsion.

    So, if analog lives long enough for long term R&D, then this may be the next big step taken by the various film companies. For all I know, it might even be in use in some products now, but I've not been able to get any concrete information on it.

    Then again, it might turn out to be a complete bust. Who knows. I thought you might be interested in knowing that there are a few things left in the R&D bag of tricks.

    PE

  2. #2
    Michel Hardy-Vallée's Avatar
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    Ron, great information as usual. Are not the Delta films already using the epitaxial technology? That's what I gather from a quick google on the subject. There's a post by Roger Hicks on pnet about it, so perhaps he will chime in.
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    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

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  3. #3

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    Quote Originally Posted by mhv View Post
    Ron, great information as usual. Are not the Delta films already using the epitaxial technology? That's what I gather from a quick google on the subject. There's a post by Roger Hicks on pnet about it, so perhaps he will chime in.
    I had indeed understood that Delta films were epitaxial, and that controlled crystal growth -- epitaxial and tabular -- was not exclusively a Kodak invention/discovery: Ilford emulsion engineers were somewhat surprised when Kodak went for the tabular technology, which they had considered but dismissed as insufficiently easily tamed (too sensitive to over- and under-exposure and development, too small a developer repertoire).

    As I recall, too, it was Agfa who did the basic research on positive hole traps, another major advance of the last 20 years, the advantages of which have yet to be fully realized.

    Of course PE was at the coal face, so to speak, whereas my information is necessarily second-hand; I'd be glad of further enlightenment, but as soon as is feasible, I'll also tap up some old Ilford contacts.

    Cheers,

    R.

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    Photo Engineer's Avatar
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    I was working with Joe Maskasky at the time he did his work although I never made any expitaxial emulsions. In fact, I did not know that Ilford was using it, as I only looked at my very old notes plus some things given to me by friends.

    Perhaps Ilford does use it, IDK. I certainly should google it and see what is said. It is certainly not an exclusive to Kodak nor are t-grains exclusive to Kodak. Each company has very carefully tailored t-grains to avoid existing patents. However, Kodak did use t-grains by taming the problems with them before they tamed the epitaxy problems. In fact, when I was working on Gold 400, there was a team behind us doing the first Gold t-grain work.

    So, sorry I didn't know that Delta was epitaxial emulsions, but rather thoguht that they were strictly tabular. But then I mentioned that I was not that sure. However, I have seen some of the Kodak work in epitaxy and it is quite outstanding. It is also over 10 years out of date.

    Controlled growth of both t-grains and epitaxial centers is a very complex field. Just as a comment here, it is related to pAg and silver/salt addition rate. The pAg must be carefully controlled and silver and salt must be added at a rate such that the surface area of the desired emulsion at the growth edges is appropriate for that specific crystal. Therefore, for controlled growth a cube must be grown at a different rate than a t-grain. The salt flow rate is dependant on silver flow rate and pAg and the other salts present.

    The correct method involves a flow rate modeled on a polynomial of the form V = A + Bt + Ct^x + Dt^y + Et^z .... for example where the Volumetric flow of the silver is V, and A, B, C, D and E are coefficients of flow, while x, y, z and etc are powers that t, or time of duration are applied to.

    pAg is the negative log of the silver ion concentration.

    The positive hole traps have never been commercialized to my knowledge due to severe problems in product stability, unlike the 2 electron sensitivity. Even the latter had a similar problem that had to be solved before commercialization.

    Maybe Simon Galley would be permitted to speak up on the subject. In the mean-time I'll do some searching and ask some additional questions myself.

    Thanks for the comments. It was a dull afternoon and I was reading some old notes and made the post without googling.

    PE

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    Mick Fagan's Avatar
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    It might have been a dull afternoon for you PE, but it made for an interesting morning for me, many thanks for another insight.

    Mick.

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    Photo Engineer's Avatar
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    Well, I googled and I find that for the most part, Joe Maskasky has the earlier dates on the patents I could find, and the rest have citations to the work of Maskasky. I also found that as I knew at the time, Kodak devoted most of its work to cubic Cl/I emulsions with epitaxy, but others devoted time to epitaxy on AgBr and AgBrI emulsions of various types.

    I might add that Mees and James make no mention of either T-Grains or epitaxy, as that work was highly secret at the time they wrote their books. Joe's work was ongoing at the time and the patents were just being examined as the book went to press IIRC.

    The statements I was able to find regarding Ilford t-grains with epitaxy are posts on the internet, so I have no factual evidence on its use. I'll take your word Roger. I have no information on Ilford products that detailed. There are Agfa, Kodak and Ilford patents, with citations in Agfa patents to the work of Maskasky. I just skimmed the Ilford patents.

    Thanks again all and thanks Mick. I had just finished a 600 page novel by one of my favorite authors, and I was at a loose end before dinner here, sooo....

    PE

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    Photo Engineer's Avatar
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    Due to the comments above on the use of epitaxy on t-grains, I looked into the matter with one of my emulsion associates (not Joe) who worked with this sort of thing.

    He says that Kodak used epitaxy on only X-Ray films, and that epitaxy generally cannot be used in t-grains due to their thinness. The epitaxy growth tends to etch the t-grain away, so as far as he knows, epitaxy has not been and probably cannot be used in t-grains.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    Due to the comments above on the use of epitaxy on t-grains, I looked into the matter with one of my emulsion associates (not Joe) who worked with this sort of thing.

    He says that Kodak used epitaxy on only X-Ray films, and that epitaxy generally cannot be used in t-grains due to their thinness. The epitaxy growth tends to etch the t-grain away, so as far as he knows, epitaxy has not been and probably cannot be used in t-grains.

    PE
    Dear PE,

    Indeed. This is one of my little crusades, just like yours about film ageing. Delta films are NOT T-grain. At least, so Ilford tells me, and so it looks from the electron micrographs. But as I understand it, both Delta and T-grain promote multiple development centres.

    Cheers,

    R.

  9. #9
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    Roger;

    You can tell an epitaxial emulsion from the characteristic knobs on each corner of the grain.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    Roger;

    You can tell an epitaxial emulsion from the characteristic knobs on each corner of the grain.

    PE
    Dear PE,

    That's what I recall from the electron micrographs though I do not seem to be able to put my hands on any at the moment.

    Cheers,

    R.

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