From: pete (temperaprint@blueyonder.co.uk)
Date: 02/12/03-02:16:45 PM Z
on Mon,10 Feb 2003, Judy wrote
>
> On Sun, 9 Feb 2003, Pete wrote:
>
>> In my opinion there are two staing processes at work here,
>> one-:the stain produced by the pigment particles themselves for instance
>> Phthalocyanine Blue can give massive stain whereas Cerulean blue does not.
>
> I take your word for your pigments, but my thalo blue is one of the
> cleanest working pigments I have. I've never used cerulean blue for a gum,
> after the first try... no covering power
Here we are looking at terminology when is a stain a tint I used the example
of phalo blue because it has a very high tinting strength. For instance in
my three colour printmaking to obtain a nearly neutral grey I have to mix
yellow one part by volume to six of egg mix,magenta one part by volume to
seven parts of egg mix however with the cyan (phalo blue ) it is one part by
volume to twelve parts of egg mix.It is obvious that as the quantities of
pigment change so will the physical viscosity of the mix.This is what I
meant in part one I am sorry my explanation was not as clear as it should
be.
>> And two-: the concentration of the pigment in mixture the more pigment mixed
>> in a given volume increases contrast and reduces emulsion sensitivity to
>> light, and also at the same time shows a corresponding reduction in the
>> apparent pigment stain
>
> What do you mean *apparent* pigment stain? Can there be an unapparent
> pigment stain? Whichever, if I follow what you're saying then you too
> agree the famous GPR test is rubbish.
Firstly of all I do not intend to discuss what you term the famous GPR test
rubbish. I have said my piece on this and do not wish to continue.
What I mean is this if a piece of paper is given a slight stain or tint. It
can be seen strongly as eye is very sensitive to light tones flesh tones in
particular> If on the other hand a large area is covered in dark tones of
grey and black the stain is not so *apparent* simultaneous contrast is the
culprit here.
>
> Though I don't know what "reduces emulsion sensitivity to light" refers
> to.... If you're saying a lot of pigment means you don't get light through
> to the base where it's got to harden to hold, so it tends to flake --
> that's what Mike Ware explained these 6 years ago & what I've been trying
> to point out since then too.
No I am not saying this. what Am saying is as the quantity of pigment
increases per standard unit of volume the emulsion gets more viscose and
because of this becomes physically thicker.
It takes light a lot longer to pass through a thick coat than it does a thin
one so what in fact can to occur is " a reduction in emulsion sensitivity to
light.
>
>> There is also the dichromate stain at work but that is another story
>
> Dichromate stain is removable... and I doubt related to gum-pigment ratio
> (especially without the dichromate).
This is a small point which I wont completely concede as I maintain that you
do not get rid of the stain just change its colour. However in certain
circumstances it can adulterate a light tint however this rarely happens and
therefore can be ignored
On Sun, 9 Feb 2003,Dave Rose wrote-;
Regarding your two points, I can't totally agree or disagree. There are too
many variables, the most important being the pigments themselves. Sorry to
be so brief. I'd be writing all night to do this subject justice.
Yes Dave so could I. Looking back at my original e-mail I think I tried to
bite off a little more than I could chew. I think this area needs a
disciplined academic approach employing scientific methods of study. At the
moment there are to many flags flying in the air.
Pete
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