<< I carefully weighed it: I use .25 gram (that's a
quarter of a gram) of ammonium dichromate in each 10 cc of working gum
solution (I dilute the gum with equal parts of water first, add pigment,
then add the powder dichromate). So, the dichromate concentration is .25
divided by 10 = .025, or 2.5%.
I use pot. dichromate, which I believe saturates at 12-14%. Using the same
amount of dicrhomate and gum, the concentration of dichromate in the final mix
is about 6-7%, still higher than your figure, but at least not as drastically
as compared to Crawford's number.
I did find, however, when I reduce the dichromate concentration to 10%, I did
lose some speed.