And it seems that Sam is not alone in encountering difficulties with Vandyke. 
Maybe it isn't quite so easy!  I too would love to hear from other Vandykers 
out there. 
I am getting very uneven Vandyke results, even after conscientiously and 
meticulously processing each piece of paper. Well, so I thought. That is, I did 
my processing following some 6 or so published sources. 
> One of the difficulties I've encountered is paper choice. Some papers just
> don't work well with Vandyke. Cranes Kid Finish works well, while Cranes
> Resume does not.
I have no access to Cranes papers but I recently did a huge batch of Vandyke 
and cyanotype prints on Fabriano artistico, with some very satisfactory results 
(if I may say so myself, some of the images worked very well) but with a huge 
Vandyke failure rate too. Many prints yellowed quickly for no reason that I 
could figure. And I can't pinpoint the problem for the life of me.
Judy also raised a question similar to Sam's, in her assessmnet of "Ansel 
Adams'" gum printing. There is a lot of untested published literature out 
there. But also as someone else pointed out, (Wayde Allen I think) the 
variables are rather overwelming! And so called scientific testing is not 
without its own problems. 
This summer (yes, northerners, we've already had ours) I got fabulous rich 
blues with the old style cyanotype (on Fabriano artistico). I was really 
thrilled with the results (at least something went right in the face of so many 
failed brown images). That blue was obtained in close to 100 percent humidity, 
and in temperatures of around 30 degress centigrade (that's Sydney in February 
for you). I am now therefore convinced that high humidity is great for old 
fashioned recipe cyanotype. But maybe the humidity and heat was what made my 
Vandyke results so unreliable... And when I get round to trying Dick's Ziatype 
(finally), somehow I will have to de-humidify my workroom. It seems that many 
on this list have the opposite problem. 
As a further query, has anyone got any thoughts on if the (unexposed) Vandyke 
solution coated onto paper, deteriorates if left overnight, or longer?
 
I think that I am going to have to go back to the drawing board, as they say.
I've got another Vandyke question but I'll save that for another post.
Cheers
Catherine