I've proven successfully (to myself) with outside exposures why there is such discrepancy between exposure times between different practitioners. Eli Ponsaing set me up to this--I blame him--with his 30 minute combined exposure time (computing to a ratio of 1:4 aquatint to positive with inverse square law of light). It makes perfect logical sense because of the following experience: when exposing any plate and then putting it in the water, the parts of the plate that should be hardened to paper white, even with gross overexposure, still give off some gunk in development if you wipe it with a finger. In other words, there is thickness to spare.
I've exposed plates at all kinds of different combinations of times and they've all produced good images (alas, step wedges only...with a few parts of images cut and pasted on the side). This parameter of exposure time is much less for KM73s, because they in general are a thinner coating on the plate (or perhaps the different ingredient--nylon?--responds differently) but both solarplate and KM73s respond to exposure by hardening a thickness (vertical) of a layer and hence, the ratio of time of aquatint to positive is really the critical one that has to be found out based on exposure time choice, light unit, etc.
To give an example, I exposed a solarplate for 1 hr each exposure (2 hr total) and still got blacks and whites in the image, and had gunk to spare that developed off the plate.
So the real issue is to settle on an aquatint time that gives a nice rich black and then choose the positive to run the tones up and down the step wedge scale accordingly.
Jon, comparing sun (point source) to UVBL, I am having much less problem getting a deep velvety black and have finally confirmed that the mottled areas of pale in the blacks is diffused light leaking under the substrate where incomplete contact occurs which is different than the black dots that are newton rings--one is incomplete contact, the other too complet contact, almost! BUT, I have not been getting either in the sun the last 10 plates with my quite lengthy exposures and am wondering if not enough exposure can contribute to either issue--just an hypothesis at this point (no pun intended). I definitely uphold your philosophy to stick with point source light--much less problematic. Ha ha, I've gotten such a velvety black outside that the entire image of a nude on a rock was so low key the highest highlight was about a Zone 4--and THAT with a combined 30 min exposure.
I also have found that a MUCH sturdier plate can be had with increasing exposure.
I also now understand why Dan Welden can be so loosey goosey with solarplate, doing workshops and exposing in the sun like he does. It actually works really well outside, so those without lightboxes, go try it. But buy an etching press first...
Elibeth Dove's screens are wonderful. Unfortunately (why do I NEVER learn????) I was doing some salt printing in my darkroom and splattered silver nitrate on my aquatint screen unbeknownst to me. I don't remember doing it at all. BUT I kept getting these white spots on my plates (a new phenomenon--white spots the size of a small drip) and finally checked my screen and saw these splatters. $30 down the drain. It is amazing where silver nitrate appears when you least expect it.
One and a half weeks left of school and counting!
PS Susan, I tried to check out that chine colle thing on Graphic Chemicals and couldn't seem to access it, if you remember posting this maybe a month ago...thanks for the URL...