Porphyrography – no darkroom required

Writer and photography / James Tanguay

Porphyrography is a long exposure dry plate process that uses a lensed camera and novel iron-based chemistry to make direct positive transparent images on glass plates. Porphyrography is often attractive to students because of its relatively simple technique and its use of inexpensive chemicals. Perhaps its most appealing aspect, however, is that it does not require a darkroom. The light-sensitive iron salt used in the process reacts only to UV light, so the mixing of the chemicals, the preparation of the plates, and the developing can all be carried out in standard artificial lighting conditions.^1,2

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The Camera

Plate cameras are ideal for porphyrography, especially if the lens board can be fitted with a fast lens. Small plate sizes, such as 4 x 5 inches or smaller, are the easiest to work with in this process.

Medium format and 35mm cameras work well. They should have a fast lens, f/4.0 or faster. The shutter should have a ‘T’ setting so it can be locked open for the long exposures. (The ‘B’ setting will do if it can be used with a locking cable release.) There should be a film pressure plate inside the back cover to hold the glass plate in place when the camera is closed. If there is a little red window for showing the exposure number, it should be covered with black electrical tape.

Homemade cameras are well suited to porphyrography. The example below shows a simple gift box, spray painted black inside and fitted with an inexpensive slide projector lens. The plate is loaded by wedging it between two matchsticks which are glued to the inside of the lid. Focusing is accomplished by peering through a tiny flap cut in the side of the box while sliding the lens in and out until a clear image is formed on the plate. The flap is then taped shut for the long exposure.

The plates used for porphyrography

Whatever camera is used, clear glass plates will be needed to fit it. If suitably sized plates are not available, you can cut them to size yourself. It is not difficult with a little practice. The tools used in the popular stained glass art hobby make the job simple. They are readily available online and in hobby stores. Alternatively, a window glass shop can cut a batch of plates to size for you.

The exact dimensions of the plates are important. Use glass that is 2mm to 3mm thick. Make sure not to sand the edges. Smooth edges can make coating the plates difficult.

For plate cameras, remember that the actual size of a plate is different than the nominal dimensions. For example, a 4×5 plate is actually only 3-7/8 x 4-7/8 inches.

For film cameras, the plates should be a little smaller than the outer edges of the film gate but larger than the space in the gate that allows the light through to form the image.

For homemade cameras, build the camera to fit any plates available. Old magic lantern blanks and 35mm slide cover glasses make good plates. Microscope slides offer another possibility.

Preparing the Plates

Clean the plates thoroughly with soapy water and then rinse them in distilled water. Drip dry them or else wipe them dry with a lintless paper product.

The Starch Slurry

In porphyrography, one side of the plate is coated with a starch slurry which is then dried before the sensitizing process.

• In a very small saucepan, mix 100 mL of distilled water and 3 g of arrowroot starch.
• Place the small pan in a larger, shallow pan. Fill the large pan with as much water as you can without causing the small pan to float. Place this double pan setup on a stove burner or hot plate set to very low heat.
• Using a very small ladle or bent soup spoon, stir the starch mixture slowly and constantly. After a few minutes, the solution will start to thicken and change from milky white to translucent. Let it continue thickening for about a minute and then turn off the heat. Never let it boil. Leave the slurry pan in the warmth of the water of the larger pan during the entire coating process. Small bubbles in the slurry are of no consequence as they will disappear when the slurry dries on the coated plate.

Coating the Plates

1. Grab one of the clean plates by the edges, hold it level over the pans and ladle on some hot slurry. The amount of slurry should be enough so that once it is spread out, it will cover the entire top surface. A couple of tries will teach you how much to use.
2. Very slowly rock the plate toward one edge, then another. Tilt the plate far enough to allow the slurry to go all the way to the edges but not so far that it goes over the sides. Avoid the temptation to use your finger or ladle to spread the slurry.
3. When the plate is completely covered, tilt it at a sharp angle toward one corner and allow any excess slurry to flow off for a few seconds. When the flow slows to a drip, hold the plate level again and wipe off the very tip of the drip corner.
4. Place the coated plate starch side up on a very level surface to dry. Coat the rest of the plates the same way.

The plates typically take about 12 hours to dry. Afterwards, the starch layer is surprisingly durable. Avoid touching it though, because the oils from the skin can cause problems during the sensitizing process. The dry coated plates will keep indefinitely, so a large batch can be made and then sensitized as needed.

