Fume box distance of plate from Iodine

[Jeremy Lynch]

If one where to fume a 20″ x 24″ plate, what would be the optimal distance for the Iodine be? And thank you in advance for any knowledge shared.

Cheers Jeremy Lynch

[greg7mdp]

I don’t see why the plate size would matter very much. I would have the plate between 2 and 5 inches away from the iodine. David Burder described making a very large daguerreotype in http://www.cdags.org/wp-content/uploads/PhotCanV29-4Burder-2%20fi%5B1%5D.pdf. It is definitely is a significant undertaking. Good luck to you!

greg

[Jeremy Lynch]

Thanks greg7mdp for the info, but unfortunately I can not excess the link, I get a “not found 404 message.” Is there any other link? As for a significant undertaking, 20 x 24″ plates, yes, one could say that, but crazy and insane is a little bit closer to the mark though. Fortunately for me I have found that Daguerreotypists are a sharing lot that enjoys solving technical problems. And with 20 x 24″ plates there will be a many problems to encounter and also many solutions.

Cheers Jeremy Lynch

[CasedImage]

That link should be http://www.cdags.org/wp-content/uploads/PhotCanV29-4Burder-2%20fi%5B1%5D.pdf , its in the “general writing” section of the resources page under “Making Daguerreotypes – My First 12 Months”. if I remember correctly David used resublimed Iodine rather than crystalline, the later off gases at a greater rate.

www.CasedImage.com

[Jeremy Lynch]

He was testing wet-iodine sensitization using Iodine in solution, resublimed Iodine, which gases at a greater rate? Interesting but I was thinking maybe it is important to get a more even fumed plate with the Iodine gas. Maybe placing a thin membrane filter 2 inches above the Iodine and 2 ” below the plate so the gas will seep, roll onto the plate in an even fashion.

I could be talking crap here and your link may be the solution and I am just complicating things here?

Cheers Jeremy Lynch

[photolytic]

Resublimed iodine is just iodine purified by sublimation (distillation from the solid state).

Outgassing as you call it is nothing more than sublimation. The amount of sublimation is primarily controlled by the vapor pressure of iodine which is a well known function of temperature. The rate of sublimation is proportional to the surface area of the iodine crystals and can be increased by grinding up the crystals to a fine powder.

In a confined space sublimation will stop when the partial pressure of iodine vapor in the air above the crystals reaches the vapor pressure of iodine and equilibrium is reached. Because iodine vapor is more than 8 times heavier than air, it tends to stratify unless the air is stirred up, so that the concentration of iodine vapor at the top of a fuming box may be a several thousand times lower than at the bottom of the box near the crystals. As a result fuming takea longer if the the distance from the iodine to the plate is greater or if a barrier or membrane is placed between them.

To ensure uniform silver iodide coatings, fuming is usally done when the concentration of iodine fumes in the box have reached equilibrium. This may take only a minute or so if the temperature is above 20C.

In his paper on iodizing Daguerreotype plates, Irving Pobboravsky reported that iodide coating thickness (XRF) increase linearly with time up to 180nM during a over a period 7 minutes. From the density of silver iodide it can be calculated that more than 20 mg of Iodine will have reacted with the silver on the surface of a wholeplate during this fuming. Thus it is important that during long fuming period sufficient intervals for iodine fume renewal be allowed.

[Andy Stockton]

Photolytic, based on what you say about stratification, is there an optimum distance between plate and iodine/bromine? Your science based answers are always so helpful to me.

I have read numerous different suggestions concerning methods for getting an even coating and some suggest that plates too close to the iodine/bromine can get “spots”. Several have said covering the iodine crystals with a cotton layer helps with evenness of the coating. How would iodine gas cause spotting regardless of the distance?

Any further thoughts you care to share would be most welcome. I am working on a new fuming box design since I need a new one and have wondered about the z dimension. The plate size sets x and y, but the height seems rather arbitrary.

[photolytic]

Andy,

There appears to be no optimum distance but the distance in most boxes seems to be 3-4 inches simply to reduce the size of the box to a more manageable size. My measurements in the attached graph were made by sampling 100cc of the vapors at the top of a closed box using a syringe.

The iodine was extracted from the vapors with ethyl alcohol and titrated with a standard solution of sodium thiosulfate using starch as the indicator.

I repeated this at several temperatures and found that the measured iodine concentration in the vapor was 4 orders of magnitude (1/10000) less that the theoretical concentration for iodine vapors above iodine crystals at that temperature.

I also did this for bromine vapors using a fuming box containing 10 grams of bromine mixed with 400grams of calcium hydroxide. In the case of Bromine I found that its concentration was also approximately 1/10000th of the theoretical concentration for bromine vapor above bromine liquid at that temperature.

[Jeremy Lynch]

Photolytic, respect! Thank you for the information, you answered all the questions I had.

Cheers Jeremy Lynch

[dagist]

If you are going to use iodine crystals in your fuming box, as most of us do, place a layer of cheesecloth over the crystals; lay it right on top of the crystals themselves. This does two things. Not only does it theoretically help to diffuse/even-out the vapors in the box, but it also helps to keep the iodine crystals from moving around and becoming unevenly distributed if you move your box from place to place.

The cheesecloth trick is especially helpful if you plan on traveling with your iodine box. My sensitizing boxes move back and forth from my lab to the portable daguerreian darkroom in my van, and without the cheesecloth, the iodine crystals would be clumped up in random piles from driving around.

I’ve had the same piece of cheesecloth (now brown and saturated with iodine) in my fuming box for over 10 years and my plates are almost always evenly sensitized from edge-to-edge. Rarely do I ever get blotchy patches on the plate from uneven sensitization, where one area turns color before another. A smooth, evenly-saturated color, covering the entire surface of the plate, is ideal.

Pre-heating your plate just before sensitization is also helpful in achieving even sensitization.

I agree with John’s suggestion of between 3 and 4 inches for the distance between the iodine crystals and the plate. That gives the box plenty of volume with which to more-evenly disperse the vapors from below.

Good luck,

Rob McElroy

Buffalo, NY

[Jeremy Lynch]

Makes sense with the cheese cloth lying on the crystals to “theoretically help to diffuse/even-out the vapors in the box”. It would be interesting to get high speed footage of the event. To see how Iodine moves, circulates over time, inside the box and over the plate. I bet you it is quite sexy and a lot could be learned.

Dreaming out loud.

Cheers Jeremy Lynch and thanks Rob for the tip.

[Andy Stockton]

Thanks photolytic. Your information is helpful as always. I will post some pictures when I am done with the fuming box.

[photolytic]

I agree with Rob that placing a layer of cheese cloth on top of the iodine will lessen the tendency for rearrangement of the crystals when the box is moved. I haven’t personally used this method recently but I haven’t notice any increase in uneven fuming. I believe most of the uneven fuming is the result of variations in roughness of the plate surface due to insufficient polishing or areas of silver worn thin on older plates. Rough areas in the silver surface have a greater surface area, allowing the iodine fumes to react faster.

Because most of the distance between the iodine/cheesecloth layer at the bottom of the box and the plate drawer at the top of the box is filled with an air/iodine mixture, any movement of the box can still cause a temporary disturbance in the iodine vapor stratification. I always allow my fuming boxes to sit a few minutes after moving them from their normal storage area to my fuming table to allow the air/iodine mixture to reestablish equilibrium.

[Author]

Posts