Pigments have a colour because they reflect and refract light in a,
for the pigment, specific way in the (for humans) visibe wavelenghts. For
instance, sticking to photograppy, the brown colour of let's say salt
prints and POP papers is explained by the fact that the (metallic) silver
particles that make the image are so small that they refract (shift) the
visible light to the red end of the colour spectrum. DOP (developed) silver
images on the other hand, by use of a developer, build around a small
silver particle a conglomerate of silver. Because of the form (rectangular
coiled ribbons) they do absorb the light and look therefore black. In short
this goes for almost all pigments. So particle size has something to do
with colour. Reds for instance should clear more easily because red
pigments tend to have a larger particle size than blue ones (the two
opposites of the colour spectrum). Since I am not a colour expert nor a
chemist, physicist I could not tell you if it would be possible for
instance to grind a red piment so small that it would become blue. (but it
do not think so, better still this is not the case..). Maybe one can find a
clue on the colour pages that are linked from CoOL,
http://palimpsest.stanford.edu.
Particle size can be measured by using a standardised block of
stainless steel that has a =B1 1cm wide groove cut in it, This groove is
"deep" at one end (=B11 mm) and becomes less deep to zero in about 15 cm.
Putting the choosen paint or whatever in the deepest point, by use of a
scraper it is spread along the groove. As long the pigment particles are
embedded in the paint medium no scratches are formed on the surface of the
paint. The moment the surfaces shows small parallel "scratches" the pigment
sticks through the paint medium and reveales its size. This can be read at
the side of the groove. In this way particle size from 1 mm to very small
sizes can be easily and accurate determined. Doubtless there are some
standardized tests for this, (DIN, TAPPI, ANSI and the like.)
Bas
Jonge Eland papierrestauratie
eland@knoware.nl
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