Cyanotype exposure time: sun, UV lamp and test strips

The correct cyanotype exposure time is not a fixed number. Sun strength, season, cloud cover, UV lamp, distance, paper, coating and negative together determine how much UV reaches the light-sensitive layer. Therefore, use times as a starting point and always make a test strip. This way you can find a reproducible setting within one session.

Practical starting times

The values below are safe starting areas, not guarantees:

• Bright summer sun: often about 3–8 minutes.
• Mild or partly cloudy sunlight: often about 8–20 minutes.
• Low winter sun or heavy cloud cover: 20 minutes or longer may be necessary.
• UV lamp or light box: often 5–20 minutes, depending on wavelength, power and distance.

A dark negative film or thick contact plate can further extend the time. Keep the first test small, because a complete print only tells you that one time worked or didn't work.

Why sunlight varies so much

The eye sees clarity, but cyanotype responds mainly to UV. Two days may seem equally light, while the UV intensity varies greatly. The sun is lower in winter, clouds filter UV and a window blocks some of the radiation. Therefore, “ten minutes” is not a universal recipe.

If you work outside, record the time, weather and UV index, if applicable. If you use a lamp, note the brand or type, distance to the print and warm-up time. Repeat the same setup before adding new variables.

This is how you make a test strip

1. Coat a strip of the same paper as the final print.
2. Let the strip dry at the same time as the real print.
3. Place the same type of object or a piece of the negative on top.
4. Cover most of the strip with opaque cardboard.
5. For example, move the cardboard an equal distance every 2 minutes.
6. Rinse the entire strip at a time and let it dry.

Choose the box with the best balance of deep blue, clean highlights and visible detail. Record the total time of that subject. With a fixed UV lamp you can often use this reference for a long time; Retesting remains sensible in sunlight.

What does the color reveal during lighting?

The coating changes from yellow-green to gray-green, bronze or dark during good lighting. Slight solarization appears on some materials. That change is an indication, but not a perfect meter: paper, recipe and humidity influence the appearance. Base your final choice on a rinsed and dried test.

Recognizing underexposure

• The image becomes much brighter or partially disappears during rinsing.
• Dark areas remain pale and lack depth.
• Halftones are weak or blend into each other.
• The print has little contrast after drying.

Increase time in small increments. Also check that the coating was completely dry and that the paper absorbs the solution evenly.

Recognizing overexposure

• Light details turn blue when they should have remained bright.
• Fine lines in a negative are closed.
• The print appears flat because different tones coincide.

Shorten the time or use a negative with more density. In a botanical photogram, semi-transparent material can deliberately produce blue or gray tones; that's not always a mistake.

Illuminate with a UV lamp

A lamp makes results more repeatable than the sun. Keep the distance constant, use a timer and prevent the setup from heating up. A small change in distance can noticeably change the intensity. Make a test strip again when using a different lamp, contact plate, paper type or negative.

Plexiglass can be practical as a light contact plate, but not every plastic transmits the same amount of UV. Read more about plexiglass, thickness and UV transmittance .

Paper, textiles and negatives

Smooth paper provides good contact with digital negatives; rough paper sometimes takes more time and is more likely to lose small details. Textiles absorb differently and can dry unevenly. For fabric, the step-by-step guide to cyanotype on textiles will help. For paper, see the paper selection and testing guide .

Frequently asked questions

Can you expose cyanotype through a window?

That is possible, but many types of glass filter UV and extend the time. A test outside and behind the window shows how much difference your glazing makes.

Should a UV lamp be exactly 365 or 395 nm?

Both ranges can be useful, but power, distance and spectrum vary per lamp. Base settings on your own test instead of just the number on the package.

Can I re-expose an underexposed print?

Only before rinsing, as long as the composition remains exactly in place. After rinsing, the unused chemistry is removed and you cannot simply continue to expose the same coating.

Do you still have a pale, spotty or too dark print? Then use the complete cyanotype troubleshooter .