Lesson in lightfast ratings Re: Problem pigment

From: Katharine Thayer ^lt;kthayer@pacifier.com>
Date: 06/04/05-04:06:08 AM Z
Message-id: <42A17CFA.6C3A@pacifier.com>

Katharine Thayer wrote:

> The people I'm worried about are those who for instance look at Winsor &
> Newton's "moderately durable" rating, which is the lowest possible
> rating a fugitive pigment can get in their rating system, and think
> that means that somehow the pigment made by W&N is more durable (less
> fugitive) than the same pigment made by Daniel Smith or Graham and rated
> more honestly by those manufacturers as IV (fugitive). All it really
> means (the different ratings) is that Winsor & Newton have just made
> their pigments look more "durable" by dropping the "fugitive" rating and
> forcing fugitive pigments into more permanent-sounding categories. It's
> like grade inflation-- just because no one's getting a D any more
> doesn't mean that no one is doing D work.

Yes, I did my Ph.D. dissertation on rating scales, so I'm probably more
attuned to these nuances than most, but I wouldn't think you'd need to
have a Ph.D. in statistics to suspect that when a company takes the
lightfastness rating scale, which for the ASTM has five points, and for
most responsible manufacturers has four points, and that company makes
it a three-point scale, and then further aggravates the offense by
labeling the points AA, A, and B instead of I, II, III, that perhaps
they're up to something?. (People think "B, well, heck, that's a good
grade, right?") I don't have a Winsor & Newton chart right here (Eric,
can I borrow yours for a minute?) so I can't say how the AA and B points
relate to the III and IV rating, but as I'm remembering it, it seems
like only the colors other manufacturers rate IV ended up in B. If
that's the case, then the A ("durable") category must include colors
that most manufacturers rate as "fair" or III and if that's so then it
seems that even AA probably includes some II pigments rather than
designating only perfectly lightfast (I) pigments.

It's all about manipulating perception, and I've seen enough confidence
expressed in the W&N ratings ("W&N says it's 'moderately durable,' and
that's good enough for me") to be convinced that the manipulation has
been altogether successful. In fact it's been so successful that many
experts have come to believe as a truism that lightfastness of a
particular pigment depends on the manufacturer; the same pigment from a
better manufacturer will be more lightfast.

To some very small extent there's something to that, but not the way
many people think. The example I gave the other day about PR 122 is a
good illustration here. Bruce MacEvoy tested 9 single-pigment PR 122
paints and found that 8 of them rated 7,8 on an 8-point scale (with 8
being most lightfast). But the Sennelier paint rated only 4,5. So every
now and then one manufacturer's paint will be significantly less
lightfast than most paints made of that pigment. But in no case (and
I've scrolled clear through handprint.com to be sure of this) does one
manufacturer rate significantly HIGHER than the rest. And when looking
at the slight differences between ratings for a given pigment, there's
no one manufacturer that's always got the highest of similar ratings.

For most pigments, the thing that decides the lightfastness is the
pigment itself, contrary to widespread belief in the idea that
lightfastness varies widely depending on the manufacturer. (Noting the
one exception already mentioned, that sometimes there is one paint that
is clearly not as lightfast as most paints made with that pigment). For
example, (using MacEvoy's ratings because they are handy, and because I
trust them) let's take the pigment alizarin crimson, PR 83. Ratings for
this pigment are dismal across all manufacturers. In a tint, all
manufacturers' PR 83 paints score 1 except, oddly, Rembrandt, which gets
a 2. In masstone, which is less useful for us because we can't get our
paints that thick, but just for the purpose of the exercise here, in
masstone there is more variation; the ratings run from 3 to 6. The 6's
are M. Graham, Rembrandt, and Daniel Smith. So the idea that many people
hold onto, that Winsor & Newton's PR 83 is more durable than Daniel
Smith's because Winsor & Newton *calls* their PR 83 "moderately durable"
and Daniel Smith calls theirs "fugitive" is simply wishful thinking.

On the other hand, for PV 19 beta (quinacridone violet) all
manufacturers' paints score 8,8. For PV19 gamma, there is slightly more
variation, but all paints score at least 6,7. And again, the paints that
score 8,8 (M. Graham, Maimeri Blu, Daniel Smith and Schminke) do not
include Winsor & Newton.

I could go on with more examples, but I think that's probably enough to
make the point. In other words, the idea I've heard asserted many times,
that the reason Winsor & newton's ratings of their own paints are higher
than other manufacturer's ratings of their paints is that Winsor &
Newton's paints are in fact better and more lightfast, is simply not
Katharine Thayer
Received on Sat Jun 4 19:40:19 2005

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