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Anthotype Exposure Tracking: A Sunlight Log for Plant Pigment Prints
Turn a plant pigment fading test into a repeatable routine: log sunlight minutes, UV index and fade rate so anthotype exposure time stays comparable.
What to take away
- Anthotype exposure time only becomes comparable when minutes, peak UV index, and cloud cover are recorded for every session.
- A control patch scanned beside each exposed strip converts fading into a percentage you can plot.
- UV index forecasts from the National Weather Service or Environment Canada let you plan a session instead of guessing at the sky.
- Cumulative UV dose, not calendar days, is what makes a June print comparable to an October print.
Why an anthotype sunlight log beats memory
An anthotype fades in ultraviolet light and keeps fading after you lift it out of the sun. A cyanotype wash stops development, but an anthotype has no such step, so timing and dark storage do the work.
Full sun in Miami and full sun in Seattle are different exposures. Without numbers, a strong July print teaches you nothing about November.
Sun printing covers a family of contact processes, and the fields below work for any pigment you choose. Anthotype printing uses a plant emulsion that bleaches under ultraviolet light, as the overview of the anthotype process describes. Track the light, and the comparison becomes repeatable.
Fields to record in an anthotype sunlight log
| Field | What to write | Why it matters |
|---|---|---|
| Session date | Day, month, year | Groups results by season |
| Exposure window | Start and stop clock time | Bounds the actual test |
| Total minutes | Sum of exposure time | The number you compare |
| Peak UV index | Highest forecast or meter value | Explains fast and slow days |
| Cloud cover | Oktas from 0 to 8 | Flags hazy sessions |
| Pigment and mordant | Plant, plant part, mordant ratio | Ties fading to one recipe |
| Control reading | Pixel value of the unexposed patch | Sets the fade baseline |
The UV index runs from 0 to 11 and higher, and a value of 8 or above will bleach a thin coating far faster than a value of 3. Write the number down even when you work under a hazy sky.
Example: one line from a beetroot exposure log
A completed line might read: June 11, strip B3, beetroot on hot press paper. Exposure ran 9:45 a.m. to 3:15 p.m., 330 minutes, peak UV index 8, cloud 2 oktas. Control patch 198, exposed strip 171, fade 14 percent.
The next week the same recipe might sit at 15 percent fade after only 200 minutes, because the UV index climbed to 10. The practical detail is set out in Anthotype Prints Without Spinach Testing Everyday.
Step by step: running a daily exposure session
- Coat and dry the test strip the evening before, then store it in a light tight envelope.
- Load the strip into a contact frame with the plant material or a step wedge negative.
- Record the start time and the day's peak UV index before you set the frame outdoors.
- Check the strip every 30 minutes after the first two hours and note the first visible change.
- Pull the strip, write the stop time, and scan it within ten minutes next to its control patch.
- Sleeve the strip in the dark logbook and record the fade percentage on the same line.
A session without a UV index reading is a session you cannot compare with any other day.
Reading fade rate against a fixed patch
Scan at fixed settings every time: 300 dpi, no auto exposure, TIFF output. Sample a one centimeter square of the coated area in free software such as ImageJ or GIMP, and record the average channel value.
Then compare that value with the control patch, which lives in a dark sleeve and never sees sunlight. Divide the difference by the control value and multiply by 100. That gives fade percent.
Paper, coating thickness, and drying time change the starting value, so keep them constant across a test series. Score each pigment on a fixed patch before you commit a full sheet, using the Anthotype Pigment Testing logbook method.
Keeping weeks and seasons comparable
Multiply the session UV index by the exposure minutes to get a rough dose. A 300 minute session at UV 8 and a 400 minute session at UV 6 land near the same dose, so the strips can be judged side by side.
Compare pigments at the dose where each one reaches the same fade percentage, not at the same number of days. Latitude and solar altitude shift that dose through the year, and a shaded porch drops it again. Note the site on every line.
Store finished prints flat in a dark box. Plant emulsions continue to shift in room light, and a logged print is easier to reprint than a remembered one.
