From: J. Wayde Allen (wallen@lug.boulder.co.us)
Date: 11/06/00-02:50:00 PM Z
On Mon, 6 Nov 2000, Judy Seigel wrote:
> Wayde, a good teacher doesn't say you are a stupid doodyhead. She softens
> the correction or deflects it, as with the "editorial" we.
Fair enough! I guess I missed that, but we obviously are on the same
page.
> But another similarity is that both use gelatin -- the carbon printing
> incorporates it with the pigment, gum printing (usually) has it as the
> paper size. (The particular gelatin used can be an *enormous* variable in
> gum printing, perhaps not in carbon.)
Yes, gelatin is usually used as a sizing agent in both cases, although
there are other sizes too. Sure it is all a variable.
> My sense of the situation, incidentally, is that the *mechanical*
> differences cause large chemical differences, and I mean within a given
> medium, too -- as a change in method often changes results beyond what
> seems "logical."
Mechanical differences don't change the chemistry, only the mechanics.
What it does do is change the "boundary conditions" that Mr. Mathias
alludes to in his post, but that is a whole other can of worms.
For example, there seems to be very little bonding energy between gelatin
and Plexiglas. That is a chemical attribute that has a mechanical
manifestation. As such, we can use the Plexiglas as a temporary support
since the developed image can be separated from the surface once it
dries. We can use this chemical effect to our advantage for the mechanics
of the process.
> Exactly. But the sum total of *ingredients* much the same. (Unless Sandy &
> co were just making that up.)
As usual, it sounds like we are in pretty violent agreement on this
point. For comparison you have:
For Carbon: For Gum:
gelatin Gum Arabic
pigment pigment
glycerine
sugar
dichromate dichromate
The differences between the Gum Arabic and gelatin are so great, that on
this point alone we're not talking about the same process.
Of course we all tend to print our alt processes on paper that may or may
not be sized with gelatin. In that sense all the alt processes are
similar.
> Firstly, explain to the backward what you mean by "highly correlated
> variables."
Say you have two variables A and B. The combination of which results in
some result Y. As long as the value of either of these variables
doesn't change the how they each affect Y one-variable-at-a-time testing
is fine. One would say that the variables are independent.
The problem I'm talking about occurs when the effect that variable A has
on Y depends on B. Such variables are said to be highly correlated. In
this case, holding one variable constant and changing only the other one
doesn't give you any information about this interaction. In this case, if
you hold variable B constant and vary A, what you see is only how A
affects Y at this particular concentration of B. If B changes, your test
data is no longer valid.
> In gum printing, changing more than one variable at a time may
> give you a good printing method, but little information about more than
> that exact combination. If we ("we") keep all conditions the same and
> change one variable, say the pigment, we ("we") at least see the changes
> wrought by that pigment in the given combo....
That is correct. I've never disagreed with you or anyone else on this
point.
> Pigment character changes with the environment, so it isn't clear why you
> scorn one-variable-at-a-time. It's not crucial to a beginner, who needs to
> find a general modus operandi-- but for fine tuning & control, also
> trouble shooting, we (I) don't see how, or why, you dismiss it.
I don't specifically scorn one-variable-at-a-time testing. It certainly
has its place. However, this group quite often laments how these
processes seem fickle. Many of the posts also deal with processes that
were working and somehow went out of control. My reason for bringing this
up is that one-variable-at-a-time testing in processes with known
inter-variable correlation will always create this problem. I'm not
saying it is bad, but rather just a fact of life. I think that the group
should know about the limitations of the one-variable-at-a-time testing
method. I also think you need to know that there are very good ways to
get around these limitations. In other words, you don't have to live with
the limitations of one-variable-at-a-time testing.
- Wayde
(wallen@lug.boulder.co.us)
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