Re: Could someone summarize that gum up or down discussion?

Well, I knew it would be dangerous to try to summarize that
discussion; anyone who tried it would probably have got something wrong.

But on reflection I think I probably was too succinct in at least
one sense; I shouldn't have left out Mike Ware. He believes that
gum on paper hardens from the bottom, and his speculations about that
have had some influence here over the years. I don't know if he's
right or not; I don't find his arguments particularly persuasive, as
I've said several times, but he should be included when answering
the question, why do some believe or postulate bottom hardening for gum.

Katharine

On Apr 13, 2006, at 6:33 PM, Katharine Thayer wrote:

>
> On Apr 13, 2006, at 5:52 PM, Joe Smigiel wrote:
>
>
>
>> Could someone please *succinctly* summarize the gum hardening from
>> the
>> top down issue for me.
>>
>>
>
> Top down rules, at least for printing on transparencies.
>
>
>
>>
>> I skimmed many of the posts but I missed seeing anything that would
>> point to hardening from the bottom up.
>>
>>
>
> Judy and Chris mentioned the fact that unreacted dichromate leaves
> the print very rapidly, as an observation supporting bottom
> hardening, but I don't see bottom hardening as a necessary
> conclusion from the observation. The hardened gum matrix is a net
> rather than a solid slab, and the dichromate can easily slip
> through the net, at least that's the way it was explained to me by
> a physical chemist. So the observation that the dichromate runs out
> easily is neither here nor there as far as which way gum hardens, IMO.
>
>
>
>
>
>> I have to admit my attention
>> span isn't what it used to be and that I lost interest in reading the
>> off-topic banter but I would be interested in any novel empirical
>> results that would lead one to believe gum hardens from the bottom
>> up.
>>
>>
>
> No, you didn't miss anything; there's been nothing of that nature,
> unless I missed it too.
>
>
>
>>
>> Marek's image on transparency helps convince that gum hardens from
>> top
>> down, but isn't that image problematic by also negating the widely
>> held
>> belief that more surface roughness (e.g., Pictorico ceramic layer vs
>> smooth side) allows for a stronger or easier to print image in a
>> single
>> layer? (A post for perhaps another day...)
>>
>>
>
> I'm puzzling about that too, but I think there's more to it than
> just smooth vs. rough, because these special inkjet-prep coatings
> interact in unexpected ways with the gum coating, as I showed with
> Pictorico and Marek showed with the HP transparency.
>
>
>
>>
>> My take on this is has always been that gum hardens from the top down
>> and that is evidenced by the often observed flaking of a too
>> strongly-pigmented and underexposed emulsion layer. It comes off in
>> chunks once the unexposed emulsion beneath it dissolves in water.
>> This
>> also seems confirmed by Marek's latest image.
>>
>>
>
> Me too.
>
>
>
>> What leads others to
>> believe or postulate the opposite?
>>
>>
>
> You'll have to look at Judy's posts for her reasons she believes
> the opposite. The only observation I've made personally that makes
> me wonder about top-down hardening is tonal inversion, such as the
> image on my website where the gum layer was so underexposed that it
> ran off the paper in development, and yet there was enough hardened
> gum left behind in the paper to form a resist against stain.
>
> http://www.pacifier.com/~kthayer/html/tonalinversion.html
>
> I'm not arguing that this observation means hardening starts from
> the bottom and goes up, but I don't think it can be explained by
> top-down theory either. I don't know how it could happen if there
> weren't some hardening at the bottom from the beginning.
>
> Katharine
>
>
>
>

On Apr 13, 2006, at 9:59 PM, Katharine Thayer wrote:

>
> On Apr 13, 2006, at 8:26 PM, Christina Z. Anderson wrote:
>
>>
>>
>
>
>>> Judy and Chris mentioned the fact that unreacted dichromate
>>> leaves the print very rapidly, as an observation supporting
>>> bottom hardening, but I don't see bottom hardening as a
>>> necessary conclusion from the observation. The hardened gum
>>> matrix is a net rather than a solid slab, and the dichromate can
>>> easily slip through the net, at least that's the way it was
>>> explained to me by a physical chemist. So the observation that
>>> the dichromate runs out easily is neither here nor there as far
>>> as which way gum hardens, IMO.
>>>
>>>
>>
>>
>> Whhhhoooooaaa leave me out of this. In fact, all I said was a
>> "this is verrrrry interesting" remark when you, Katharine,
>> mentioned Mike Ware's theory of bottoms up, by remarking that
>> sometimes I observe when brushing on a layer the dichromate seems
>> to be separate and at the bottom of the brushed swathe.
>>
>>
>
>
> Chris, it was the below sequence that I was going by, not the part
> about the dichromate separating from the pigment/gum during coating
> that you refer to above. I read that sentence (below) to mean that
> you were agreeing with Judy on that point; apparently I was mistaken.
>
> Katharine
>
>
>
>
> On Apr 6, 2006, at 8:34 PM, Judy Seigel wrote:
>
>>
>> I'm going to mention two observations that suggest to me that
>> hardening in gum isn't top down.
>>
>> First is the fact that most of the unexposed dichromate seems to
>> come out of the print in the first 10 minutes (in fact some
>> manuals point that out & instruct accordingly).
>>
>
>
>
> On Apr 8, 2006, at 8:59 AM, Christina Z. Anderson wrote:
>
>>
>>
>> In reference to Judy's post, too, I find it odd that dichromate,
>> when you place an exposed print in water to develop, immediately
>> leaches out of the print in the first 30 seconds.
>>
>
>
>
>
Received on Sat Apr 15 20:02:45 2006