Terry - I wish I could say what reliably gave the intense blue!
As I've only been making these for a month, sporadically, I've a lot to learn about the variables, although there is no doubt hydrogen peroxide and acidic washing are significant factors.
One tip, be careful if you use white vinegar at 10% dilution- I tried this and watched as the prussian blue colour blissfully drifted off the surface of the print! Try 1 or 2 %.
The most intense blue has only occurred a few times and I was very lax at keeping accurate notes of the conditions and processing during my earlier attempts.
The UV light source has now eliminated the vagaries of daylight exposure, so that's one variable sorted.
The paper I've used so far has been Daler Aquafine Watercolour 250gsm and most recently Scribble & Dot Cold Press Watercolour Premium 300gsm (
https://www.scribbleanddot.com/produ...atercolour-pad).
Both are textured more one side than the other. I've also tried ordinary thick smooth paper, but its light weight makes it trickier to handle (tends to curl).
My chemicals are the standard 'beginner's mix' - potassium ferricyanide 10g in 100ml, ammonium ferric citrate, 25g in 100ml. Distilled water used in both cases. Mixed 1+1 just prior to coating with a foam brush. The mix I used today was made up on October 11th, so 1 month old.
My UV light is 50W, and placed about 40-50cm away from the contact frame is giving an exposure of 5 minutes with a digital negative on Pictorico film.
The first one shown here was made today, as I said, chemicals were around 2 months old, mixed and coated about three days ago. Paper is the Scribble & Dot stuff. Negative was a translated digital file printed onto Pictorico using a standard inkjet printer (Canon - pigment ink).
Contact exposure 5 minutes, developed in a dish with 1gm of citric acid added to 1000ml tap water, then washed under cold tap water briefly before soaking in a hydrogen peroxide bath, around 5ml of 9% H2O2 to 1000ml tap water. Washed again, briefly, as I know that continued washing in my hard water will start to bleach the print. The paper seems not to hold much chemical residue, but only time will tell. It's a bit light, but ok. I expect it to darken more over the next few days, but it's not the intense blue I got with...the second example...
This pineapple still life was the first cyanotype I made. Daler paper, same chemical mix, freshly made back then, coated with an old kitchen sponge! (Maybe that's the secret?
).
It's quite dark, exposure was in sunlight from a half-plate silver negative shot on my Gandolfi. The silver image is much denser than you get with inkjet and I think the exposure was 20 minutes in weak sun. It needed more.
The blue is as intense as the one shown previously of the bird stone ornament - also a silver negative, also fresh chemicals.
I am mixing a new batch tomorrow and will coat and use some paper immediately to see if freshness helps.
I do have plenty of failures and the third image shows the blue variation of four prints made with the same paper (Daler), the same mix, at the same time, with different exposures trying to get the image to print well (...so far I have failed to). The darkest blue is notably using the reverse of the paper, which has a different texture. Perhaps the paper absorption is different?
At least watercolour paper is
slightly cheaper than photographic paper!
Endless fun ahead...