Pontillistic effects (was: Re: Fresson's beauty )

[Katharine Thayer <kthayer@pacifier.com>]

Peter Marshall wrote:
>Ive often looked at colour Fressons and thought how similar they were to
> some experiments I did years back using 4 colour gum applied using random
> halftone screens (made by printing photographic grain onto high contrast
> film), which created a similar pointillistic effect, although somewhat
> coarser as the halftone screens I made were rather coarse.
> 
> 


That's interesting. I've never been interested in halftones, but I once
experimented with darkening the sky area on a group  of paper negatives
(tricolor separations) with a chunk of graphite. As I recall, it was a
desperate attempt to disguise some banding I was getting on digital
negatives. Anyway I got a similar broken effect with the
color all separated into bits in the final tricolor gum print. It wasn't
an effect that pleased me and I didn't show that piece but kept it with
a stack of things I've used in demonstrations and talks, to show the
different unusual effects you can get with gum if you want to.

It's always puzzled me why some autochromes have that pointillist,
broken color effect and why most don't. Does anyone know what causes it
-- does it have something to do with how thickly the starch grains are
applied or what? 
Katharine Thayer