Lumen printing, how?

After having read a recent thread here on APUG I got interested in lumen printing. I think I got the basic idea:
- You place an object - often a plant - on a photographic paper under a piece of glass and leave it for a significant time in the sun.
- You remove the object, wash and fix the paper. Toning is optional.

Is there anything more I should know before trying? One question. How do I know how long to expose the paper? Do I check it during the day to see how it changes, or is it trial and error?

thanks
Erik

Exposure times can vary depending on the amount of UV light and the paper you use (That I've noticed).
I would recommend toning unless you like muddy, ugly looking prints. Different toners can have differing effects with papers.
Also, when you tone a print that was exposed for 3 hours you can get different colors than a print that was exposed for 5 hours, then toned.

Experiment and have fun with it!

You can create a Photogram by placing your object on normal photographic paper, and using a timed light source expose the paper as desired. Develop and fix it as a nomal print, tone to taste. The only picture I have in the gallery is one of these.

Diffused light can create soft shadow edges, whereas a pinpoint light source can help to sharpen them up.

Thanks for your tips! It looks like experimentation is the way to go. Unfortunately summer is already over here in Sweden, but bright autumn days would also work for lumen prints.

I use a cheap UV meter that gives me a number on the UV Index and base my stuff on that. Don't know if the cheap ones are any accurate though.