Nsurit, can't speak for BFK but various pages about Johannot says: (a.) 75% cotton + 25% esparto and (b.) Acid free / *buffered*.
(a.) It's best to stay away from papers which are formed by a mixture of fibers; absorption characteristics of different fibers may differ considerably, and that may cause increased graininess. (Cyanotype et al. are extremely affected by the absorption characteristics of the substrate...) Cyanotype (by its nature) gives more grainy results than ...say... pt/pd, therefore it's best to avoid anything that may exacerbate this characteristic.
(b.) Cyanotype - and any other iron / iron-silver process - is pretty sensitive to alkali conditions; you have to avoid alkali buffered papers at all costs - if you're after best possible results, optimum longevity, with minimum frustration.
Look for pure cotton, *unbuffered / neutral pH*, *watercolor* papers. (N.B. Not printmaking papers, "watercolor" papers; printmaking papers are usually weakly sized / more absorptive compared to watercolor papers. Too much absorption may lessen the definition of the image, and weak sizing may cause handling problems - while wet processing - such as creasing or tearing, especially so with lightweight papers and - relatively - big image sizes...)
Hope this helps,
Loris.
