Re: Gum variables
Hamish Stewart & Sophie Colmont (Hamish.Sophie@wanadoo.fr)
Wed, 17 Jun 1998 07:13:30 +0200
Dave wrote..
>
>Rounding the dichromate concentration to 30% (for easier discussion), the 2:1
>mix has 20% dichromate, and the 1:1 mix has 15% dichromate. Both 20% and 15%
>are pretty high concentration and more than enough to harden the gum, so I
>suspect that that is that is not the reason why contrast change. The
>reason is
>more because of the dilution of the whole mix.
>
>Now think about it, when you have a diluted mix, if you brush it on normally,
>you have a thinner coat on the paper. If you try to imagine and compare the
>thinner coat and the thicker coats in your mind (like a mind microscope), you
>can actually know that the 2:1 will indeed give you a lower contrast.
>
Thanks for this input, but just to further raise questions, I know from
experience that a coating with more pigment (tube watercolour) is
thinner, but if the dichromate concentration is higher the contrast is
lower and higher if there is less dichromate (the 2:1 or 1:1 dilutions).
But the thinning of the coating, or the lowering of the gum viscosity,
can perhaps be making the coating more susceptible to staining, which
will lower contrast. Sam's method interests me, I would like to try using
less dichromate.
Hamish