after having found the way to do an almost perfect cyanotype (blueprint is ok - I'm going to experiment with toning now), I also started making VDB prints.
I'm using PDN system to calibrate my R2400 Epson printer and my UV exposure unit. I use Canson transparencies as negatives. Well... after many test prints, I have now a satisfactory color table scale... The problem is the pure black. It is not deep, no matter for how long I set the exposure. To make easier for you to understand, put on your monitor a 0,0,0 RGB square near a 40,40,40 gray block. The brownish black in my finished prints is even lighter...
I normally expose my VDB for 6'. Have tried different papers. I always expose papers when they're bone dry (I dry them with an hairdryer at cool setting)
What it could be? Developing perhaps?
I develop my prints this way
1) 90" bath in water acidified with winegar (ph <7)
2) 30" in 3% sodium iposolphyte fixer
3) 30" in water acidified with acetic acid (stop bath)
4) 90" in 1% sodium sulphyte (hypo-clearing bath)
5) wash for 25'
prolonged or reduced fixing bath won't accomplish better results...
I don't think there's something wrong in the PDN digital calibrated negative... Even with a Stouffer exposure scale or a paper exposed without any negative on it the black will remain the same.
I'm going literally nuts...