The Sensitizer

The sensitizer is a combination of two common, inexpensive iron salts. Whenever handling the sensitizer or the sensitizer chemicals, always work under artificial lighting and exercise all necessary safety precautions.

Ferric Ammonium Oxalate – also known as FAO. This is the key ingredient used in “New Cyanotype.” It comes as greenish crystals or as a powder. Don’t confuse it with the ferric ammonium citrate used in traditional cyanotype. Use the oxalate form.

Anhydrous Ferric Chloride – FeCl₃. This is a dark, grayish-black, crystalline powder. Most photochemical suppliers don’t stock it, but it is readily available online. Do not use other forms of this chemical. It must be the anhydrous type. Also, do not use the premixed ferric chloride solutions intended for etching circuit boards.

Making the Sensitizer

1. Measure 100 mL of distilled water into a beaker or other mixing container.
2. Add 10.2 g of FAO (Ferric Ammonium Oxalate) and stir with a plastic spoon or glass rod until dissolved.
3. Slowly and carefully add 3 g of FeCl₃. Stir until dissolved.
4. Pour the mixture into a labeled amber glass bottle.
5. Allow the sensitizer to sit for a couple of hours. There will be impurities that settle to the bottom.
6. Filter the sensitizer into a new labeled amber bottle using a funnel and a moistened coffee filter.

If stored in a dark, cool place, the sensitizer will be viable for a year or more.

Sensitizing the Plates

1. Find a small, shallow tray with enough room to fit a plate and still have some room around the edges for your fingers. Square petri plates are ideal. You’ll also need a very small, very thin, narrow plastic spatula. You can cut one from a plastic food or drink container.
2. In the proper lighting, pour more than enough sensitizer into the tray to cover one of the plates. Do this before you put the plate into the tray. The starch layer becomes soft when wet, so pouring liquid over it may damage it. You can use the same tray of sensitizer to sensitize many plates.
3. Gently slide a plate into the sensitizer or lower it in with the spatula.
4. Let the plate absorb the sensitizer for about 30 seconds, then scoop it out gently with the help of the spatula. Tilt it toward one corner to let the excess sensitizer drip off. Wipe the back with a paper towel, being careful not to disturb the starch layer on the front, then set the plate on a level surface while you sensitize any remaining plates.
5. Dry each plate by holding it starch side up with your hands above an alcohol burner or stovetop. Don’t worry about overheating the plate. As long as your hands are not too hot, the sensitizer is safe. Plates are best used the day they are sensitized, but they usually remain viable for at least 3 or 4 days.

Making a Porphyrograph

For your first image, try something simple. Use bright outdoor lighting. Choose a subject that has a lot of detail and one that’s noticeably brighter than its surroundings. A close-up with an uncluttered background can be a good choice.

If you’re using a camera with a viewfinder, you can load it before you go out into the sunlight. If you’re using a field camera, you can, of course, load in full sun by just sliding in a plate holder. When using other types of cameras, you can load while placing yourself, the camera and a darkbox containing the plate under a large dark cloth or blackout curtain. The little bit of light reflecting up under the dark cloth is not enough to affect the plate while you load the camera. You can swap out plates afield in the same way.

The Four Exposure Factors

There are four factors to consider in determining the correct exposure time for a porphyrograph. The first three correspond to the same ones used in conventional film photography – shutter, lens and lighting.³ You can learn about them from countless online articles and film photography books. However, as explained below, there are some special considerations for these factors in regard to porphyrography. The fourth factor, the UV Factor, is somewhat unique to porphyrography. When the four factors have been determined, they are simply multiplied together to get the exposure time.

Factor 1: The Speed Factor

This corresponds to shutter speed. For the sensitizer formula in this article, the Speed Factor is 96 minutes. This means that a plate must be exposed for 96 minutes at standard conditions in order for a white object to be white but still with significant detail in the developed image.

Factor 2: The Lens Factor

This adjusts the exposure to compensate for a difference in f/stop from the standard. In film photography, the standard is usually f/16. In porphyrography, the standard is f/2.5. You can calculate the Lens Factor by using the usual principles of aperture adjustment (remember the Sunny 16 Rule).³ Alternatively, you can obtain the Lens Factor by simply dividing the f/stop you are using by 2.5 and then squaring the result. Example: If you are using f/4.0, the Lens Factor would be (4.0/2.5)² = 2.56.

