OK, I finally got around to popping my fumed silica cherry last weekend. My views follow:
I used the wet version as supplied by Bostick + Sullivan.
I first printed an image on Arches Platine (the paper I am using somewhat reluctantly for a large ongoing project) without using any fumed silica. Once I got an acceptable image I coated another piece with fumed silica by way of a coating rod (same as I coat sensitiser). As a starting point I used 12 drops of fumed silica over an area about 6x8 inches (for neg size 5x7).
The coating went fairly well, except that I couldn't see anything. By looking at an angle I was able to determine what was wet and what was not. Three or four passes max and it looked like a good even coating. The paper was let sit for a few minutes then dried with a cool blow dryer.
I then coated and exposed the paper exactly as for the previous print. As per expectations I observed an increase in Dmax, a slight overall contrast increase (though less than I had expected). So it all seemed good. Who wouldn't want more Dmax with Platine, although I agree with someone else's comment (can't remember whose) about the extra highlight contrast being undesirable in some images. I did notice some of what I think Loris is referring to when she says "raised nap", although I've always found this to a degree with Platine and sometimes 'spot it' in darker areas.
I then tried another negative (4x5) and got very similare results although I noticed that an uneven coating of fumed silica results in patches or streaks of uneven Dmax, which looks crap.
I then tried another paper that a friend had been told worked well for platinum. He had never tried it so I took one for the team. Strathmore 500 series plate finish. Quite thin, about 120-130gsm I imagine. Not quite as white as platine but with a smoother finish. The first sheet creased from being too wet. I halved the drop count and coated again.
This is where it gets interesting, and frustrating. The first two prints, barring a few small areas of patchy Dmax, were stunning. To the point where I instantly had to consider re-printing my 4 year project. The Dmax was as good as platine or better, low separation and mid-tone contrast were better, and the image was sharper with 0 raised nap. It made the print on platine look quite ordinary.
Then, to overcome the patchiness in the fumed silica coat my friend suggested we coat the whole sheet. We did so, and it did not crease too badly as it was all wet, rather than a wet rectangle in an otherwise dry sheet.
This print looked terrible. There was barely a latent image, and when developed it looked flat and underexposed by about 3 stops. I figured I must have stuffed up the mix and tried again.
The next three prints were all the same, flat, pale and lifeless. Was it coating the whole sheet? One last print where I coated just around the image area as before. Same result, well slightly better see attached image. The earlier print is on the right, the problem print on the left. Same mix, same chemistry, same exposure unit and time, same developer. It's got me buggered. Since its dry the dud print also has a distinctly powdery feel to it which is absent on the other prints that worked. My apologies for the crappy phone pics, my scanner is packed away due to renovations.
I didn't get time to re-try with platine to check my process and I'm dumbfounded as to what happened or what changed. If it were repeatable the results I got on the Strathmore paper with fumed silica would force to consider re-printing my whole show.
Jon
