Re: gloy for tricolor on yupo?

On Mar 30, 2006, at 12:39 PM, Loris Medici wrote:

> I assumed development of each layer will take 30 minutes. (I had
> developed my tests on paper for 1 hour - I expose "a la Christina"; at
> least 6 minutes - making relatively sturdy layers...) How do you
> develop
> so quickly? (Hot water, abrading w/ paint pad or brush, water
> jets...???)
>

Sorry, that was a mental lapse; I was thinking of something else when
I wrote that. I was thinking about a project I'd done that I
described here recently, where I was coating yupo with pigmented
dichromated gum, and doing two layers from coating of the first
layer through final dry of the second layer in less than 15 minutes.
BUT, the crucial thing there was that it didn't involve an image;
there was only one value: DMax (for that particular pigment mix) so
there was nothing to develop, and it only had to be in the water long
enough to wash out the remaining unreacted dichromated, which really
only took a couple or three vigorous swishes through the water. The
gum layer was totally hardened after a few seconds in direct sun, so
there wasn't any soluble gum/pigment left in the layer to be
developed out. So, sorry, bad example. Yes, when you figure in
developing an image, it would be more like an hour and 45 minutes
than 30 minutes to make a tricolor gum on yupo. Like I said, a slight
brain malfunction. sorry.

But I must take some mild issue with your statement "I expose ala
Christina, at least 6 minutes-- making relatively sturdy layers." The
exposure time required to make "relatively sturdy layers" is a
function of so many things (light source, negative, type of
dichromate and concentration thereof, and relative humidity, for
starters) that it is simply meaningless to state an absolute exposure
time that will result in "relatively sturdy layers."

  My exposure times are almost never more than 2 or 3 minutes, but my
gum layers are extremely sturdy; I've demonstrated this by pouring
boiling water on them from a height without disturbing the hardened
gum. But I use saturated ammonium dichromate and I live in a very
humid climate, which together probably account for most of the fact
that I can print a very sturdy layer in a short time. If I were
using potassium dichromate, or diluted ammonium dichromate, or if I
were living in a drier climate, exposure times to make a sturdy layer
would be longer (and in fact my times are longer during rare dry
spells here. So, like I say, a statement to the effect "I expose x
minutes, making a relatively sturdy gum layer" is a meaningless
statement. With some lights, with some dichromates, in some
environments, 6 minutes would cook the emulsion to a fare-thee-well,
while with a very different combination, 6 minutes wouldn't be nearly
enough to create a sturdy gum layer, and 20 minutes might more like
it. So, it all depends.

>
> I don't see why dichromated PVA wouldn't adhere to Yupo - I mean when
> thinking it is used to "glue" things. (Bookbinding, package
> manufacturing, self adhesive labels...) Of course I'm speculating
> assuming PVA will be more adhesive than (or equally adhesive as) whole
> egg (read as temperapring - I know temperaprinters can make as much
> as 9
> layers).
>
>

Loris, I didn't ask if you thought it should work, I asked if you
knew anyone who had done it. But I think maybe a couple of things are
being confused here. As to coating, I can coat yupo with gum easily;
the issue for me with gum and yupo isn't the adhesion of the coating
to the yupo but the adhesion of the hardened gum to the yupo, which
is a completely different issue. (Note: I'm talking about a normal
gum coat, not the extremely thin dry coats that I recently discovered
adhere very well to yupo through development but are difficult to
coat smoothly. Whereas a normal gum coat, in my experience, can be
coated quite smoothly but won't stick to the yupo through development).
Katharine
Received on Thu Mar 30 18:44:31 2006