Ask A Scientist General Science Archive

name         David M.
status       other
age          50s

Question -   What is the difference between a dye and a pigment?
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Dyes are dissolved in whatever solvent is used. Pigments are insoluble.
Often a pigment is made by attaching a dye molecule to an insoluble
particle.

Vince Calder
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Hi David,

According to my sources a pigment is a coloring matter, usually in the form
of an insoluble powder that is mixed with oil, water, etc. to make paint.
The pigment lays on the surface and becomes part of the dry film.

A dye is soluble in water and is used to color cloth, basket reed and other
porous materials.

Hope this helps,
Martha Croll
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Definitions are not well established, but mine is:

A dye is a molecule or chemical which absorbs light more at some visible 
wavelengths than at others.
Added to a clear medium, it gives clear colors.  Transparent, not even 
milky or translucent.
Added to an opaque medium such as concrete, the opacity remains, some 
color is added,
and the net gray-equivalent brightness is always reduced, because a dye 
can only absorb light.

A pigment is a mixture of dye and an opacifying agent,
such as white oxide powders which scatter light or dark colored powders 
which both absorb and scatter.
So the colors they add are opaque; i.e. it looks more like paint, 
regardless of whether the original medium was clear or opaque.
A white or light-colored pigment can sometimes make a dark medium lighter, 
provided
the original medium was more translucent than the pigment.

This translucency can be quantified.  Virtually every opaque material 
becomes transparent or translucent if sliced thin enough.
For light-scattering materials, the thickness which deflects 63% of the 
light can be called the "scattering length".
In effect, this is about the minimum thickness which would fuzz-up your 
view and stop you from reading a newspaper through it,
if held a certain distance above the paper.
Adding enough white or silver or black powder to a medium shortens the 
scattering length.
This enables light-colored pigments to seize control of the color of 
medium-gray concrete and make it white or light colors.
A pure dye could never do that, could only make the gray into dark or 
perhaps medium colors.

Textiles already have plenty of scattering for their uses, so to color 
them all you need is dye.
Adding powder would not endure anyway; it would soon wash out, causing the 
color you added to change.

In the field of resins for fiberglass lamination or casting, the words are 
used interchangeably,
even though some color additives are clear and others have opacifying 
agents in them.
I find this unfortunate.

The freeware 3-D photo-realistic rendering software "POV-Ray" uses the 
word "pigment"
to label a collection of color properties which can be applied to a surface.
I don't yet understand exactly which properties they allow their term to 
include,
but it's probably more complex than just colored absorption, and results 
in some opacity.
I find learning to use POV-Ray "visually educating".

Many dye additives, as delivered, are so concentrated and absorbing that 
they have a color-tinged black appearance.
but if you spread one drop thinly over a surface you could read fine print 
through it.
Food coloring is a water-soluble example of what I would call dye.

A tube of white toothpaste would be a white pigment for some imaginary use.
The same toothpaste, with a red dye added, would be a pink or red pigment.
With an overwhelming concentration of dyes, or with black powder added, it 
could be a black pigment.
Silver or metallic colorants can only be pigment.

"Stain" tends to mean a dye with selective uptake: some parts of the 
stained object get darker than others by retaining more of the colorant.
Wood stains intensify the visibility of the wood's grain; biological 
stains selectively color certain substances for viewing in the microscope.

Hope that adds something relevant to your field.
Jim Swenson
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