It used to be good, but they changed their formula apparently and now it’s buffered and needs to be acidified. I didn’t want to go through the hassle so I returned a batch of the Stonehenge to B&S a few years ago. They told me I could use an oxalic acid bath, and I’m sure if you search the web you’d find a formula.
However, an authoritative source is Christina Anderson’s book “Cyanotype,” and she recommends sulfamic acid (available at any hardware store as heavy duty cleaner crystals, apparently) . Her formula is as follows for a 10% solution, but she says others use anywhere from 1%-5%:
- 100g sulfamic acid to 1000ml water
- Add crystals to 750ml to start (crystals to water, not the other way around) and stir
- Add remaining water to make 1000ml.
- Keep stirring until dissolved, which will take a while
- Pre-soak paper in plain water for 20-30 minutes
- Transfer the paper to the acid bath for 15-20 minutes, interleaving sheets every few minutes
- Once fizzing stops, transfer paper to a water bath and wash for 20-30 minutes or longer until it is cleared completely of the acid.
The acid is caustic so wear gloves and eye pro when you do this.