Siderotype processes using ammonium ferric oxalate as the sensitizer, such as New Cyanotype, New Chrysotype, and Malde-Ware platinum-palladium, are reported -on occasion- to show a defect called “blotching or splotching”, in which irregular pale patches give a “fibrous, woolly, or cloudy” appearance to the image pigment. This is not due to crystallization, but is attributable to some non-uniformity in the paper sheet being disclosed and visually amplified by the sensitive photochemistry of these printing-out processes, which give very fine tonal separation.
In paper manufacturing today it has become almost universal practice to add so-called “retention aids” to the pulp at the wet end of the mill. These additives are likely to cause image “blotching” because they are highly polymeric organic polyelectrolytes, such as cationic polyacrylamides (CPAM), or cationic starch: they consist of long polymer chains bearing many positively-charged groups that will exert a strong attraction on the small, mobile, negatively-charged anions present in ammonium ferric oxalate photochemistry. These anions are liable to be trapped and rendered less reactive by local, non-uniform concentrations of the huge cationic polymer retention aid, thus wreaking havoc with our sensitive printed-out images.
Retention aids will interact much less with the sensitizers of traditional development siderotypes, like ferric oxalate or ferric ammonium citrate, whose larger molecules carry little ionic charge; this explains why the historic formulae like Classic Cyanotype and development Platinotype don’t suffer from blotching.
Although the makers of papers for siderotype now publicise the importance of excluding alkaline buffers like chalk from their product, they do not tell us whether they still include retention aids. It would be useful to know this. The handmade ‘Buxton’ paper that I commissioned from Ruscombe Mill in 1992 (specifically without chalk or retention aids) was used for 20 years with my ammonium ferric oxalate processes without ever showing a trace of blotching.
For anyone interested, I’ll attach a 4-page pdf which sets out the arguments in detail.