This past March, at one of our Special collections meetings, the Lab received a photographic project from UC’s Classics Library. This was a large German collection comprised of 16 series, each series with approximately 300 photographs, a total of about 4800 photographs!! These are silver gelatin photographs that depict ancient sculptures. The photographs are important since in some cases they show sculptures that may have been destroyed during the WWII.
Before Treatment – Series 5 of the 16 series of photographs
Before Treatment – Original enclosure – Book board with cloth ties
Before Treatment – Curled photographs
All the photographs were curled, some showed silver mirroring, and minor tears along the edges, or creases. Most of the conservation treatment focuses on humidification and flattening of each photograph. With such a large number of photographs, the project was divided between Chris (Senior Conservation Technician), Hyacinth (Conservation Technician), and myself (Assistant Conservator).
Before Treatment – Photographs as they were received in the Preservation Lab. Overall, curled with small tears along the edges.
We each took a series of photographs to work on. Ahead of starting the project, we conducted some tests, along with Ashleigh (Conservator), to understand how long we should humidify the photographs, we create the pressing stacks that would be used for the flattening, and some guidelines that we could all refer to throughout the projects.
Before Treatment – Curled photograph, tear, and creased corners (Recto) Before Treatment – Curled photograph, tear, and creased corners (Verso)
We concluded that we would obtain the best results by only humidifying the photographs for a maximum of 20-30 mins and then pressing them. First, we pressed the photographs between pressing stacks of thin Hollytex, blotter, Rising Museum Photomount mat board and binders board for two days. Then we pressed them in a book press or under weights between Photomount mat board until the compression enclosure is created. Before humidification, each photograph was surface cleaned with a hydrophilic sponges.
During Treatment – Small batch of Photographs being humidified in a cold humidity chamber.
During Treatment – Photographs being prepared for flattening by being pressed in a pressing stack.
After being humidified, small cracks on the emulsion and small tears were repaired.
Chris is usually faster with any treatment, so his batch has been fully treated, and now he is in the process of making an enclosure, a cloth clamshell compression enclosure to ensure the photographs don’t start to curl again.
During Treatment – Chris working on the cloth clamshell compression enclosure.
I am still working on my batch. I currently have one-fifth of the photographs being pressed and the rest are awaiting humidification and flattening. This is a long project that requires constant monitoring and time for pressing, but it is so satisfying to see the photographs slowly relaxing and flattening. It will probably take us a few years or more to fully complete the entire 16 series, but once the project is complete each series will be safely housed and repair.
Before Treatment – Photographs as they were received in the Preservation Lab. Overall, curled with small tears along the edges.After treatment – Photographs have been humidified and flattened, each received minor stabilization treatment After treatment – Photographs flattened.
About two years ago, I set upon a mission to gain expertise in the area of identification and treatment of photographic materials. Under the guidance of our conservator, Ashleigh, I developed an education plan that was split between the theory of learning the ins and outs of photograph identification, and the hands-on work of treating pieces that came into the Lab. Of course, these two things go hand in hand. If you can’t identify a piece, you can’t treat it correctly, right?
Fast forward to last year. With the start of the pandemic and the transition to working from home, my education plan changed radically. If I’m not in the Lab, I can’t spend much time on treatment, so I had to get a little creative and work on other ways to learn more.
I am about halfway through the series; a triumph for me, as I have never been one for the study of chemistry. I will say that while it is still very technical, I’ve had a lot of good pegs to hang the information on, owing both to my earlier studies in photograph conservation and my personal history with film photography. It’s been a tremendous thing, viewing things that I learned as a photography student from a different angle. So far, it’s been a great journey.
In this series, I will share with you some of the most fascinating things that I’ve learned so far. My aim will be to keep the technical as simple as possible, for those of you who are like me, still coming to terms with the deeper science. The small bites help it all make sense, I promise. Hopefully, you’ll find it all as interesting as I have.
Before we can understand anything else, we need to talk about halides. What are those and why are they used in photography? Good questions! Halide salts are derived from halogens, which occupy group 7A (column 17) of the Periodic Table of Elements (see below.) Halide salts are used in photographic emulsions that are spread over a substrate (such as paper or film) before the substrate is exposed to light. The silver halides react to the light to form an image when developed.
I should note here that silver gelatin prints, albumen, and collodion photographs all utilize silver halides in their chemical composition. However, silver gelatin is unique among the three in that it is the only one that uses a true emulsion; in albumen and collodion coatings, the halides rest on the surface.
