I’ve been thinking of making some cyanotypes after reading an article in Amateur Photographer a few weeks ago. Then someone turned up at my local camera club with some excellent examples, really nice work. I think their creator said she’d read the same article.
One thing puzzled me and that was that the process is mainly sensitive to UV light, yet a common technique is to hold things flat with a sheet of glass. I thought plain glass blocked UV so wondered if longer exposure times would be necessary when using a glass sheet. There was a brief discussion at the group about this but I was left none-the-wiser, really.
I’ve now ordered the chemicals to have a go at this and have done some research (a quick Google!) to understand this better for when I get around to creating my own cyanotypes. It turns out that normal glass blocks nearly all UVB (which causes sunburn) but allows 75% or so of UVA to pass through (you won’t get sunburn from UVA, but it can cause skin cancer). The UV spectrum consists of UVC radiation (200 nm to 280 nm), UVB radiation (280 nm to 315 nm) and UVA radiation (315 nm to 400 nm) according to a research paper I found.
The research indicates that the sensitised paper is sensitive over the UVB range, but also to UVA until you get into the visible wavelengths. This suggests that the glass does increase exposure times somewhat… it will be interesting to experiment with the cyanotype process. Now I just have to find the time to do so.