Our tap water is indeed around ph 9.5 - very alkaline. The addition of mild
acid in the wash does help the Cyanotype prints from fading during the
wash. All it takes is a few drops of stop bath in a tray of water. The
prints are technically "soaked" and not "washed" in several changes of this
slightly acidic water (Those of us who laundered our Blue-Printed T-shirts
can quickly tell you what alkaline detergent does to the blue).
The 3.5 ph (or there abouts) is the acidity of the Cyanotype solution. As
Sil mentioned, it is extremely acidic (acid in our stomach is around ph 3).
Which explains the fact that adding acid to the basic formula will make
hardly any difference.
Sam
@ the foothills of the Great Smokies Mountains (for Risa)