Photograph – silver gelatin process
Photograph – albumin process
Photograph – collodion process
In forming the silver gelatin emulsion, halide salts are combined with silver nitrate and water to form silver halides, the compound at the core of silver gelatin photography. Silver nitrate is pretty much universally used regardless of halide salt, as it is water soluble (it dissolves) but not too much so. The freed silver will look for a bond partner, and the halides in halide salt fits the bill. As a result, silver nitrate, when combined with a halide salt in water, will result in silver halide and a left over salt.
This reaction, which seems like a lot, I know, is referred to for our purposes as “The Emulsification Equation.” To refresh our memories a bit, an emulsification is a liquid (here, gelatin) that contains fine particles of another liquid (the silver halide) without fully combining. Think mayonnaise, or butter. (This isn’t perfectly analogous, as silver halides are crystalline solids and not liquid fats, but the basic idea is the same.)
Chemically speaking, that reaction looks like this:
Equation for emulsification
As a quick reminder, Ag = silver, N = Nitrogen, O = Oxygen, K = Potassium, and Cl = Chlorine.
Now, if you’ll look at the image of the halogen column of the table below, you’ll see a number of options for salts to combine with silver nitrate. Older emulsions involved bromine or iodine; more modern emulsions tend toward chlorine. Crystals formed from silver chloride salts are much more uniform in structure, which makes its use outcomes much more predictable.
Salts that will combine with silver nitrate
I’m sure you’ve noticed that we’ve got a couple of halogens unaccounted for, namely fluorine and astatine. Neither of these are used for this kind of work, and for good reason. Fluorine, for its part, is very water soluble. Very water soluble. To put it in perspective, sodium chloride (regular table salt) is about 35% water soluble. I’m sure that in the course of cooking, we’ve all dissolved salt in water, and you can recall how relatively simple that is to do, though not without some small effort. Well, fluorine salts are about 172% water soluble! You could use it for your emulsion, but moments after developing an image in a water-based solution, you’d see it dissolve before your eyes.
I’ll note here very briefly that chlorine, bromine, and iodine are also more soluble than table salt, but not nearly as much as fluorine, making them perfect partners for our silver ions.
Meanwhile, astatine is…well, it’s radioactive. I think you can see the problem with this one.
And there you have it, a short and hopefully painless explanation of the humble halide in silver-based photography. In the coming months, we’ll be looking at other fascinating aspects of halides and our Emulsification Equation.
Hyacinth Tucker (UCL) —- Bindery and Conservation Technician
At the end of last year the lab purchased a modified UV-Vis-IR Nikon through MaxMax so that we can start to play around with infrared photography. Infrared photography (IR) is commonly used in fine art conservation as an examination tool. Reflected IR can be helpful when trying to identify pigments, inks, coatings, etc. and transmitted IR can helpful for viewing watermarks, underdrawings, and linings. We’ve only just started dabbling with IR photography, but I wanted to share some photos from my most recent session with reflected IR.
This is a full leather photo album from the Public Library of Cincinnati & Hamilton County’s collection. This early 1900s photo album contains hand-colored silver gelatin photographs taken by A. Nielen. The photographs appear to depict his travels through the US and Canada, and various landmarks and neighborhoods of Cincinnati are represented.
Before treatment
After treatment
This seemed like a good object for reflected IR because of the hand-coloring on the photographs and the white ink inscription below each photograph. I began by taking a representative visible light image (first image below) using our modified UV-Vis-IR camera, incandescent lighting, and the X-Nite CC1 filter on our 50mm lens. Then, being careful not to move the position of the camera or the object, I switched to the X-Nite 830 filter (830nm) and converted that image to grayscale in Photoshop (second image below). Then I took my visible light image and my reflected IR image into Photoshop to create the false-color image (third image below). The digital false-color image is a combined representation of the visible and infrared images, and it’s actually quite simple to make. You basically copy and paste the various channels for the VIS and IR image as follows, green to blue, red to green, and IR to red. The false-color image allows you to better differentiate and characterize the various materials (pigments, inks, etc.) and potentially even identify them if you have sufficient known samples to use as references.
Normal illumination
Near infrared with wavelength at roughly 830nm
False color
Like I said, we’ve only just started using IR and we’ve got a long way to go, but I’m looking forward to experimenting and learning more about it as time goes on.