Factor 3: The Illumination Factor

This corresponds to the light reading in Exposure Value units (EV) in film photography. The standard in porphyrography is EV15.0, a cloudless, sunny day.⁴ Because the Speed Factor is based on white objects, it is best to take a light reading from the brightest object in the scene you’re shooting. If you calculate the exposure time with this reading, the object will be white in the final image. If you want the object to appear darker, reduce the exposure time by up to 50%. There are several excellent light meter apps which you can download to your phone that measure in EV units.

Factor 4: The UV Factor

Remember that porphyrographic sensitizer is sensitive only to UV light. Because our eyes and our light meters don’t pick up on much UV, an adjustment has to be calculated and then figured into the usual exposure calculations. This is the UV Factor.

Latitude N or S	Latitude Component
0-15	0.80
20-25	0.87
30	0.98
32	1.00
35	1.07
40	1.18
45	1.31
50	1.46
55	1.64
60	1.94
65	2.21
70	2.54

Month	SC Northern Hemisphere	SC Southern Hemisphere
January	1.45	1.00
February	1.33	1.00
March	1.17	1.04
April	1.06	1.13
May	1.01	1.28
June	1.00	1.43
July	1.00	1.45
August	1.00	1.32
September	1.04	1.17
October	1.13	1.06
November	1.28	1.01
December	1.43	1.00

UV light intensity changes most significantly with two things – first, the time of year, the Seasonal Component (SC), and, second, the angular distance from the equator, the Latitude Component ( LC).⁵  These two components are shown in the tables. To get the UV Factor, simply multiply the figures for your current month and latitude together. It is important to note that the porphyrographic UV Factor is not at all the same as the UV Index used by medical professionals to indicate the danger of ultraviolet radiation to human skin.

Example UV Factor Calculation — It is October in Los Angeles, California, USA (34°N). The tables indicate the Seasonal Component is 1.13 and the Latitude Component is about 1.07.

UV Factor = SC x LC = 1.13 x 1.07 = 1.21

The figure will stay the same until the next month or until the location changes significantly north or south.

A Real-Life Example of Exposure Time Calculation

1. You set up a shot with a 4×5 field camera and slide in a plate holder. Your subject is a lonely garden chair. The camera doesn’t have a shutter, so you simply pull the dark slide from the plate holder to start the exposure. You start the stopwatch on your phone and then begin calculating the exposure time.
2. You’re using the sensitizer formula from this article which has a Speed Factor of 96 minutes.
3. Your lens is set to f/3.5. Because f/3.5 is one stop up from the standard of f/2.5, you decide to double the exposure time. Verifying by calculation, you get (3.5/2.5)² = 1.96 or about 2. Your Lens Factor is 2.0.
4. The chair is white and you want it to appear that way in the final image so you take your light reading from the brightest part of the chair. It reads EV16.0. This is twice as bright as the standard of EV15.0, so you’re going to cut the exposure time by half. Your Illumination Factor then is 0.5.
5. You’re shooting in September in Tucson, Arizona, USA. Your phone tells you that the latitude there is 32°N. Using the tables, you find that your Seasonal Component is 1.04 and your Latitude Component is 1.00. You multiply the two components together to get your UV Factor of 1.04.
6. You now multiply the above four factors together to get the correct exposure time. 96 minutes x 2.0 x 0.5 x1.04 = 99.84 minutes. Rounding off, you find the
   Exposure Time = 100 minutes or 1 Hour 40 Minutes.

During the long exposure, you sit under a shade tree and read a couple of interesting articles on AlternativePhotography.com (this website!). You put the dark slide back in place at the 100-minute mark and go to your kitchen to develop the image.

The Developer

The developer is a simple solution of Potassium Iodide, KI. It will keep indefinitely.

Exercise all necessary safety precautions when mixing or using the developer.

• Add 41.5 g of potassium iodide to a labeled clear glass bottle.
• Add 1 litre of distilled water. Cap and shake until all of the KI is dissolved.

Developing

Developing the plates is quick and easy. Set up in suitable lighting.

