Zinc white

Loris Medici wrote, last Thursday:

"Errr... This is bad news - especially when reading it while holding the
guache white paint I purchased to make a one coat gum on black Fabriano
cardboard (using a positive). Because the label says PW6/PW5 (a mixture
of
"Titanium Dioxide" and "Coprecipitate of zinc sulphide and barium
sulphate")
I hope PW5 is different from the zinc white you're referring to (could
your
"zinc white" be PW4?)."

Loris,
I apologize for not getting to this sooner, and also for not following
my own usual rule of using pigment numbers to identify a pigment,
although I don't always do that when the pigment is straightforward and
can't be easily confused with something else. I thought zinc white was
straightforward in this way; it's zinc oxide, PW4, which is clearly
distinguishable from the other white, titanium white (PW6) that is
commonly used in good watercolor lines, although your note shows me that
it's not so straightforward as I thought. But anyway, yes, it's PW4 that
I'm talking about.

It took me a while to find any information about this PW5 that's in your
paint; it's not listed in any of my best sources as a pigment that's
used in watercolors. I finally found it listed as "PW5. Lithopone. A
semi-opaque to opaque white, widely used as an extender or filler. Mixed
precipitate of PW7, Zinc Sulphide, PW22 and Barytes."

So I looked up PW7 Zinc Sulphide, which is also not listed as a pigment
used in good watercolor paints. The source says this pigment "reflects
in the ultraviolet"

PW22 Baryte is listed as "a heavy white to greyish white powder obtained
from the mineral baryte. Used extensively as a filler for cheaper
paints... rarely used alone as a pigment."

One might suspect that they've extended the titanium white with this
other because the titanium is more expensive; If it were me I'd want to
use straight PW6 and not mess around with these filler whites, but
that's just me,
 
Katharine

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, June 17, 2004 2:11 PM
Subject: Re: dichromate stain

> Sorry, another clarification:
>
> Katharine Thayer wrote:
> >
>
> >
> > (2) One thing I learned yesterday is that zinc white gouache in two
> > different brands with two different gums caused dichromate stain every
> > time. I also learned from handprint.com that zinc white blocks UV below
> > 370. So I guess I would say that this is the first pigment that I've
> > found, at least in gouache formulations, that probably doesn't work very
> > well for gum.
Received on Wed Jun 23 11:15:32 2004