1. Pour enough developer into a shallow tray to amply cover the plate size you’re using. The temperature of the developer is not important. Fill a second tray with distilled water for rinsing. Gently slide a plate into the developer or lower it in with a spatula. Leave it in the developer for 15 to 30 seconds, but no longer. The image will appear very quickly.
2. Lift the plate from the developing tray and put it the rinse tray. Leave it there for a minute while rocking the tray only very gently. Remember that the starch layer is easily damaged when wet.
3. Remove the plate, change out the water and repeat the rinse, then do a third rinse.
4. Remove the plate, wipe off the back and either leave it in a dust free area to air dry or dry it over a heat source as you did at the end of the sensitizing procedure. The plate is no longer light-sensitive. No fixing process is necessary.
5. Discard the developer in the first tray and repeat the steps above for any additional plates.

Redeveloping

A peculiar habit of porphyrographs is that they slowly change color. When a developed image is first pulled from the final rinse, it has the distinct blue colors often associated with cyanotype images. After the plate ages several days, the colors shift toward purple. This purple stage lasts for a long time, usually months, even a year or more. The purple stage ends when a final, gradual shift toward sepia begins. Many find the erstwhile look of the sepia stage very appealing. In fact, for some, this stage is the goal of making a porphyrograph to begin with. If, however, the original cyan hues are preferred, the image can simply be redeveloped at any time. It will immediately return to its original appearance.

Variations

Porphyrography lends itself to experimentation. Variations in the relative proportions of the iron salts in the sensitizer and the overall combined concentration of both these salts can be used to control contrast, speed and density. Manipulation of the starch concentration in the slurry used to coat the plates can also lead to interesting artistic effects. In the example image of the bowl, the relative proportion of FAO and FeCl₃ was kept the same while the concentration of both salts was halved. The result was a sort of watercolor effect with much lower density and lower contrast along with a doubling of the speed, 45 minutes instead of 96 minutes.

The image of the chair and table was created by using a 4% starch coating instead of the usual 3% along with a sensitizer in which the proportion of FAO to FeCl₃ was increased by 33%. A huge increase in both density and contrast was observed. The speed was only slightly faster—84 minutes instead of 96 minutes.

Footnotes

1. Pozdnyakov, Ivan & Kel, Oksana & Plyusnin, Victor & Grivin, V. & Bazhin, Nikolai. 2008. “New Insight into Photochemistry of Ferrioxalate.” The journal of physical chemistry. A. 112. 8316-22. 10.1021/jp8040583. https://www.researchgate.net/publication/ 23174616_New_Insight_into_Photochemistry_of_Ferrioxalate#pf3
2. Papiewski, John. “What Light Bulbs Do Not Emit UV Radiation?” sciencing.com, https:// sciencing.com/light-bulbs-not-emit-uv-radiation-15925.html. 15 May 2022. https://sciencing.com/ light-bulbs-not-emit-uv-radiation-15925.html
3. For those new to exposure calculations, a good way to gain an understanding of the factors involved is to study “The Sunny 16 Rule.” There are many resources available. Here is one example, “How to Master the Sunny 16 Rule” https://www.photographytalk.com/beginner- photography-tips/how-to-master-the-sunny-16-rule .
4. EV15 corresponds to a sunny, cloudless day. EV14 – partly cloudy, EV13 – cloudy, EV12 – full shade on a sunny day. White objects, snow and sand will often read one EV unit or more greater than the general surroundings. Use of a light meter or app reading in EV units is recommended.
5. UV intensity is affected by the time of year and latitude because both change the angle of the sun with respect to the atmosphere. For general discussion of the phenomenon, see “What Determines How Much Ultraviolet Radiation Reaches the Earth’s Surface?” https:// earthobservatory.nasa.gov/features/UVB/uvb_radiation3.php and “How Does Latitude Affect Sunlight Intensity” https://solarenergyhub.com/how-does-latitude-affect-sunlight-intensity/ . For a detailed discussion of the pertinent calculations, see the following: https://solarsena.com/solar- declination-angle-calculator/ , https://en.wikipedia.org/wiki/Position_of_the_Sun , Mantas Grigalavicius MSc, Johan Moan, PhD, Arne Dahlback, PhD, and Asta Juzeniene, PhD “Daily, Seasonal, and Latitudinal Variations in Solar Ultraviolet A and B Radiation in Relation to Vitamin D Production and Risk for Skin Cancer.” International Journal of Dermatology 2016, 55, e23–e28

James Tanguay is a photographer, musician and science educator living in the Southwestern United States. His artistic endeavors include adapting alternative photographic printing processes for use in wet and dry plate cameras. James Tanguay can be contacted at  porphyrography@gmail.com